interpreter.js

/**
 * @license
 * JavaScript Interpreter
 *
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * @fileoverview Interpreting JavaScript in JavaScript.
 * @author fraser@google.com (Neil Fraser)
 */
'use strict';

/**
 * Create a new interpreter.
 * @param {string|!Object} code Raw JavaScript text or AST.
 * @param {Function=} opt_initFunc Optional initialization function.  Used to
 *     define APIs.  When called it is passed the interpreter object and the
 *     global scope object.
 * @constructor
 */
var Interpreter = function(code, opt_initFunc) {
  if (typeof code === 'string') {
    code = acorn.parse(code, Interpreter.PARSE_OPTIONS);
  }
  this.ast = code;
  this.initFunc_ = opt_initFunc;
  this.paused_ = false;
  this.polyfills_ = [];
  // Unique identifier for native functions.  Used in serialization.
  this.functionCounter_ = 0;
  // Map node types to our step function names; a property lookup is faster
  // than string concatenation with "step" prefix.
  this.stepFunctions_ = Object.create(null);
  var stepMatch = /^step([A-Z]\w*)$/;
  var m;
  for (var methodName in this) {
    if ((typeof this[methodName] === 'function') &&
        (m = methodName.match(stepMatch))) {
      this.stepFunctions_[m[1]] = this[methodName].bind(this);
    }
  }
  // Create and initialize the global scope.
  this.global = this.createScope(this.ast, null);
  // Run the polyfills.
  this.ast = acorn.parse(this.polyfills_.join('\n'), Interpreter.PARSE_OPTIONS);
  this.polyfills_ = undefined;  // Allow polyfill strings to garbage collect.
  this.stripLocations_(this.ast, undefined, undefined);
  var state = new Interpreter.State(this.ast, this.global);
  state.done = false;
  this.stateStack = [state];
  this.run();
  this.value = undefined;
  // Point at the main program.
  this.ast = code;
  var state = new Interpreter.State(this.ast, this.global);
  state.done = false;
  this.stateStack.length = 0;
  this.stateStack[0] = state;
  // Get a handle on Acorn's node_t object.  It's tricky to access.
  this.nodeConstructor = state.node.constructor;
  // Preserve publicly properties from being pruned/renamed by JS compilers.
  // Add others as needed.
  this['stateStack'] = this.stateStack;
};

/**
 * @const {!Object} Configuration used for all Acorn parsing.
 */
Interpreter.PARSE_OPTIONS = {
  ecmaVersion: 5
};

/**
 * Property descriptor of readonly properties.
 */
Interpreter.READONLY_DESCRIPTOR = {
  configurable: true,
  enumerable: true,
  writable: false
};

/**
 * Property descriptor of non-enumerable properties.
 */
Interpreter.NONENUMERABLE_DESCRIPTOR = {
  configurable: true,
  enumerable: false,
  writable: true
};

/**
 * Property descriptor of readonly, non-enumerable properties.
 */
Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR = {
  configurable: true,
  enumerable: false,
  writable: false
};

/**
 * Property descriptor of variables.
 */
Interpreter.VARIABLE_DESCRIPTOR = {
  configurable: false,
  enumerable: true,
  writable: true
};

/**
 * Unique symbol for indicating that a step has encountered an error, has
 * added it to the stack, and will be thrown within the user's program.
 * When STEP_ERROR is thrown in the JS-Interpreter, the error can be ignored.
 */
Interpreter.STEP_ERROR = {};

/**
 * Unique symbol for indicating that a reference is a variable on the scope,
 * not an object property.
 */
Interpreter.SCOPE_REFERENCE = {};

/**
 * Unique symbol for indicating, when used as the value of the value
 * parameter in calls to setProperty and friends, that the value
 * should be taken from the property descriptor instead.
 */
Interpreter.VALUE_IN_DESCRIPTOR = {};

/**
 * For cycle detection in array to string and error conversion;
 * see spec bug github.com/tc39/ecma262/issues/289
 * Since this is for atomic actions only, it can be a class property.
 */
Interpreter.toStringCycles_ = [];

/**
 * Add more code to the interpreter.
 * @param {string|!Object} code Raw JavaScript text or AST.
 */
Interpreter.prototype.appendCode = function(code) {
  var state = this.stateStack[0];
  if (!state || state.node['type'] !== 'Program') {
    throw Error('Expecting original AST to start with a Program node.');
  }
  if (typeof code === 'string') {
    code = acorn.parse(code, Interpreter.PARSE_OPTIONS);
  }
  if (!code || code['type'] !== 'Program') {
    throw Error('Expecting new AST to start with a Program node.');
  }
  this.populateScope_(code, state.scope);
  // Append the new program to the old one.
  for (var i = 0, node; (node = code['body'][i]); i++) {
    state.node['body'].push(node);
  }
  state.done = false;
};

/**
 * Execute one step of the interpreter.
 * @return {boolean} True if a step was executed, false if no more instructions.
 */
Interpreter.prototype.step = function() {
  var stack = this.stateStack;
  var state = stack[stack.length - 1];
  if (!state) {
    return false;
  }
  var node = state.node, type = node['type'];
  if (type === 'Program' && state.done) {
    return false;
  } else if (this.paused_) {
    return true;
  }
  try {
    var nextState = this.stepFunctions_[type](stack, state, node);
  } catch (e) {
    // Eat any step errors.  They have been thrown on the stack.
    if (e !== Interpreter.STEP_ERROR) {
      // Uh oh.  This is a real error in the JS-Interpreter.  Rethrow.
      throw e;
    }
  }
  if (nextState) {
    stack.push(nextState);
  }
  if (!node['end']) {
    // This is polyfill code.  Keep executing until we arrive at user code.
    return this.step();
  }
  return true;
};

/**
 * Execute the interpreter to program completion.  Vulnerable to infinite loops.
 * @return {boolean} True if a execution is asynchronously blocked,
 *     false if no more instructions.
 */
Interpreter.prototype.run = function() {
  while (!this.paused_ && this.step()) {}
  return this.paused_;
};

/**
 * Initialize the global scope with buitin properties and functions.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initGlobalScope = function(scope) {
  // Initialize uneditable global properties.
  this.setProperty(scope, 'NaN', NaN,
                   Interpreter.READONLY_DESCRIPTOR);
  this.setProperty(scope, 'Infinity', Infinity,
                   Interpreter.READONLY_DESCRIPTOR);
  this.setProperty(scope, 'undefined', undefined,
                   Interpreter.READONLY_DESCRIPTOR);
  this.setProperty(scope, 'window', scope,
                   Interpreter.READONLY_DESCRIPTOR);
  this.setProperty(scope, 'this', scope,
                   Interpreter.READONLY_DESCRIPTOR);
  this.setProperty(scope, 'self', scope); // Editable.

  // Create the objects which will become Object.prototype and
  // Function.prototype, which are needed to bootstrap everything else.
  this.OBJECT_PROTO = new Interpreter.Object(null);
  this.FUNCTION_PROTO = new Interpreter.Object(this.OBJECT_PROTO);
  // Initialize global objects.
  this.initFunction(scope);
  this.initObject(scope);
  // Unable to set scope's parent prior (OBJECT did not exist).
  // Note that in a browser this would be 'Window', whereas in Node.js it would
  // be 'Object'.  This interpreter is closer to Node in that it has no DOM.
  scope.proto = this.OBJECT_PROTO;
  this.setProperty(scope, 'constructor', this.OBJECT);
  this.initArray(scope);
  this.initString(scope);
  this.initBoolean(scope);
  this.initNumber(scope);
  this.initDate(scope);
  this.initRegExp(scope);
  this.initError(scope);
  this.initMath(scope);
  this.initJSON(scope);

  // Initialize global functions.
  var thisInterpreter = this;
  var func = this.createNativeFunction(
      function(x) {throw EvalError("Can't happen");}, false);
  func.eval = true;
  this.setProperty(scope, 'eval', func);

  this.setProperty(scope, 'parseInt',
      this.createNativeFunction(parseInt, false));
  this.setProperty(scope, 'parseFloat',
      this.createNativeFunction(parseFloat, false));

  this.setProperty(scope, 'isNaN',
      this.createNativeFunction(isNaN, false));

  this.setProperty(scope, 'isFinite',
      this.createNativeFunction(isFinite, false));

  var strFunctions = [
    [escape, 'escape'], [unescape, 'unescape'],
    [decodeURI, 'decodeURI'], [decodeURIComponent, 'decodeURIComponent'],
    [encodeURI, 'encodeURI'], [encodeURIComponent, 'encodeURIComponent']
  ];
  for (var i = 0; i < strFunctions.length; i++) {
    var wrapper = (function(nativeFunc) {
      return function(str) {
        try {
          return nativeFunc(str);
        } catch (e) {
          // decodeURI('%xy') will throw an error.  Catch and rethrow.
          thisInterpreter.throwException(thisInterpreter.URI_ERROR, e.message);
        }
      };
    })(strFunctions[i][0]);
    this.setProperty(scope, strFunctions[i][1],
        this.createNativeFunction(wrapper, false),
        Interpreter.NONENUMERABLE_DESCRIPTOR);
  }
  // Preserve publicly properties from being pruned/renamed by JS compilers.
  // Add others as needed.
  this['OBJECT'] = this.OBJECT; this['OBJECT_PROTO'] = this.OBJECT_PROTO;
  this['FUNCTION'] = this.FUNCTION; this['FUNCTION_PROTO'] = this.FUNCTION_PROTO;
  this['ARRAY'] = this.ARRAY; this['ARRAY_PROTO'] = this.ARRAY_PROTO;
  this['REGEXP'] = this.REGEXP; this['REGEXP_PROTO'] = this.REGEXP_PROTO;
  this['DATE'] = this.DATE; this['DATE_PROTO'] = this.DATE_PROTO;
  // The following properties are obsolete.  Do not use.
  this['UNDEFINED'] = undefined; this['NULL'] = null; this['NAN'] = NaN;
  this['TRUE'] = true; this['FALSE'] = false; this['STRING_EMPTY'] = '';
  this['NUMBER_ZERO'] = 0; this['NUMBER_ONE'] = 1;

  // Run any user-provided initialization.
  if (this.initFunc_) {
    this.initFunc_(this, scope);
  }
};

/**
 * Initialize the Function class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initFunction = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  var identifierRegexp = /^[A-Za-z_$][\w$]*$/;
  // Function constructor.
  wrapper = function(var_args) {
    if (thisInterpreter.calledWithNew()) {
      // Called as new Function().
      var newFunc = this;
    } else {
      // Called as Function().
      var newFunc =
          thisInterpreter.createObjectProto(thisInterpreter.FUNCTION_PROTO);
    }
    if (arguments.length) {
      var code = String(arguments[arguments.length - 1]);
    } else {
      var code = '';
    }
    var argsStr = Array.prototype.slice.call(arguments, 0, -1).join(',').trim();
    if (argsStr) {
      var args = argsStr.split(/\s*,\s*/);
      for (var i = 0; i < args.length; i++) {
        var name = args[i];
        if (!identifierRegexp.test(name)) {
          thisInterpreter.throwException(thisInterpreter.SYNTAX_ERROR,
              'Invalid function argument: ' + name);
        }
      }
      argsStr = args.join(', ');
    }
    // Interestingly, the scope for constructed functions is the global scope,
    // even if they were constructed in some other scope.
    newFunc.parentScope = thisInterpreter.global;
    // Acorn needs to parse code in the context of a function or else 'return'
    // statements will be syntax errors.
    try {
      var ast = acorn.parse('(function(' + argsStr + ') {' + code + '})',
          Interpreter.PARSE_OPTIONS);
    } catch (e) {
      // Acorn threw a SyntaxError.  Rethrow as a trappable error.
      thisInterpreter.throwException(thisInterpreter.SYNTAX_ERROR,
          'Invalid code: ' + e.message);
    }
    if (ast['body'].length !== 1) {
      // Function('a', 'return a + 6;}; {alert(1);');
      thisInterpreter.throwException(thisInterpreter.SYNTAX_ERROR,
          'Invalid code in function body.');
    }
    newFunc.node = ast['body'][0]['expression'];
    thisInterpreter.setProperty(newFunc, 'length', newFunc.node['length'],
        Interpreter.READONLY_DESCRIPTOR);
    return newFunc;
  };
  wrapper.id = this.functionCounter_++;
  this.FUNCTION = this.createObjectProto(this.FUNCTION_PROTO);

  this.setProperty(scope, 'Function', this.FUNCTION);
  // Manually setup type and prototype because createObj doesn't recognize
  // this object as a function (this.FUNCTION did not exist).
  this.setProperty(this.FUNCTION, 'prototype', this.FUNCTION_PROTO);
  this.FUNCTION.nativeFunc = wrapper;

  // Configure Function.prototype.
  this.setProperty(this.FUNCTION_PROTO, 'constructor', this.FUNCTION,
                   Interpreter.NONENUMERABLE_DESCRIPTOR);
  this.FUNCTION_PROTO.nativeFunc = function() {};
  this.FUNCTION_PROTO.nativeFunc.id = this.functionCounter_++;
  this.setProperty(this.FUNCTION_PROTO, 'length', 0,
      Interpreter.READONLY_DESCRIPTOR);

  var boxThis = function(value) {
    // In non-strict mode 'this' must be an object.
    if ((!value || !value.isObject) && !thisInterpreter.getScope().strict) {
      if (value === undefined || value === null) {
        // 'Undefined' and 'null' are changed to global object.
        value = thisInterpreter.global;
      } else {
        // Primitives must be boxed in non-strict mode.
        var box = thisInterpreter.createObjectProto(
            thisInterpreter.getPrototype(value));
        box.data = value;
        value = box;
      }
    }
    return value;
  };

  wrapper = function(thisArg, args) {
    var state =
        thisInterpreter.stateStack[thisInterpreter.stateStack.length - 1];
    // Rewrite the current 'CallExpression' to apply a different function.
    state.func_ = this;
    // Assign the 'this' object.
    state.funcThis_ = boxThis(thisArg);
    // Bind any provided arguments.
    state.arguments_ = [];
    if (args !== null && args !== undefined) {
      if (args.isObject) {
        state.arguments_ = thisInterpreter.arrayPseudoToNative(args);
      } else {
        thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
            'CreateListFromArrayLike called on non-object');
      }
    }
    state.doneExec_ = false;
  };
  this.setNativeFunctionPrototype(this.FUNCTION, 'apply', wrapper);

  wrapper = function(thisArg /*, var_args */) {
    var state =
        thisInterpreter.stateStack[thisInterpreter.stateStack.length - 1];
    // Rewrite the current 'CallExpression' to call a different function.
    state.func_ = this;
    // Assign the 'this' object.
    state.funcThis_ = boxThis(thisArg);
    // Bind any provided arguments.
    state.arguments_ = [];
    for (var i = 1; i < arguments.length; i++) {
      state.arguments_.push(arguments[i]);
    }
    state.doneExec_ = false;
  };
  this.setNativeFunctionPrototype(this.FUNCTION, 'call', wrapper);

  this.polyfills_.push(
// Polyfill copied from:
// developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Function/bind
"Object.defineProperty(Function.prototype, 'bind',",
    "{configurable: true, writable: true, value:",
  "function(oThis) {",
    "if (typeof this !== 'function') {",
      "throw TypeError('What is trying to be bound is not callable');",
    "}",
    "var aArgs   = Array.prototype.slice.call(arguments, 1),",
        "fToBind = this,",
        "fNOP    = function() {},",
        "fBound  = function() {",
          "return fToBind.apply(this instanceof fNOP",
                 "? this",
                 ": oThis,",
                 "aArgs.concat(Array.prototype.slice.call(arguments)));",
        "};",
    "if (this.prototype) {",
      "fNOP.prototype = this.prototype;",
    "}",
    "fBound.prototype = new fNOP();",
    "return fBound;",
  "}",
"});",
"");

  // Function has no parent to inherit from, so it needs its own mandatory
  // toString and valueOf functions.
  wrapper = function() {
    return this.toString();
  };
  this.setNativeFunctionPrototype(this.FUNCTION, 'toString', wrapper);
  this.setProperty(this.FUNCTION, 'toString',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);
  wrapper = function() {
    return this.valueOf();
  };
  this.setNativeFunctionPrototype(this.FUNCTION, 'valueOf', wrapper);
  this.setProperty(this.FUNCTION, 'valueOf',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);
};

/**
 * Initialize the Object class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initObject = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // Object constructor.
  wrapper = function(value) {
    if (value === undefined || value === null) {
      // Create a new object.
      if (thisInterpreter.calledWithNew()) {
        // Called as new Object().
        return this;
      } else {
        // Called as Object().
        return thisInterpreter.createObjectProto(thisInterpreter.OBJECT_PROTO);
      }
    }
    if (!value.isObject) {
      // Wrap the value as an object.
      var box = thisInterpreter.createObjectProto(
          thisInterpreter.getPrototype(value));
      box.data = value;
      return box;
    }
    // Return the provided object.
    return value;
  };
  this.OBJECT = this.createNativeFunction(wrapper, true);
  // Throw away the created prototype and use the root prototype.
  this.setProperty(this.OBJECT, 'prototype', this.OBJECT_PROTO);
  this.setProperty(this.OBJECT_PROTO, 'constructor', this.OBJECT);
  this.setProperty(scope, 'Object', this.OBJECT);

  /**
   * Checks if the provided value is null or undefined.
   * If so, then throw an error in the call stack.
   * @param {Interpreter.Value} value Value to check.
   */
  var throwIfNullUndefined = function(value) {
    if (value === undefined || value === null) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          "Cannot convert '" + value + "' to object");
    }
  };

  // Static methods on Object.
  wrapper = function(obj) {
    throwIfNullUndefined(obj);
    var props = obj.isObject ? obj.properties : obj;
    return thisInterpreter.arrayNativeToPseudo(
        Object.getOwnPropertyNames(props));
  };
  this.setProperty(this.OBJECT, 'getOwnPropertyNames',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  wrapper = function(obj) {
    throwIfNullUndefined(obj);
    if (obj.isObject) {
      obj = obj.properties;
    }
    return thisInterpreter.arrayNativeToPseudo(Object.keys(obj));
  };
  this.setProperty(this.OBJECT, 'keys',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  wrapper = function(proto) {
    // Support for the second argument is the responsibility of a polyfill.
    if (proto === null) {
      return thisInterpreter.createObjectProto(null);
    }
    if (proto === undefined || !proto.isObject) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          'Object prototype may only be an Object or null');
    }
    return thisInterpreter.createObjectProto(proto);
  };
  this.setProperty(this.OBJECT, 'create',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  // Add a polyfill to handle create's second argument.
  this.polyfills_.push(
"(function() {",
  "var create_ = Object.create;",
  "Object.create = function(proto, props) {",
    "var obj = create_(proto);",
    "props && Object.defineProperties(obj, props);",
    "return obj;",
  "};",
"})();",
"");

  wrapper = function(obj, prop, descriptor) {
    prop = String(prop);
    if (!obj || !obj.isObject) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          'Object.defineProperty called on non-object');
    }
    if (!descriptor || !descriptor.isObject) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          'Property description must be an object');
    }
    if (!obj.properties[prop] && obj.preventExtensions) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          "Can't define property '" + prop + "', object is not extensible");
    }
    // The polyfill guarantees no inheritance and no getter functions.
    // Therefore the descriptor properties map is the native object needed.
    thisInterpreter.setProperty(obj, prop, Interpreter.VALUE_IN_DESCRIPTOR,
                                descriptor.properties);
    return obj;
  };
  this.setProperty(this.OBJECT, 'defineProperty',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  this.polyfills_.push(
// Flatten the descriptor to remove any inheritance or getter functions.
"(function() {",
  "var defineProperty_ = Object.defineProperty;",
  "Object.defineProperty = function(obj, prop, d1) {",
    "var d2 = {};",
    "if ('configurable' in d1) d2.configurable = d1.configurable;",
    "if ('enumerable' in d1) d2.enumerable = d1.enumerable;",
    "if ('writable' in d1) d2.writable = d1.writable;",
    "if ('value' in d1) d2.value = d1.value;",
    "if ('get' in d1) d2.get = d1.get;",
    "if ('set' in d1) d2.set = d1.set;",
    "return defineProperty_(obj, prop, d2);",
  "};",
"})();",

"Object.defineProperty(Object, 'defineProperties',",
    "{configurable: true, writable: true, value:",
  "function(obj, props) {",
    "var keys = Object.keys(props);",
    "for (var i = 0; i < keys.length; i++) {",
      "Object.defineProperty(obj, keys[i], props[keys[i]]);",
    "}",
    "return obj;",
  "}",
"});",
"");

  wrapper = function(obj, prop) {
    if (!obj || !obj.isObject) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR,
          'Object.getOwnPropertyDescriptor called on non-object');
    }
    prop = String(prop);
    if (!(prop in obj.properties)) {
      return undefined;
    }
    var descriptor = Object.getOwnPropertyDescriptor(obj.properties, prop);
    var getter = obj.getter[prop];
    var setter = obj.setter[prop];

    if (getter || setter) {
      descriptor.get = getter;
      descriptor.set = setter;
      delete descriptor.value;
      delete descriptor.writable;
    }
    // Preserve value, but remove it for the nativeToPseudo call.
    var value = descriptor.value;
    var hasValue = 'value' in descriptor;
    delete descriptor.value;
    var pseudoDescriptor = thisInterpreter.nativeToPseudo(descriptor);
    if (hasValue) {
      thisInterpreter.setProperty(pseudoDescriptor, 'value', value);
    }
    return pseudoDescriptor;
  };
  this.setProperty(this.OBJECT, 'getOwnPropertyDescriptor',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  wrapper = function(obj) {
    throwIfNullUndefined(obj);
    return thisInterpreter.getPrototype(obj);
  };
  this.setProperty(this.OBJECT, 'getPrototypeOf',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  wrapper = function(obj) {
    return Boolean(obj) && !obj.preventExtensions;
  };
  this.setProperty(this.OBJECT, 'isExtensible',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  wrapper = function(obj) {
    if (obj && obj.isObject) {
      obj.preventExtensions = true;
    }
    return obj;
  };
  this.setProperty(this.OBJECT, 'preventExtensions',
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  // Instance methods on Object.
  this.setNativeFunctionPrototype(this.OBJECT, 'toString',
      Interpreter.Object.prototype.toString);
  this.setNativeFunctionPrototype(this.OBJECT, 'toLocaleString',
      Interpreter.Object.prototype.toString);
  this.setNativeFunctionPrototype(this.OBJECT, 'valueOf',
      Interpreter.Object.prototype.valueOf);

  wrapper = function(prop) {
    throwIfNullUndefined(this);
    if (!this.isObject) {
      return this.hasOwnProperty(prop);
    }
    return String(prop) in this.properties;
  };
  this.setNativeFunctionPrototype(this.OBJECT, 'hasOwnProperty', wrapper);

  wrapper = function(prop) {
    throwIfNullUndefined(this);
    if (!this.isObject) {
      return this.propertyIsEnumerable(prop);
    }
    return Object.prototype.propertyIsEnumerable.call(this.properties, prop);
  };
  this.setNativeFunctionPrototype(this.OBJECT, 'propertyIsEnumerable', wrapper);

  wrapper = function(obj) {
    while (true) {
      // Note, circular loops shouldn't be possible.
      obj = thisInterpreter.getPrototype(obj);
      if (!obj) {
        // No parent; reached the top.
        return false;
      }
      if (obj === this) {
        return true;
      }
    }
  };
  this.setNativeFunctionPrototype(this.OBJECT, 'isPrototypeOf',  wrapper);
};

/**
 * Initialize the Array class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initArray = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // Array constructor.
  wrapper = function(var_args) {
    if (thisInterpreter.calledWithNew()) {
      // Called as new Array().
      var newArray = this;
    } else {
      // Called as Array().
      var newArray =
          thisInterpreter.createObjectProto(thisInterpreter.ARRAY_PROTO);
    }
    var first = arguments[0];
    if (arguments.length === 1 && typeof first === 'number') {
      if (isNaN(Interpreter.legalArrayLength(first))) {
        thisInterpreter.throwException(thisInterpreter.RANGE_ERROR,
                                       'Invalid array length');
      }
      newArray.properties.length = first;
    } else {
      for (var i = 0; i < arguments.length; i++) {
        newArray.properties[i] = arguments[i];
      }
      newArray.properties.length = i;
    }
    return newArray;
  };
  this.ARRAY = this.createNativeFunction(wrapper, true);
  this.ARRAY_PROTO = this.ARRAY.properties['prototype'];
  this.setProperty(scope, 'Array', this.ARRAY);

  // Static methods on Array.
  wrapper = function(obj) {
    return obj && obj.class === 'Array';
  };
  this.setProperty(this.ARRAY, 'isArray',
                   this.createNativeFunction(wrapper, false),
                   Interpreter.NONENUMERABLE_DESCRIPTOR);

  // Instance methods on Array.
  wrapper = function() {
    return Array.prototype.pop.call(this.properties);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'pop', wrapper);

  wrapper = function(var_args) {
    return Array.prototype.push.apply(this.properties, arguments);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'push', wrapper);

  wrapper = function() {
    return Array.prototype.shift.call(this.properties);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'shift', wrapper);

  wrapper = function(var_args) {
    return Array.prototype.unshift.apply(this.properties, arguments);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'unshift', wrapper);

  wrapper = function() {
    Array.prototype.reverse.call(this.properties);
    return this;
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'reverse', wrapper);

  wrapper = function(index, howmany /*, var_args*/) {
    var list = Array.prototype.splice.apply(this.properties, arguments);
    return thisInterpreter.arrayNativeToPseudo(list);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'splice', wrapper);

  wrapper = function(opt_begin, opt_end) {
    var list = Array.prototype.slice.call(this.properties, opt_begin, opt_end);
    return thisInterpreter.arrayNativeToPseudo(list);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'slice', wrapper);

  wrapper = function(opt_separator) {
    return Array.prototype.join.call(this.properties, opt_separator);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'join', wrapper);

  wrapper = function(var_args) {
    var list = [];
    var length = 0;
    // Start by copying the current array.
    var iLength = thisInterpreter.getProperty(this, 'length');
    for (var i = 0; i < iLength; i++) {
      if (thisInterpreter.hasProperty(this, i)) {
        var element = thisInterpreter.getProperty(this, i);
        list[length] = element;
      }
      length++;
    }
    // Loop through all arguments and copy them in.
    for (var i = 0; i < arguments.length; i++) {
      var value = arguments[i];
      if (thisInterpreter.isa(value, thisInterpreter.ARRAY)) {
        var jLength = thisInterpreter.getProperty(value, 'length');
        for (var j = 0; j < jLength; j++) {
          if (thisInterpreter.hasProperty(value, j)) {
            list[length] = thisInterpreter.getProperty(value, j);
          }
          length++;
        }
      } else {
        list[length] = value;
      }
    }
    return thisInterpreter.arrayNativeToPseudo(list);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'concat', wrapper);

  wrapper = function(searchElement, opt_fromIndex) {
    return Array.prototype.indexOf.apply(this.properties, arguments);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'indexOf', wrapper);

  wrapper = function(searchElement, opt_fromIndex) {
    return Array.prototype.lastIndexOf.apply(this.properties, arguments);
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'lastIndexOf', wrapper);

  wrapper = function() {
    Array.prototype.sort.call(this.properties);
    return this;
  };
  this.setNativeFunctionPrototype(this.ARRAY, 'sort', wrapper);

  this.polyfills_.push(
// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/every
"Object.defineProperty(Array.prototype, 'every',",
    "{configurable: true, writable: true, value:",
  "function(callbackfn, thisArg) {",
    "if (!this || typeof callbackfn !== 'function') throw TypeError();",
    "var T, k;",
    "var O = Object(this);",
    "var len = O.length >>> 0;",
    "if (arguments.length > 1) T = thisArg;",
    "k = 0;",
    "while (k < len) {",
      "if (k in O && !callbackfn.call(T, O[k], k, O)) return false;",
      "k++;",
    "}",
    "return true;",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/filter
"Object.defineProperty(Array.prototype, 'filter',",
    "{configurable: true, writable: true, value:",
  "function(fun/*, thisArg*/) {",
    "if (this === void 0 || this === null || typeof fun !== 'function') throw TypeError();",
    "var t = Object(this);",
    "var len = t.length >>> 0;",
    "var res = [];",
    "var thisArg = arguments.length >= 2 ? arguments[1] : void 0;",
    "for (var i = 0; i < len; i++) {",
      "if (i in t) {",
        "var val = t[i];",
        "if (fun.call(thisArg, val, i, t)) res.push(val);",
      "}",
    "}",
    "return res;",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach
"Object.defineProperty(Array.prototype, 'forEach',",
    "{configurable: true, writable: true, value:",
  "function(callback, thisArg) {",
    "if (!this || typeof callback !== 'function') throw TypeError();",
    "var T, k;",
    "var O = Object(this);",
    "var len = O.length >>> 0;",
    "if (arguments.length > 1) T = thisArg;",
    "k = 0;",
    "while (k < len) {",
      "if (k in O) callback.call(T, O[k], k, O);",
      "k++;",
    "}",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/map
"Object.defineProperty(Array.prototype, 'map',",
    "{configurable: true, writable: true, value:",
  "function(callback, thisArg) {",
    "if (!this || typeof callback !== 'function') new TypeError;",
    "var T, A, k;",
    "var O = Object(this);",
    "var len = O.length >>> 0;",
    "if (arguments.length > 1) T = thisArg;",
    "A = new Array(len);",
    "k = 0;",
    "while (k < len) {",
      "if (k in O) A[k] = callback.call(T, O[k], k, O);",
      "k++;",
    "}",
    "return A;",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce
"Object.defineProperty(Array.prototype, 'reduce',",
    "{configurable: true, writable: true, value:",
  "function(callback /*, initialValue*/) {",
    "if (!this || typeof callback !== 'function') throw TypeError();",
    "var t = Object(this), len = t.length >>> 0, k = 0, value;",
    "if (arguments.length === 2) {",
      "value = arguments[1];",
    "} else {",
      "while (k < len && !(k in t)) k++;",
      "if (k >= len) {",
        "throw TypeError('Reduce of empty array with no initial value');",
      "}",
      "value = t[k++];",
    "}",
    "for (; k < len; k++) {",
      "if (k in t) value = callback(value, t[k], k, t);",
    "}",
    "return value;",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/ReduceRight
"Object.defineProperty(Array.prototype, 'reduceRight',",
    "{configurable: true, writable: true, value:",
  "function(callback /*, initialValue*/) {",
    "if (null === this || 'undefined' === typeof this || 'function' !== typeof callback) throw TypeError();",
    "var t = Object(this), len = t.length >>> 0, k = len - 1, value;",
    "if (arguments.length >= 2) {",
      "value = arguments[1];",
    "} else {",
      "while (k >= 0 && !(k in t)) k--;",
      "if (k < 0) {",
        "throw TypeError('Reduce of empty array with no initial value');",
      "}",
      "value = t[k--];",
    "}",
    "for (; k >= 0; k--) {",
      "if (k in t) value = callback(value, t[k], k, t);",
    "}",
    "return value;",
  "}",
"});",

// Polyfill copied from:
// developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Array/some
"Object.defineProperty(Array.prototype, 'some',",
    "{configurable: true, writable: true, value:",
  "function(fun/*, thisArg*/) {",
    "if (!this || typeof fun !== 'function') throw TypeError();",
    "var t = Object(this);",
    "var len = t.length >>> 0;",
    "var thisArg = arguments.length >= 2 ? arguments[1] : void 0;",
    "for (var i = 0; i < len; i++) {",
      "if (i in t && fun.call(thisArg, t[i], i, t)) {",
        "return true;",
      "}",
    "}",
    "return false;",
  "}",
"});",


"(function() {",
  "var sort_ = Array.prototype.sort;",
  "Array.prototype.sort = function(opt_comp) {",
    // Fast native sort.
    "if (typeof opt_comp !== 'function') {",
      "return sort_.call(this);",
    "}",
    // Slow bubble sort.
    "for (var i = 0; i < this.length; i++) {",
      "var changes = 0;",
      "for (var j = 0; j < this.length - i - 1; j++) {",
        "if (opt_comp(this[j], this[j + 1]) > 0) {",
          "var swap = this[j];",
          "this[j] = this[j + 1];",
          "this[j + 1] = swap;",
          "changes++;",
        "}",
      "}",
      "if (!changes) break;",
    "}",
    "return this;",
  "};",
"})();",

"Object.defineProperty(Array.prototype, 'toLocaleString',",
    "{configurable: true, writable: true, value:",
  "function() {",
    "var out = [];",
    "for (var i = 0; i < this.length; i++) {",
      "out[i] = (this[i] === null || this[i] === undefined) ? '' : this[i].toLocaleString();",
    "}",
    "return out.join(',');",
  "}",
"});",
"");
};

/**
 * Initialize the String class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initString = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // String constructor.
  wrapper = function(value) {
    value = String(value);
    if (thisInterpreter.calledWithNew()) {
      // Called as new String().
      this.data = value;
      return this;
    } else {
      // Called as String().
      return value;
    }
  };
  this.STRING = this.createNativeFunction(wrapper, true);
  this.setProperty(scope, 'String', this.STRING);

  // Static methods on String.
  this.setProperty(this.STRING, 'fromCharCode',
      this.createNativeFunction(String.fromCharCode, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  // Instance methods on String.
  // Methods with exclusively primitive arguments.
  var functions = ['charAt', 'charCodeAt', 'concat', 'indexOf', 'lastIndexOf',
      'slice', 'substr', 'substring', 'toLocaleLowerCase', 'toLocaleUpperCase',
      'toLowerCase', 'toUpperCase', 'trim'];
  for (var i = 0; i < functions.length; i++) {
    this.setNativeFunctionPrototype(this.STRING, functions[i],
                                    String.prototype[functions[i]]);
  }

  wrapper = function(compareString, locales, options) {
    locales = locales ? thisInterpreter.pseudoToNative(locales) : undefined;
    options = options ? thisInterpreter.pseudoToNative(options) : undefined;
    return String(this).localeCompare(compareString, locales, options);
  };
  this.setNativeFunctionPrototype(this.STRING, 'localeCompare', wrapper);

  wrapper = function(separator, limit) {
    if (thisInterpreter.isa(separator, thisInterpreter.REGEXP)) {
      separator = separator.data;
    }
    var jsList = String(this).split(separator, limit);
    return thisInterpreter.arrayNativeToPseudo(jsList);
  };
  this.setNativeFunctionPrototype(this.STRING, 'split', wrapper);

  wrapper = function(regexp) {
    if (thisInterpreter.isa(regexp, thisInterpreter.REGEXP)) {
      regexp = regexp.data;
    }
    var m = String(this).match(regexp);
    return m && thisInterpreter.arrayNativeToPseudo(m);
  };
  this.setNativeFunctionPrototype(this.STRING, 'match', wrapper);

  wrapper = function(regexp) {
    if (thisInterpreter.isa(regexp, thisInterpreter.REGEXP)) {
      regexp = regexp.data;
    }
    return String(this).search(regexp);
  };
  this.setNativeFunctionPrototype(this.STRING, 'search', wrapper);

  wrapper = function(substr, newSubstr) {
    // Support for function replacements is the responsibility of a polyfill.
    if (thisInterpreter.isa(substr, thisInterpreter.REGEXP)) {
      substr = substr.data;
    }
    return String(this).replace(substr, newSubstr);
  };
  this.setNativeFunctionPrototype(this.STRING, 'replace', wrapper);
  // Add a polyfill to handle replace's second argument being a function.
  this.polyfills_.push(
"(function() {",
  "var replace_ = String.prototype.replace;",
  "String.prototype.replace = function(substr, newSubstr) {",
    "if (typeof newSubstr !== 'function') {",
      // string.replace(string|regexp, string)
      "return replace_.call(this, substr, newSubstr);",
    "}",
    "var str = this;",
    "if (substr instanceof RegExp) {",  // string.replace(regexp, function)
      "var subs = [];",
      "var m = substr.exec(str);",
      "while (m) {",
        "m.push(m.index, str);",
        "var inject = newSubstr.apply(null, m);",
        "subs.push([m.index, m[0].length, inject]);",
        "m = substr.global ? substr.exec(str) : null;",
      "}",
      "for (var i = subs.length - 1; i >= 0; i--) {",
        "str = str.substring(0, subs[i][0]) + subs[i][2] + " +
            "str.substring(subs[i][0] + subs[i][1]);",
      "}",
    "} else {",                         // string.replace(string, function)
      "var i = str.indexOf(substr);",
      "if (i !== -1) {",
        "var inject = newSubstr(str.substr(i, substr.length), i, str);",
        "str = str.substring(0, i) + inject + " +
            "str.substring(i + substr.length);",
      "}",
    "}",
    "return str;",
  "};",
"})();",
"");
};

/**
 * Initialize the Boolean class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initBoolean = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // Boolean constructor.
  wrapper = function(value) {
    value = Boolean(value);
    if (thisInterpreter.calledWithNew()) {
      // Called as new Boolean().
      this.data = value;
      return this;
    } else {
      // Called as Boolean().
      return value;
    }
  };
  this.BOOLEAN = this.createNativeFunction(wrapper, true);
  this.setProperty(scope, 'Boolean', this.BOOLEAN);
};

/**
 * Initialize the Number class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initNumber = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // Number constructor.
  wrapper = function(value) {
    value = Number(value);
    if (thisInterpreter.calledWithNew()) {
      // Called as new Number().
      this.data = value;
      return this;
    } else {
      // Called as Number().
      return value;
    }
  };
  this.NUMBER = this.createNativeFunction(wrapper, true);
  this.setProperty(scope, 'Number', this.NUMBER);

  var numConsts = ['MAX_VALUE', 'MIN_VALUE', 'NaN', 'NEGATIVE_INFINITY',
                   'POSITIVE_INFINITY'];
  for (var i = 0; i < numConsts.length; i++) {
    this.setProperty(this.NUMBER, numConsts[i], Number[numConsts[i]],
        Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  }

  // Instance methods on Number.
  wrapper = function(fractionDigits) {
    try {
      return Number(this).toExponential(fractionDigits);
    } catch (e) {
      // Throws if fractionDigits isn't within 0-20.
      thisInterpreter.throwException(thisInterpreter.ERROR, e.message);
    }
  };
  this.setNativeFunctionPrototype(this.NUMBER, 'toExponential', wrapper);

  wrapper = function(digits) {
    try {
      return Number(this).toFixed(digits);
    } catch (e) {
      // Throws if digits isn't within 0-20.
      thisInterpreter.throwException(thisInterpreter.ERROR, e.message);
    }
  };
  this.setNativeFunctionPrototype(this.NUMBER, 'toFixed', wrapper);

  wrapper = function(precision) {
    try {
      return Number(this).toPrecision(precision);
    } catch (e) {
      // Throws if precision isn't within range (depends on implementation).
      thisInterpreter.throwException(thisInterpreter.ERROR, e.message);
    }
  };
  this.setNativeFunctionPrototype(this.NUMBER, 'toPrecision', wrapper);

  wrapper = function(radix) {
    try {
      return Number(this).toString(radix);
    } catch (e) {
      // Throws if radix isn't within 2-36.
      thisInterpreter.throwException(thisInterpreter.ERROR, e.message);
    }
  };
  this.setNativeFunctionPrototype(this.NUMBER, 'toString', wrapper);

  wrapper = function(locales, options) {
    locales = locales ? thisInterpreter.pseudoToNative(locales) : undefined;
    options = options ? thisInterpreter.pseudoToNative(options) : undefined;
    return Number(this).toLocaleString(locales, options);
  };
  this.setNativeFunctionPrototype(this.NUMBER, 'toLocaleString', wrapper);
};

/**
 * Initialize the Date class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initDate = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // Date constructor.
  wrapper = function(value, var_args) {
    if (!thisInterpreter.calledWithNew()) {
      // Called as Date().
      // Calling Date() as a function returns a string, no arguments are heeded.
      return Date();
    }
    // Called as new Date().
    var args = [null].concat(Array.from(arguments));
    this.data = new (Function.prototype.bind.apply(Date, args));
    return this;
  };
  this.DATE = this.createNativeFunction(wrapper, true);
  this.DATE_PROTO = this.DATE.properties['prototype'];
  this.setProperty(scope, 'Date', this.DATE);

  // Static methods on Date.
  this.setProperty(this.DATE, 'now', this.createNativeFunction(Date.now, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  this.setProperty(this.DATE, 'parse',
      this.createNativeFunction(Date.parse, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  this.setProperty(this.DATE, 'UTC', this.createNativeFunction(Date.UTC, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  // Instance methods on Date.
  var functions = ['getDate', 'getDay', 'getFullYear', 'getHours',
      'getMilliseconds', 'getMinutes', 'getMonth', 'getSeconds', 'getTime',
      'getTimezoneOffset', 'getUTCDate', 'getUTCDay', 'getUTCFullYear',
      'getUTCHours', 'getUTCMilliseconds', 'getUTCMinutes', 'getUTCMonth',
      'getUTCSeconds', 'getYear',
      'setDate', 'setFullYear', 'setHours', 'setMilliseconds',
      'setMinutes', 'setMonth', 'setSeconds', 'setTime', 'setUTCDate',
      'setUTCFullYear', 'setUTCHours', 'setUTCMilliseconds', 'setUTCMinutes',
      'setUTCMonth', 'setUTCSeconds', 'setYear',
      'toDateString', 'toISOString', 'toJSON', 'toGMTString',
      'toLocaleDateString', 'toLocaleString', 'toLocaleTimeString',
      'toTimeString', 'toUTCString'];
  for (var i = 0; i < functions.length; i++) {
    wrapper = (function(nativeFunc) {
      return function(var_args) {
        var args = [];
        for (var i = 0; i < arguments.length; i++) {
          args[i] = thisInterpreter.pseudoToNative(arguments[i]);
        }
        return this.data[nativeFunc].apply(this.data, args);
      };
    })(functions[i]);
    this.setNativeFunctionPrototype(this.DATE, functions[i], wrapper);
  }
};

/**
 * Initialize Regular Expression object.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initRegExp = function(scope) {
  var thisInterpreter = this;
  var wrapper;
  // RegExp constructor.
  wrapper = function(pattern, flags) {
    if (thisInterpreter.calledWithNew()) {
      // Called as new RegExp().
      var rgx = this;
    } else {
      // Called as RegExp().
      var rgx = thisInterpreter.createObjectProto(thisInterpreter.REGEXP_PROTO);
    }
    pattern = pattern ? pattern.toString() : '';
    flags = flags ? flags.toString() : '';
    thisInterpreter.populateRegExp(rgx, new RegExp(pattern, flags));
    return rgx;
  };
  this.REGEXP = this.createNativeFunction(wrapper, true);
  this.REGEXP_PROTO = this.REGEXP.properties['prototype'];
  this.setProperty(scope, 'RegExp', this.REGEXP);

  this.setProperty(this.REGEXP.properties['prototype'], 'global', undefined,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(this.REGEXP.properties['prototype'], 'ignoreCase', undefined,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(this.REGEXP.properties['prototype'], 'multiline', undefined,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(this.REGEXP.properties['prototype'], 'source', '(?:)',
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);

  wrapper = function(str) {
    return this.data.test(str);
  };
  this.setNativeFunctionPrototype(this.REGEXP, 'test', wrapper);

  wrapper = function(str) {
    str = str.toString();
    // Get lastIndex from wrapped regex, since this is settable.
    this.data.lastIndex =
        Number(thisInterpreter.getProperty(this, 'lastIndex'));
    var match = this.data.exec(str);
    thisInterpreter.setProperty(this, 'lastIndex', this.data.lastIndex);

    if (match) {
      var result =
          thisInterpreter.createObjectProto(thisInterpreter.ARRAY_PROTO);
      for (var i = 0; i < match.length; i++) {
        thisInterpreter.setProperty(result, i, match[i]);
      }
      // match has additional properties.
      thisInterpreter.setProperty(result, 'index', match.index);
      thisInterpreter.setProperty(result, 'input', match.input);
      return result;
    }
    return null;
  };
  this.setNativeFunctionPrototype(this.REGEXP, 'exec', wrapper);
};

/**
 * Initialize the Error class.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initError = function(scope) {
  var thisInterpreter = this;
  // Error constructor.
  this.ERROR = this.createNativeFunction(function(opt_message) {
    if (thisInterpreter.calledWithNew()) {
      // Called as new Error().
      var newError = this;
    } else {
      // Called as Error().
      var newError = thisInterpreter.createObject(thisInterpreter.ERROR);
    }
    if (opt_message) {
      thisInterpreter.setProperty(newError, 'message', String(opt_message),
          Interpreter.NONENUMERABLE_DESCRIPTOR);
    }
    return newError;
  }, true);
  this.setProperty(scope, 'Error', this.ERROR);
  this.setProperty(this.ERROR.properties['prototype'], 'message', '',
      Interpreter.NONENUMERABLE_DESCRIPTOR);
  this.setProperty(this.ERROR.properties['prototype'], 'name', 'Error',
      Interpreter.NONENUMERABLE_DESCRIPTOR);

  var createErrorSubclass = function(name) {
    var constructor = thisInterpreter.createNativeFunction(
        function(opt_message) {
          if (thisInterpreter.calledWithNew()) {
            // Called as new XyzError().
            var newError = this;
          } else {
            // Called as XyzError().
            var newError = thisInterpreter.createObject(constructor);
          }
          if (opt_message) {
            thisInterpreter.setProperty(newError, 'message',
                String(opt_message), Interpreter.NONENUMERABLE_DESCRIPTOR);
          }
          return newError;
        }, true);
    thisInterpreter.setProperty(constructor, 'prototype',
        thisInterpreter.createObject(thisInterpreter.ERROR));
    thisInterpreter.setProperty(constructor.properties['prototype'], 'name',
        name, Interpreter.NONENUMERABLE_DESCRIPTOR);
    thisInterpreter.setProperty(scope, name, constructor);

    return constructor;
  };

  this.EVAL_ERROR = createErrorSubclass('EvalError');
  this.RANGE_ERROR = createErrorSubclass('RangeError');
  this.REFERENCE_ERROR = createErrorSubclass('ReferenceError');
  this.SYNTAX_ERROR = createErrorSubclass('SyntaxError');
  this.TYPE_ERROR = createErrorSubclass('TypeError');
  this.URI_ERROR = createErrorSubclass('URIError');
};

/**
 * Initialize Math object.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initMath = function(scope) {
  var thisInterpreter = this;
  var myMath = this.createObjectProto(this.OBJECT_PROTO);
  this.setProperty(scope, 'Math', myMath);
  var mathConsts = ['E', 'LN2', 'LN10', 'LOG2E', 'LOG10E', 'PI',
                    'SQRT1_2', 'SQRT2'];
  for (var i = 0; i < mathConsts.length; i++) {
    this.setProperty(myMath, mathConsts[i], Math[mathConsts[i]],
        Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  }
  var numFunctions = ['abs', 'acos', 'asin', 'atan', 'atan2', 'ceil', 'cos',
                      'exp', 'floor', 'log', 'max', 'min', 'pow', 'random',
                      'round', 'sin', 'sqrt', 'tan'];
  for (var i = 0; i < numFunctions.length; i++) {
    this.setProperty(myMath, numFunctions[i],
        this.createNativeFunction(Math[numFunctions[i]], false),
        Interpreter.NONENUMERABLE_DESCRIPTOR);
  }
};

/**
 * Initialize JSON object.
 * @param {!Interpreter.Object} scope Global scope.
 */
Interpreter.prototype.initJSON = function(scope) {
  var thisInterpreter = this;
  var myJSON = thisInterpreter.createObjectProto(this.OBJECT_PROTO);
  this.setProperty(scope, 'JSON', myJSON);

  var wrapper = function(text) {
    try {
      var nativeObj = JSON.parse(text.toString());
    } catch (e) {
      thisInterpreter.throwException(thisInterpreter.SYNTAX_ERROR, e.message);
    }
    return thisInterpreter.nativeToPseudo(nativeObj);
  };
  this.setProperty(myJSON, 'parse', this.createNativeFunction(wrapper, false));

  wrapper = function(value) {
    var nativeObj = thisInterpreter.pseudoToNative(value);
    try {
      var str = JSON.stringify(nativeObj);
    } catch (e) {
      thisInterpreter.throwException(thisInterpreter.TYPE_ERROR, e.message);
    }
    return str;
  };
  this.setProperty(myJSON, 'stringify',
      this.createNativeFunction(wrapper, false));
};

/**
 * Is an object of a certain class?
 * @param {Interpreter.Value} child Object to check.
 * @param {Interpreter.Object} constructor Constructor of object.
 * @return {boolean} True if object is the class or inherits from it.
 *     False otherwise.
 */
Interpreter.prototype.isa = function(child, constructor) {
  if (child === null || child === undefined || !constructor) {
    return false;
  }
  var proto = constructor.properties['prototype'];
  if (child === proto) {
    return true;
  }
  // The first step up the prototype chain is harder since the child might be
  // a primitive value.  Subsequent steps can just follow the .proto property.
  child = this.getPrototype(child);
  while (child) {
    if (child === proto) {
      return true;
    }
    child = child.proto;
  }
  return false;
};

/**
 * Is a value a legal integer for an array length?
 * @param {Interpreter.Value} x Value to check.
 * @return {number} Zero, or a positive integer if the value can be
 *     converted to such.  NaN otherwise.
 */
Interpreter.legalArrayLength = function(x) {
  var n = x >>> 0;
  // Array length must be between 0 and 2^32-1 (inclusive).
  return (n === Number(x)) ? n : NaN;
};

/**
 * Is a value a legal integer for an array index?
 * @param {Interpreter.Value} x Value to check.
 * @return {number} Zero, or a positive integer if the value can be
 *     converted to such.  NaN otherwise.
 */
Interpreter.legalArrayIndex = function(x) {
  var n = x >>> 0;
  // Array index cannot be 2^32-1, otherwise length would be 2^32.
  // 0xffffffff is 2^32-1.
  return (String(n) === String(x) && n !== 0xffffffff) ? n : NaN;
};

/**
 * Typedef for JS values.
 * @typedef {!Interpreter.Object|boolean|number|string|undefined|null}
 */
Interpreter.Value;

/**
 * Class for an object.
 * @param {Interpreter.Object} proto Prototype object or null.
 * @constructor
 */
Interpreter.Object = function(proto) {
  this.getter = Object.create(null);
  this.setter = Object.create(null);
  this.properties = Object.create(null);
  this.proto = proto;
};

/** @type {Interpreter.Object} */
Interpreter.Object.prototype.proto = null;

/** @type {boolean} */
Interpreter.Object.prototype.isObject = true;

/** @type {string} */
Interpreter.Object.prototype.class = 'Object';

/** @type {Date|RegExp|boolean|number|string|undefined|null} */
Interpreter.Object.prototype.data = null;

/**
 * Convert this object into a string.
 * @return {string} String value.
 * @override
 */
Interpreter.Object.prototype.toString = function() {
  if (this.class === 'Array') {
    // Array
    var cycles = Interpreter.toStringCycles_;
    cycles.push(this);
    try {
      var strs = [];
      for (var i = 0; i < this.properties.length; i++) {
        var value = this.properties[i];
        strs[i] = (value && value.isObject && cycles.indexOf(value) !== -1) ?
            '...' : value;
      }
    } finally {
      cycles.pop();
    }
    return strs.join(',');
  }
  if (this.class === 'Error') {
    var cycles = Interpreter.toStringCycles_;
    if (cycles.indexOf(this) !== -1) {
      return '[object Error]';
    }
    var name, message;
    // Bug: Does not support getters and setters for name or message.
    var obj = this;
    do {
      if ('name' in obj.properties) {
        name = obj.properties['name'];
        break;
      }
    } while ((obj = obj.proto));
    var obj = this;
    do {
      if ('message' in obj.properties) {
        message = obj.properties['message'];
        break;
      }
    } while ((obj = obj.proto));
    cycles.push(this);
    try {
      name = name && name.toString();
      message = message && message.toString();
    } finally {
      cycles.pop();
    }
    return message ? name + ': ' + message : String(name);
  }

  // RegExp, Date, and boxed primitives.
  if (this.data !== null) {
    return String(this.data);
  }

  return '[object ' + this.class + ']';
};

/**
 * Return the object's value.
 * @return {Interpreter.Value} Value.
 * @override
 */
Interpreter.Object.prototype.valueOf = function() {
  if (this.data === undefined || this.data === null ||
      this.data instanceof RegExp) {
    return this; // An Object.
  }
  if (this.data instanceof Date) {
    return this.data.valueOf();  // Milliseconds.
  }
  return /** @type {(boolean|number|string)} */ (this.data);  // Boxed primitive.
};

/**
 * Create a new data object based on a constructor's prototype.
 * @param {Interpreter.Object} constructor Parent constructor function,
 *     or null if scope object.
 * @return {!Interpreter.Object} New data object.
 */
Interpreter.prototype.createObject = function(constructor) {
  return this.createObjectProto(constructor &&
                                constructor.properties['prototype']);
};

/**
 * Create a new data object based on a prototype.
 * @param {Interpreter.Object} proto Prototype object.
 * @return {!Interpreter.Object} New data object.
 */
Interpreter.prototype.createObjectProto = function(proto) {
  if (typeof proto !== 'object') {
    throw Error('Non object prototype');
  }
  var obj = new Interpreter.Object(proto);
  // Functions have prototype objects.
  if (this.isa(obj, this.FUNCTION)) {
    this.setProperty(obj, 'prototype',
                     this.createObjectProto(this.OBJECT_PROTO || null));
    obj.class = 'Function';
  }
  // Arrays have length.
  if (this.isa(obj, this.ARRAY)) {
    this.setProperty(obj, 'length', 0,
        {configurable: false, enumerable: false, writable: true});
    obj.class = 'Array';
  }
  if (this.isa(obj, this.ERROR)) {
    obj.class = 'Error';
  }
  return obj;
};

/**
 * Initialize a pseudo regular expression object based on a native regular
 * expression object.
 * @param {!Interpreter.Object} pseudoRegexp The existing object to set.
 * @param {!RegExp} nativeRegexp The native regular expression.
 */
Interpreter.prototype.populateRegExp = function(pseudoRegexp, nativeRegexp) {
  pseudoRegexp.data = nativeRegexp;
  // lastIndex is settable, all others are read-only attributes
  this.setProperty(pseudoRegexp, 'lastIndex', nativeRegexp.lastIndex,
      Interpreter.NONENUMERABLE_DESCRIPTOR);
  this.setProperty(pseudoRegexp, 'source', nativeRegexp.source,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(pseudoRegexp, 'global', nativeRegexp.global,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(pseudoRegexp, 'ignoreCase', nativeRegexp.ignoreCase,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
  this.setProperty(pseudoRegexp, 'multiline', nativeRegexp.multiline,
      Interpreter.READONLY_NONENUMERABLE_DESCRIPTOR);
};

/**
 * Create a new function.
 * @param {!Object} node AST node defining the function.
 * @param {!Object} scope Parent scope.
 * @return {!Interpreter.Object} New function.
 */
Interpreter.prototype.createFunction = function(node, scope) {
  var func = this.createObjectProto(this.FUNCTION_PROTO);
  func.parentScope = scope;
  func.node = node;
  this.setProperty(func, 'length', func.node['params'].length,
      Interpreter.READONLY_DESCRIPTOR);
  return func;
};

/**
 * Create a new native function.
 * @param {!Function} nativeFunc JavaScript function.
 * @param {boolean=} opt_constructor If true, the function's
 * prototype will have its constructor property set to the function.
 * If false, the function cannot be called as a constructor (e.g. escape).
 * Defaults to undefined.
 * @return {!Interpreter.Object} New function.
 */
Interpreter.prototype.createNativeFunction =
    function(nativeFunc, opt_constructor) {
  var func = this.createObjectProto(this.FUNCTION_PROTO);
  func.nativeFunc = nativeFunc;
  nativeFunc.id = this.functionCounter_++;
  this.setProperty(func, 'length', nativeFunc.length,
      Interpreter.READONLY_DESCRIPTOR);
  if (opt_constructor) {
    this.setProperty(func.properties['prototype'], 'constructor',
        func, Interpreter.NONENUMERABLE_DESCRIPTOR);
  } else if (opt_constructor === false) {
    func.illegalConstructor = true;
    this.setProperty(func, 'prototype', undefined);
  }
  return func;
};

/**
 * Create a new native asynchronous function.
 * @param {!Function} asyncFunc JavaScript function.
 * @return {!Interpreter.Object} New function.
 */
Interpreter.prototype.createAsyncFunction = function(asyncFunc) {
  var func = this.createObjectProto(this.FUNCTION_PROTO);
  func.asyncFunc = asyncFunc;
  asyncFunc.id = this.functionCounter_++;
  this.setProperty(func, 'length', asyncFunc.length,
      Interpreter.READONLY_DESCRIPTOR);
  return func;
};

/**
 * Converts from a native JS object or value to a JS interpreter object.
 * Can handle JSON-style values, does NOT handle cycles.
 * @param {*} nativeObj The native JS object to be converted.
 * @return {Interpreter.Value} The equivalent JS interpreter object.
 */
Interpreter.prototype.nativeToPseudo = function(nativeObj) {
  if ((typeof nativeObj !== 'object' && typeof nativeObj !== 'function') ||
      nativeObj === null) {
    return nativeObj;
  }

  if (nativeObj instanceof RegExp) {
    var pseudoRegexp = this.createObjectProto(this.REGEXP_PROTO);
    this.populateRegExp(pseudoRegexp, nativeObj);
    return pseudoRegexp;
  }

  if (nativeObj instanceof Date) {
    var pseudoDate = this.createObjectProto(this.DATE_PROTO);
    pseudoDate.data = nativeObj;
    return pseudoDate;
  }

  if (nativeObj instanceof Function) {
    var interpreter = this;
    var wrapper = function() {
      return interpreter.nativeToPseudo(
        nativeObj.apply(interpreter,
          Array.prototype.slice.call(arguments)
          .map(function(i) {
            return interpreter.pseudoToNative(i);
          })
        )
      );
    };
    return this.createNativeFunction(wrapper, undefined);
  }

  var pseudoObj;
  if (Array.isArray(nativeObj)) {  // Array.
    pseudoObj = this.createObjectProto(this.ARRAY_PROTO);
    for (var i = 0; i < nativeObj.length; i++) {
      if (i in nativeObj) {
        this.setProperty(pseudoObj, i, this.nativeToPseudo(nativeObj[i]));
      }
    }
  } else {  // Object.
    pseudoObj = this.createObjectProto(this.OBJECT_PROTO);
    for (var key in nativeObj) {
      this.setProperty(pseudoObj, key, this.nativeToPseudo(nativeObj[key]));
    }
  }
  return pseudoObj;
};

/**
 * Converts from a JS interpreter object to native JS object.
 * Can handle JSON-style values, plus cycles.
 * @param {Interpreter.Value} pseudoObj The JS interpreter object to be
 * converted.
 * @param {Object=} opt_cycles Cycle detection (used in recursive calls).
 * @return {*} The equivalent native JS object or value.
 */
Interpreter.prototype.pseudoToNative = function(pseudoObj, opt_cycles) {
  if ((typeof pseudoObj !== 'object' && typeof pseudoObj !== 'function') ||
      pseudoObj === null) {
    return pseudoObj;
  }

  if (this.isa(pseudoObj, this.REGEXP)) {  // Regular expression.
    return pseudoObj.data;
  }

  if (this.isa(pseudoObj, this.DATE)) {  // Date.
    return pseudoObj.data;
  }

  var cycles = opt_cycles || {
    pseudo: [],
    native: []
  };
  var i = cycles.pseudo.indexOf(pseudoObj);
  if (i !== -1) {
    return cycles.native[i];
  }
  cycles.pseudo.push(pseudoObj);
  var nativeObj;
  if (this.isa(pseudoObj, this.ARRAY)) {  // Array.
    nativeObj = [];
    cycles.native.push(nativeObj);
    var length = this.getProperty(pseudoObj, 'length');
    for (var i = 0; i < length; i++) {
      if (this.hasProperty(pseudoObj, i)) {
        nativeObj[i] =
            this.pseudoToNative(this.getProperty(pseudoObj, i), cycles);
      }
    }
  } else {  // Object.
    nativeObj = {};
    cycles.native.push(nativeObj);
    var val;
    for (var key in pseudoObj.properties) {
      val = pseudoObj.properties[key];
      nativeObj[key] = this.pseudoToNative(val, cycles);
    }
  }
  cycles.pseudo.pop();
  cycles.native.pop();
  return nativeObj;
};

/**
 * Converts from a native JS array to a JS interpreter array.
 * Does handle non-numeric properties (like str.match's index prop).
 * Does NOT recurse into the array's contents.
 * @param {!Array} nativeArray The JS array to be converted.
 * @return {!Interpreter.Object} The equivalent JS interpreter array.
 */
Interpreter.prototype.arrayNativeToPseudo = function(nativeArray) {
  var pseudoArray = this.createObjectProto(this.ARRAY_PROTO);
  var props = Object.getOwnPropertyNames(nativeArray);
  for (var i = 0; i < props.length; i++) {
    this.setProperty(pseudoArray, props[i], nativeArray[props[i]]);
  }
  return pseudoArray;
};

/**
 * Converts from a JS interpreter array to native JS array.
 * Does handle non-numeric properties (like str.match's index prop).
 * Does NOT recurse into the array's contents.
 * @param {!Interpreter.Object} pseudoArray The JS interpreter array,
 *     or JS interpreter object pretending to be an array.
 * @return {!Array} The equivalent native JS array.
 */
Interpreter.prototype.arrayPseudoToNative = function(pseudoArray) {
  var nativeArray = [];
  for (var key in pseudoArray.properties) {
    nativeArray[key] = this.getProperty(pseudoArray, key);
  }
  // pseudoArray might be an object pretending to be an array.  In this case
  // it's possible that length is non-existent, invalid, or smaller than the
  // largest defined numeric property.  Set length explicitly here.
  nativeArray.length = Interpreter.legalArrayLength(
      this.getProperty(pseudoArray, 'length')) || 0;
  return nativeArray;
};

/**
 * Look up the prototype for this value.
 * @param {Interpreter.Value} value Data object.
 * @return {Interpreter.Object} Prototype object, null if none.
 */
Interpreter.prototype.getPrototype = function(value) {
  switch (typeof value) {
    case 'number':
      return this.NUMBER.properties['prototype'];
    case 'boolean':
      return this.BOOLEAN.properties['prototype'];
    case 'string':
      return this.STRING.properties['prototype'];
  }
  if (value) {
    return value.proto;
  }
  return null;
};

/**
 * Fetch a property value from a data object.
 * @param {Interpreter.Value} obj Data object.
 * @param {Interpreter.Value} name Name of property.
 * @return {Interpreter.Value} Property value (may be undefined).
 */
Interpreter.prototype.getProperty = function(obj, name) {
  name = String(name);
  if (obj === undefined || obj === null) {
    this.throwException(this.TYPE_ERROR,
                        "Cannot read property '" + name + "' of " + obj);
  }
  if (name === 'length') {
    // Special cases for magic length property.
    if (this.isa(obj, this.STRING)) {
      return String(obj).length;
    }
  } else if (name.charCodeAt(0) < 0x40) {
    // Might have numbers in there?
    // Special cases for string array indexing
    if (this.isa(obj, this.STRING)) {
      var n = Interpreter.legalArrayIndex(name);
      if (!isNaN(n) && n < String(obj).length) {
        return String(obj)[n];
      }
    }
  }
  do {
    if (obj.properties && name in obj.properties) {
      var getter = obj.getter[name];
      if (getter) {
        // Flag this function as being a getter and thus needing immediate
        // execution (rather than being the value of the property).
        getter.isGetter = true;
        return getter;
      }
      return obj.properties[name];
    }
  } while ((obj = this.getPrototype(obj)));
  return undefined;
};

/**
 * Does the named property exist on a data object.
 * @param {Interpreter.Value} obj Data object.
 * @param {Interpreter.Value} name Name of property.
 * @return {boolean} True if property exists.
 */
Interpreter.prototype.hasProperty = function(obj, name) {
  if (!obj.isObject) {
    throw TypeError('Primitive data type has no properties');
  }
  name = String(name);
  if (name === 'length' && this.isa(obj, this.STRING)) {
    return true;
  }
  if (this.isa(obj, this.STRING)) {
    var n = Interpreter.legalArrayIndex(name);
    if (!isNaN(n) && n < String(obj).length) {
      return true;
    }
  }
  do {
    if (obj.properties && name in obj.properties) {
      return true;
    }
  } while ((obj = this.getPrototype(obj)));
  return false;
};

/**
 * Set a property value on a data object.
 * @param {!Interpreter.Object} obj Data object.
 * @param {Interpreter.Value} name Name of property.
 * @param {Interpreter.Value} value New property value.
 *     Use Interpreter.VALUE_IN_DESCRIPTOR if value is handled by
 *     descriptor instead.
 * @param {Object=} opt_descriptor Optional descriptor object.
 * @return {!Interpreter.Object|undefined} Returns a setter function if one
 *     needs to be called, otherwise undefined.
 */
Interpreter.prototype.setProperty = function(obj, name, value, opt_descriptor) {
  name = String(name);
  if (obj === undefined || obj === null) {
    this.throwException(this.TYPE_ERROR,
                        "Cannot set property '" + name + "' of " + obj);
  }
  if (opt_descriptor && ('get' in opt_descriptor || 'set' in opt_descriptor) &&
      ('value' in opt_descriptor || 'writable' in opt_descriptor)) {
    this.throwException(this.TYPE_ERROR, 'Invalid property descriptor. ' +
        'Cannot both specify accessors and a value or writable attribute');
  }
  var strict = !this.stateStack || this.getScope().strict;
  if (!obj.isObject) {
    if (strict) {
      this.throwException(this.TYPE_ERROR, "Can't create property '" + name +
                          "' on '" + obj + "'");
    }
    return;
  }
  if (this.isa(obj, this.STRING)) {
    var n = Interpreter.legalArrayIndex(name);
    if (name === 'length' || (!isNaN(n) && n < String(obj).length)) {
      // Can't set length or letters on String objects.
      if (strict) {
        this.throwException(this.TYPE_ERROR, "Cannot assign to read only " +
            "property '" + name + "' of String '" + obj.data + "'");
      }
      return;
    }
  }
  if (obj.class === 'Array') {
    // Arrays have a magic length variable that is bound to the elements.
    var length = obj.properties.length;
    var i;
    if (name === 'length') {
      // Delete elements if length is smaller.
      if (opt_descriptor) {
        if (!('value' in opt_descriptor)) {
          return;
        }
        value = opt_descriptor.value;
      }
      value = Interpreter.legalArrayLength(value);
      if (isNaN(value)) {
        this.throwException(this.RANGE_ERROR, 'Invalid array length');
      }
      if (value < length) {
        for (i in obj.properties) {
          i = Interpreter.legalArrayIndex(i);
          if (!isNaN(i) && value <= i) {
            delete obj.properties[i];
          }
        }
      }
    } else if (!isNaN(i = Interpreter.legalArrayIndex(name))) {
      // Increase length if this index is larger.
      obj.properties.length = Math.max(length, i + 1);
    }
  }
  if (obj.preventExtensions && !(name in obj.properties)) {
    if (strict) {
      this.throwException(this.TYPE_ERROR, "Can't add property '" + name +
                          "', object is not extensible");
    }
    return;
  }
  if (opt_descriptor) {
    // Define the property.
    if ('get' in opt_descriptor) {
      if (opt_descriptor.get) {
        obj.getter[name] = opt_descriptor.get;
      } else {
        delete obj.getter[name];
      }
    }
    if ('set' in opt_descriptor) {
      if (opt_descriptor.set) {
        obj.setter[name] = opt_descriptor.set;
      } else {
        delete obj.setter[name];
      }
    }
    var descriptor = {};
    if ('configurable' in opt_descriptor) {
      descriptor.configurable = opt_descriptor.configurable;
    }
    if ('enumerable' in opt_descriptor) {
      descriptor.enumerable = opt_descriptor.enumerable;
    }
    if ('writable' in opt_descriptor) {
      descriptor.writable = opt_descriptor.writable;
      delete obj.getter[name];
      delete obj.setter[name];
    }
    if ('value' in opt_descriptor) {
      descriptor.value = opt_descriptor.value;
      delete obj.getter[name];
      delete obj.setter[name];
    } else if (value !== Interpreter.VALUE_IN_DESCRIPTOR) {
      descriptor.value = value;
      delete obj.getter[name];
      delete obj.setter[name];
    }
    try {
      Object.defineProperty(obj.properties, name, descriptor);
    } catch (e) {
      this.throwException(this.TYPE_ERROR, 'Cannot redefine property: ' + name);
    }
  } else {
    // Set the property.
    if (value === Interpreter.VALUE_IN_DESCRIPTOR) {
      throw ReferenceError('Value not specified.');
    }
    // Determine the parent (possibly self) where the property is defined.
    var defObj = obj;
    while (!(name in defObj.properties)) {
      defObj = this.getPrototype(defObj);
      if (!defObj) {
        // This is a new property.
        defObj = obj;
        break;
      }
    }
    if (defObj.setter && defObj.setter[name]) {
      return defObj.setter[name];
    }
    if (defObj.getter && defObj.getter[name]) {
      if (strict) {
        this.throwException(this.TYPE_ERROR, "Cannot set property '" + name +
            "' of object '" + obj + "' which only has a getter");
      }
    } else {
      // No setter, simple assignment.
      try {
        obj.properties[name] = value;
      } catch (e) {
        if (strict) {
          this.throwException(this.TYPE_ERROR, "Cannot assign to read only " +
              "property '" + name + "' of object '" + obj + "'");
        }
      }
    }
  }
};

/**
 * Convenience method for adding a native function as a non-enumerable property
 * onto an object's prototype.
 * @param {!Interpreter.Object} obj Data object.
 * @param {Interpreter.Value} name Name of property.
 * @param {!Function} wrapper Function object.
 */
Interpreter.prototype.setNativeFunctionPrototype =
    function(obj, name, wrapper) {
  this.setProperty(obj.properties['prototype'], name,
      this.createNativeFunction(wrapper, false),
      Interpreter.NONENUMERABLE_DESCRIPTOR);
};

/**
 * Returns the current scope from the stateStack.
 * @return {!Interpreter.Object} Current scope dictionary.
 */
Interpreter.prototype.getScope = function() {
  var scope = this.stateStack[this.stateStack.length - 1].scope;
  if (!scope) {
    throw Error('No scope found.');
  }
  return scope;
};

/**
 * Create a new scope dictionary.
 * @param {!Object} node AST node defining the scope container
 *     (e.g. a function).
 * @param {Interpreter.Object} parentScope Scope to link to.
 * @return {!Interpreter.Object} New scope.
 */
Interpreter.prototype.createScope = function(node, parentScope) {
  var scope = this.createObjectProto(null);
  scope.parentScope = parentScope;
  if (!parentScope) {
    this.initGlobalScope(scope);
  }
  this.populateScope_(node, scope);

  // Determine if this scope starts with 'use strict'.
  scope.strict = false;
  if (parentScope && parentScope.strict) {
    scope.strict = true;
  } else {
    var firstNode = node['body'] && node['body'][0];
    if (firstNode && firstNode.expression &&
        firstNode.expression['type'] === 'Literal' &&
        firstNode.expression.value === 'use strict') {
      scope.strict = true;
    }
  }
  return scope;
};

/**
 * Create a new special scope dictionary. Similar to createScope(), but
 * doesn't assume that the scope is for a function body.
 * This is used for 'catch' clauses and 'with' statements.
 * @param {!Interpreter.Object} parentScope Scope to link to.
 * @param {Interpreter.Object=} opt_scope Optional object to transform into
 *     scope.
 * @return {!Interpreter.Object} New scope.
 */
Interpreter.prototype.createSpecialScope = function(parentScope, opt_scope) {
  if (!parentScope) {
    throw Error('parentScope required');
  }
  var scope = opt_scope || this.createObjectProto(null);
  scope.parentScope = parentScope;
  scope.strict = parentScope.strict;
  return scope;
};

/**
 * Retrieves a value from the scope chain.
 * @param {string} name Name of variable.
 * @return {Interpreter.Value} Any value.
 *   May be flagged as being a getter and thus needing immediate execution
 *   (rather than being the value of the property).
 */
Interpreter.prototype.getValueFromScope = function(name) {
  var scope = this.getScope();
  while (scope && scope !== this.global) {
    if (name in scope.properties) {
      return scope.properties[name];
    }
    scope = scope.parentScope;
  }
  // The root scope is also an object which has inherited properties and
  // could also have getters.
  if (scope === this.global && this.hasProperty(scope, name)) {
    return this.getProperty(scope, name);
  }
  // Typeof operator is unique: it can safely look at non-defined variables.
  var prevNode = this.stateStack[this.stateStack.length - 1].node;
  if (prevNode['type'] === 'UnaryExpression' &&
      prevNode['operator'] === 'typeof') {
    return undefined;
  }
  this.throwException(this.REFERENCE_ERROR, name + ' is not defined');
};

/**
 * Sets a value to the current scope.
 * @param {string} name Name of variable.
 * @param {Interpreter.Value} value Value.
 * @return {!Interpreter.Object|undefined} Returns a setter function if one
 *     needs to be called, otherwise undefined.
 */
Interpreter.prototype.setValueToScope = function(name, value) {
  var scope = this.getScope();
  var strict = scope.strict;
  while (scope && scope !== this.global) {
    if (name in scope.properties) {
      scope.properties[name] = value;
      return undefined;
    }
    scope = scope.parentScope;
  }
  // The root scope is also an object which has readonly properties and
  // could also have setters.
  if (scope === this.global && (!strict || this.hasProperty(scope, name))) {
    return this.setProperty(scope, name, value);
  }
  this.throwException(this.REFERENCE_ERROR, name + ' is not defined');
};

/**
 * Create a new scope for the given node.
 * @param {!Object} node AST node (program or function).
 * @param {!Interpreter.Object} scope Scope dictionary to populate.
 * @private
 */
Interpreter.prototype.populateScope_ = function(node, scope) {
  if (node['type'] === 'VariableDeclaration') {
    for (var i = 0; i < node['declarations'].length; i++) {
      this.setProperty(scope, node['declarations'][i]['id']['name'],
          undefined, Interpreter.VARIABLE_DESCRIPTOR);
    }
  } else if (node['type'] === 'FunctionDeclaration') {
    this.setProperty(scope, node['id']['name'],
        this.createFunction(node, scope), Interpreter.VARIABLE_DESCRIPTOR);
    return;  // Do not recurse into function.
  } else if (node['type'] === 'FunctionExpression') {
    return;  // Do not recurse into function.
  } else if (node['type'] === 'ExpressionStatement') {
    return;  // Expressions can't contain variable/function declarations.
  }
  var nodeClass = node['constructor'];
  for (var name in node) {
    var prop = node[name];
    if (prop && typeof prop === 'object') {
      if (Array.isArray(prop)) {
        for (var i = 0; i < prop.length; i++) {
          if (prop[i] && prop[i].constructor === nodeClass) {
            this.populateScope_(prop[i], scope);
          }
        }
      } else {
        if (prop.constructor === nodeClass) {
          this.populateScope_(prop, scope);
        }
      }
    }
  }
};

/**
 * Remove start and end values from AST, or set start and end values to a
 * constant value.  Used to remove highlighting from polyfills and to set
 * highlighting in an eval to cover the entire eval expression.
 * @param {!Object} node AST node.
 * @param {number=} start Starting character of all nodes, or undefined.
 * @param {number=} end Ending character of all nodes, or undefined.
 * @private
 */
Interpreter.prototype.stripLocations_ = function(node, start, end) {
  if (start) {
    node['start'] = start;
  } else {
    delete node['start'];
  }
  if (end) {
    node['end'] = end;
  } else {
    delete node['end'];
  }
  for (var name in node) {
    if (node.hasOwnProperty(name)) {
      var prop = node[name];
      if (prop && typeof prop === 'object') {
        this.stripLocations_(prop, start, end);
      }
    }
  }
};

/**
 * Is the current state directly being called with as a construction with 'new'.
 * @return {boolean} True if 'new foo()', false if 'foo()'.
 */
Interpreter.prototype.calledWithNew = function() {
  return this.stateStack[this.stateStack.length - 1].isConstructor;
};

/**
 * Gets a value from the scope chain or from an object property.
 * @param {!Array} ref Name of variable or object/propname tuple.
 * @return {Interpreter.Value} Any value.
 *   May be flagged as being a getter and thus needing immediate execution
 *   (rather than being the value of the property).
 */
Interpreter.prototype.getValue = function(ref) {
  if (ref[0] === Interpreter.SCOPE_REFERENCE) {
    // A null/varname variable lookup.
    return this.getValueFromScope(ref[1]);
  } else {
    // An obj/prop components tuple (foo.bar).
    return this.getProperty(ref[0], ref[1]);
  }
};

/**
 * Sets a value to the scope chain or to an object property.
 * @param {!Array} ref Name of variable or object/propname tuple.
 * @param {Interpreter.Value} value Value.
 * @return {!Interpreter.Object|undefined} Returns a setter function if one
 *     needs to be called, otherwise undefined.
 */
Interpreter.prototype.setValue = function(ref, value) {
  if (ref[0] === Interpreter.SCOPE_REFERENCE) {
    // A null/varname variable lookup.
    return this.setValueToScope(ref[1], value);
  } else {
    // An obj/prop components tuple (foo.bar).
    return this.setProperty(ref[0], ref[1], value);
  }
};

/**
  * Completion Value Types.
  * @enum {number}
  */
 Interpreter.Completion = {
   NORMAL: 0,
   BREAK: 1,
   CONTINUE: 2,
   RETURN: 3,
   THROW: 4
 };

/**
 * Throw an exception in the interpreter that can be handled by an
 * interpreter try/catch statement.  If unhandled, a real exception will
 * be thrown.  Can be called with either an error class and a message, or
 * with an actual object to be thrown.
 * @param {!Interpreter.Object} errorClass Type of error (if message is
 *   provided) or the value to throw (if no message).
 * @param {string=} opt_message Message being thrown.
 */
Interpreter.prototype.throwException = function(errorClass, opt_message) {
  if (opt_message === undefined) {
    var error = errorClass;  // This is a value to throw, not an error class.
  } else {
    var error = this.createObject(errorClass);
    this.setProperty(error, 'message', opt_message,
        Interpreter.NONENUMERABLE_DESCRIPTOR);
  }
  this.unwind(Interpreter.Completion.THROW, error, undefined);
  // Abort anything related to the current step.
  throw Interpreter.STEP_ERROR;
};

/**
 * Unwind the stack to the innermost relevant enclosing TryStatement,
 * For/ForIn/WhileStatement or Call/NewExpression.  If this results in
 * the stack being completely unwound the thread will be terminated
 * and the appropriate error being thrown.
 * @param {Interpreter.Completion} type Completion type.
 * @param {Interpreter.Value=} value Value computed, returned or thrown.
 * @param {string=} label Target label for break or return.
 */
Interpreter.prototype.unwind = function(type, value, label) {
  if (type === Interpreter.Completion.NORMAL) {
    throw TypeError('Should not unwind for NORMAL completions');
  }

  for (var stack = this.stateStack; stack.length > 0; stack.pop()) {
    var state = stack[stack.length - 1];
    switch (state.node['type']) {
      case 'TryStatement':
        state.cv = {type: type, value: value, label: label};
        return;
      case 'CallExpression':
      case 'NewExpression':
        if (type === Interpreter.Completion.RETURN) {
          state.value = value;
          return;
        } else if (type !== Interpreter.Completion.THROW) {
          throw Error('Unsynatctic break/continue not rejected by Acorn');
        }
    }
    if (type === Interpreter.Completion.BREAK) {
      if (label ? (state.labels && state.labels.indexOf(label) !== -1) :
          (state.isLoop || state.isSwitch)) {
        stack.pop();
        return;
      }
    } else if (type === Interpreter.Completion.CONTINUE) {
      if (label ? (state.labels && state.labels.indexOf(label) !== -1) :
          state.isLoop) {
        return;
      }
    }
  }

  // Unhandled completion.  Throw a real error.
  var realError;
  if (this.isa(value, this.ERROR)) {
    var errorTable = {
      'EvalError': EvalError,
      'RangeError': RangeError,
      'ReferenceError': ReferenceError,
      'SyntaxError': SyntaxError,
      'TypeError': TypeError,
      'URIError': URIError
    };
    var name = this.getProperty(value, 'name').toString();
    var message = this.getProperty(value, 'message').valueOf();
    var type = errorTable[name] || Error;
    realError = type(message);
  } else {
    realError = String(value);
  }
  throw realError;
};

/**
 * Create a call to a getter function.
 * @param {!Interpreter.Object} func Function to execute.
 * @param {!Interpreter.Object|!Array} left
 *     Name of variable or object/propname tuple.
 * @private
 */
Interpreter.prototype.createGetter_ = function(func, left) {
  // Normally 'this' will be specified as the object component (o.x).
  // Sometimes 'this' is explicitly provided (o).
  var funcThis = Array.isArray(left) ? left[0] : left;
  var node = new this.nodeConstructor();
  node['type'] = 'CallExpression';
  var state = new Interpreter.State(node,
      this.stateStack[this.stateStack.length - 1].scope);
  state.doneCallee_ = true;
  state.funcThis_ = funcThis;
  state.func_ = func;
  state.doneArgs_ = true;
  state.arguments_ = [];
  return state;
};

/**
 * Create a call to a setter function.
 * @param {!Interpreter.Object} func Function to execute.
 * @param {!Interpreter.Object|!Array} left
 *     Name of variable or object/propname tuple.
 * @param {Interpreter.Value} value Value to set.
 * @private
 */
Interpreter.prototype.createSetter_ = function(func, left, value) {
  // Normally 'this' will be specified as the object component (o.x).
  // Sometimes 'this' is implicitly the global object (x).
  var funcThis = Array.isArray(left) ? left[0] : this.global;
  var node = new this.nodeConstructor();
  node['type'] = 'CallExpression';
  var state = new Interpreter.State(node,
      this.stateStack[this.stateStack.length - 1].scope);
  state.doneCallee_ = true;
  state.funcThis_ = funcThis;
  state.func_ = func;
  state.doneArgs_ = true;
  state.arguments_ = [value];
  return state;
};

/**
 * Class for a state.
 * @param {!Object} node AST node for the state.
 * @param {!Interpreter.Object} scope Scope object for the state.
 * @constructor
 */
Interpreter.State = function(node, scope) {
  this.node = node;
  this.scope = scope;
};


///////////////////////////////////////////////////////////////////////////////
// Functions to handle each node type.
///////////////////////////////////////////////////////////////////////////////

Interpreter.prototype['stepArrayExpression'] = function(stack, state, node) {
  var elements = node['elements'];
  var n = state.n_ || 0;
  if (!state.array_) {
    state.array_ = this.createObjectProto(this.ARRAY_PROTO);
    state.array_.properties.length = elements.length;
  } else {
    this.setProperty(state.array_, n, state.value);
    n++;
  }
  while (n < elements.length) {
    // Skip missing elements - they're not defined, not undefined.
    if (elements[n]) {
      state.n_ = n;
      return new Interpreter.State(elements[n], state.scope);
    }
    n++;
  }
  stack.pop();
  stack[stack.length - 1].value = state.array_;
};

Interpreter.prototype['stepAssignmentExpression'] =
    function(stack, state, node) {
  if (!state.doneLeft_) {
    state.doneLeft_ = true;
    var nextState = new Interpreter.State(node['left'], state.scope);
    nextState.components = true;
    return nextState;
  }
  if (!state.doneRight_) {
    if (!state.leftReference_) {
      state.leftReference_ = state.value;
    }
    if (state.doneGetter_) {
      state.leftValue_ = state.value;
    }
    if (!state.doneGetter_ && node['operator'] !== '=') {
      var leftValue = this.getValue(state.leftReference_);
      state.leftValue_ = leftValue;
      if (leftValue && typeof leftValue === 'object' && leftValue.isGetter) {
        // Clear the getter flag and call the getter function.
        leftValue.isGetter = false;
        state.doneGetter_ = true;
        var func = /** @type {!Interpreter.Object} */ (leftValue);
        return this.createGetter_(func, state.leftReference_);
      }
    }
    state.doneRight_ = true;
    return new Interpreter.State(node['right'], state.scope);
  }
  if (state.doneSetter_) {
    // Return if setter function.
    // Setter method on property has completed.
    // Ignore its return value, and use the original set value instead.
    stack.pop();
    stack[stack.length - 1].value = state.setterValue_;
    return;
  }
  var value = state.leftValue_;
  var rightValue = state.value;
  switch (node['operator']) {
    case '=':    value =    rightValue; break;
    case '+=':   value +=   rightValue; break;
    case '-=':   value -=   rightValue; break;
    case '*=':   value *=   rightValue; break;
    case '/=':   value /=   rightValue; break;
    case '%=':   value %=   rightValue; break;
    case '<<=':  value <<=  rightValue; break;
    case '>>=':  value >>=  rightValue; break;
    case '>>>=': value >>>= rightValue; break;
    case '&=':   value &=   rightValue; break;
    case '^=':   value ^=   rightValue; break;
    case '|=':   value |=   rightValue; break;
    default:
      throw SyntaxError('Unknown assignment expression: ' + node['operator']);
  }
  var setter = this.setValue(state.leftReference_, value);
  if (setter) {
    state.doneSetter_ = true;
    state.setterValue_ = value;
    return this.createSetter_(setter, state.leftReference_, value);
  }
  // Return if no setter function.
  stack.pop();
  stack[stack.length - 1].value = value;
};

Interpreter.prototype['stepBinaryExpression'] = function(stack, state, node) {
  if (!state.doneLeft_) {
    state.doneLeft_ = true;
    return new Interpreter.State(node['left'], state.scope);
  }
  if (!state.doneRight_) {
    state.doneRight_ = true;
    state.leftValue_ = state.value;
    return new Interpreter.State(node['right'], state.scope);
  }
  stack.pop();
  var leftValue = state.leftValue_;
  var rightValue = state.value;
  var value;
  switch (node['operator']) {
    case '==':  value = leftValue ==  rightValue; break;
    case '!=':  value = leftValue !=  rightValue; break;
    case '===': value = leftValue === rightValue; break;
    case '!==': value = leftValue !== rightValue; break;
    case '>':   value = leftValue >   rightValue; break;
    case '>=':  value = leftValue >=  rightValue; break;
    case '<':   value = leftValue <   rightValue; break;
    case '<=':  value = leftValue <=  rightValue; break;
    case '+':   value = leftValue +   rightValue; break;
    case '-':   value = leftValue -   rightValue; break;
    case '*':   value = leftValue *   rightValue; break;
    case '/':   value = leftValue /   rightValue; break;
    case '%':   value = leftValue %   rightValue; break;
    case '&':   value = leftValue &   rightValue; break;
    case '|':   value = leftValue |   rightValue; break;
    case '^':   value = leftValue ^   rightValue; break;
    case '<<':  value = leftValue <<  rightValue; break;
    case '>>':  value = leftValue >>  rightValue; break;
    case '>>>': value = leftValue >>> rightValue; break;
    case 'in':
      if (!rightValue || !rightValue.isObject) {
        this.throwException(this.TYPE_ERROR,
            "'in' expects an object, not '" + rightValue + "'");
      }
      value = this.hasProperty(rightValue, leftValue);
      break;
    case 'instanceof':
      if (!this.isa(rightValue, this.FUNCTION)) {
        this.throwException(this.TYPE_ERROR,
            'Right-hand side of instanceof is not an object');
      }
      value = leftValue.isObject ? this.isa(leftValue, rightValue) : false;
      break;
    default:
      throw SyntaxError('Unknown binary operator: ' + node['operator']);
  }
  stack[stack.length - 1].value = value;
};

Interpreter.prototype['stepBlockStatement'] = function(stack, state, node) {
  var n = state.n_ || 0;
  var expression = node['body'][n];
  if (expression) {
    state.n_ = n + 1;
    return new Interpreter.State(expression, state.scope);
  }
  stack.pop();
};

Interpreter.prototype['stepBreakStatement'] = function(stack, state, node) {
  var label = node['label'] && node['label']['name'];
  this.unwind(Interpreter.Completion.BREAK, undefined, label);
};

Interpreter.prototype['stepCallExpression'] = function(stack, state, node) {
  if (!state.doneCallee_) {
    state.doneCallee_ = 1;
    // Components needed to determine value of 'this'.
    var nextState = new Interpreter.State(node['callee'], state.scope);
    nextState.components = true;
    return nextState;
  }
  if (state.doneCallee_ === 1) {
    // Determine value of the function.
    state.doneCallee_ = 2;
    var func = state.value;
    if (Array.isArray(func)) {
      state.func_ = this.getValue(func);
      if (func[0] === Interpreter.SCOPE_REFERENCE) {
        // (Globally or locally) named function.  Is it named 'eval'?
        state.directEval_ = (func[1] === 'eval');
      } else {
        // Method function, 'this' is object (ignored if invoked as 'new').
        state.funcThis_ = func[0];
      }
      func = state.func_;
      if (func && typeof func === 'object' && func.isGetter) {
        // Clear the getter flag and call the getter function.
        func.isGetter = false;
        state.doneCallee_ = 1;
        return this.createGetter_(/** @type {!Interpreter.Object} */ (func),
                         state.value);
      }
    } else {
      // Already evaluated function: (function(){...})();
      state.func_ = func;
    }
    state.arguments_ = [];
    state.n_ = 0;
  }
  var func = state.func_;
  if (!state.doneArgs_) {
    if (state.n_ !== 0) {
      state.arguments_.push(state.value);
    }
    if (node['arguments'][state.n_]) {
      return new Interpreter.State(node['arguments'][state.n_++], state.scope);
    }
    // Determine value of 'this' in function.
    if (node['type'] === 'NewExpression') {
      if (func.illegalConstructor) {
        // Illegal: new escape();
        this.throwException(this.TYPE_ERROR, func + ' is not a constructor');
      }
      // Constructor, 'this' is new object.
      var proto = func.properties['prototype'];
      if (typeof proto !== 'object' || proto === null) {
        // Non-object prototypes default to Object.prototype.
        proto = this.OBJECT_PROTO;
      }
      state.funcThis_ = this.createObjectProto(proto);
      state.isConstructor = true;
    } else if (state.funcThis_ === undefined) {
      // Global function, 'this' is global object (or 'undefined' if strict).
      state.funcThis_ = state.scope.strict ? undefined : this.global;
    }
    state.doneArgs_ = true;
  }
  if (!state.doneExec_) {
    state.doneExec_ = true;
    if (!func || !func.isObject) {
      this.throwException(this.TYPE_ERROR, func + ' is not a function');
    }
    var funcNode = func.node;
    if (funcNode) {
      var scope = this.createScope(funcNode['body'], func.parentScope);
      // Add all arguments.
      for (var i = 0; i < funcNode['params'].length; i++) {
        var paramName = funcNode['params'][i]['name'];
        var paramValue = state.arguments_.length > i ? state.arguments_[i] :
            undefined;
        this.setProperty(scope, paramName, paramValue);
      }
      // Build arguments variable.
      var argsList = this.createObjectProto(this.ARRAY_PROTO);
      for (var i = 0; i < state.arguments_.length; i++) {
        this.setProperty(argsList, i, state.arguments_[i]);
      }
      this.setProperty(scope, 'arguments', argsList);
      // Add the function's name (var x = function foo(){};)
      var name = funcNode['id'] && funcNode['id']['name'];
      if (name) {
        this.setProperty(scope, name, func);
      }
      this.setProperty(scope, 'this', state.funcThis_,
                       Interpreter.READONLY_DESCRIPTOR);
      state.value = undefined;  // Default value if no explicit return.
      return new Interpreter.State(funcNode['body'], scope);
    } else if (func.eval) {
      var code = state.arguments_[0];
      if (typeof code !== 'string') {
        // JS does not parse String objects:
        // eval(new String('1 + 1')) -> '1 + 1'
        state.value = code;
      } else {
        try {
          var ast = acorn.parse(code.toString(), Interpreter.PARSE_OPTIONS);
        } catch (e) {
          // Acorn threw a SyntaxError.  Rethrow as a trappable error.
          this.throwException(this.SYNTAX_ERROR, 'Invalid code: ' + e.message);
        }
        var evalNode = new this.nodeConstructor();
        evalNode['type'] = 'EvalProgram_';
        evalNode['body'] = ast['body'];
        this.stripLocations_(evalNode, node['start'], node['end']);
        // Create new scope and update it with definitions in eval().
        var scope = state.directEval_ ? state.scope : this.global;
        if (scope.strict) {
          // Strict mode get its own scope in eval.
          scope = this.createScope(ast, scope);
        } else {
          // Non-strict mode pollutes the current scope.
          this.populateScope_(ast, scope);
        }
        this.value = undefined;  // Default value if no code.
        return new Interpreter.State(evalNode, scope);
      }
    } else if (func.nativeFunc) {
      state.value = func.nativeFunc.apply(state.funcThis_, state.arguments_);
    } else if (func.asyncFunc) {
      var thisInterpreter = this;
      var callback = function(value) {
        state.value = value;
        thisInterpreter.paused_ = false;
      };
      var argsWithCallback = state.arguments_.concat(callback);
      this.paused_ = true;
      func.asyncFunc.apply(state.funcThis_, argsWithCallback);
      return;
    } else {
      /* A child of a function is a function but is not callable.  For example:
      var F = function() {};
      F.prototype = escape;
      var f = new F();
      f();
      */
      this.throwException(this.TYPE_ERROR, func.class + ' is not a function');
    }
  } else {
    // Execution complete.  Put the return value on the stack.
    stack.pop();
    if (state.isConstructor && typeof state.value !== 'object') {
      stack[stack.length - 1].value = state.funcThis_;
    } else {
      stack[stack.length - 1].value = state.value;
    }
  }
};

Interpreter.prototype['stepCatchClause'] = function(stack, state, node) {
  if (!state.done_) {
    state.done_ = true;
    // Create an empty scope.
    var scope = this.createSpecialScope(state.scope);
    // Add the argument.
    this.setProperty(scope, node['param']['name'], state.throwValue);
    // Execute catch clause.
    return new Interpreter.State(node['body'], scope);
  } else {
    stack.pop();
  }
};

Interpreter.prototype['stepConditionalExpression'] =
    function(stack, state, node) {
  var mode = state.mode_ || 0;
  if (mode === 0) {
    state.mode_ = 1;
    return new Interpreter.State(node['test'], state.scope);
  }
  if (mode === 1) {
    state.mode_ = 2;
    var value = Boolean(state.value);
    if (value && node['consequent']) {
      // Execute 'if' block.
      return new Interpreter.State(node['consequent'], state.scope);
    } else if (!value && node['alternate']) {
      // Execute 'else' block.
      return new Interpreter.State(node['alternate'], state.scope);
    }
    // eval('1;if(false){2}') -> undefined
    this.value = undefined;
  }
  stack.pop();
  if (node['type'] === 'ConditionalExpression') {
    stack[stack.length - 1].value = state.value;
  }
};

Interpreter.prototype['stepContinueStatement'] = function(stack, state, node) {
  var label = node['label'] && node['label']['name'];
  this.unwind(Interpreter.Completion.CONTINUE, undefined, label);
};

Interpreter.prototype['stepDebuggerStatement'] = function(stack, state, node) {
  // Do nothing.  May be overridden by developers.
  stack.pop();
};

Interpreter.prototype['stepDoWhileStatement'] = function(stack, state, node) {
  if (node['type'] === 'DoWhileStatement' && state.test_ === undefined) {
    // First iteration of do/while executes without checking test.
    state.value = true;
    state.test_ = true;
  }
  if (!state.test_) {
    state.test_ = true;
    return new Interpreter.State(node['test'], state.scope);
  }
  if (!state.value) {  // Done, exit loop.
    stack.pop();
  } else if (node['body']) {  // Execute the body.
    state.test_ = false;
    state.isLoop = true;
    return new Interpreter.State(node['body'], state.scope);
  }
};

Interpreter.prototype['stepEmptyStatement'] = function(stack, state, node) {
  stack.pop();
};

Interpreter.prototype['stepEvalProgram_'] = function(stack, state, node) {
  var n = state.n_ || 0;
  var expression = node['body'][n];
  if (expression) {
    state.n_ = n + 1;
    return new Interpreter.State(expression, state.scope);
  }
  stack.pop();
  stack[stack.length - 1].value = this.value;
};

Interpreter.prototype['stepExpressionStatement'] = function(stack, state, node) {
  if (!state.done_) {
    state.done_ = true;
    return new Interpreter.State(node['expression'], state.scope);
  }
  stack.pop();
  // Save this value to interpreter.value for use as a return value if
  // this code is inside an eval function.
  this.value = state.value;
};

Interpreter.prototype['stepForInStatement'] = function(stack, state, node) {
  // First, initialize a variable if exists.  Only do so once, ever.
  if (!state.doneInit_) {
    state.doneInit_ = true;
    if (node['left']['declarations'] &&
        node['left']['declarations'][0]['init']) {
      if (state.scope.strict) {
        this.throwException(this.SYNTAX_ERROR,
            'for-in loop variable declaration may not have an initializer.');
      }
      // Variable initialization: for (var x = 4 in y)
      return new Interpreter.State(node['left'], state.scope);
    }
  }
  // Second, look up the object.  Only do so once, ever.
  if (!state.doneObject_) {
    state.doneObject_ = true;
    if (!state.variable_) {
      state.variable_ = state.value;
    }
    return new Interpreter.State(node['right'], state.scope);
  }
  if (!state.isLoop) {
    // First iteration.
    state.isLoop = true;
    state.object_ = state.value;
    state.visited_ = Object.create(null);
  }
  // Third, find the property name for this iteration.
  if (state.name_ === undefined) {
    done: do {
      if (state.object_ && state.object_.isObject) {
        if (!state.props_) {
          state.props_ = Object.getOwnPropertyNames(state.object_.properties);
        }
        do {
          var prop = state.props_.shift();
        } while (prop && (state.visited_[prop] ||
            !Object.prototype.hasOwnProperty.call(state.object_.properties,
                                                  prop)));
        if (prop) {
          state.visited_[prop] = true;
          if (Object.prototype.propertyIsEnumerable.call(
              state.object_.properties, prop)) {
            state.name_ = prop;
            break done;
          }
        }
      } else if (state.object_ !== null) {
        if (!state.props_) {
          state.props_ = Object.getOwnPropertyNames(state.object_);
        }
        do {
          var prop = state.props_.shift();
        } while (prop && state.visited_[prop]);
        if (prop) {
          state.visited_[prop] = true;
          state.name_ = prop;
          break done;
        }
      }
      state.object_ = this.getPrototype(state.object_);
      state.props_ = null;
    } while (state.object_ !== null);
    if (state.object_ === null) {
      // Done, exit loop.
      stack.pop();
      return;
    }
  }
  // Fourth, find the variable
  if (!state.doneVariable_) {
    state.doneVariable_ = true;
    var left = node['left'];
    if (left['type'] === 'VariableDeclaration') {
      // Inline variable declaration: for (var x in y)
      state.variable_ =
          [Interpreter.SCOPE_REFERENCE, left['declarations'][0]['id']['name']];
    } else {
      // Arbitrary left side: for (foo().bar in y)
      state.variable_ = null;
      var nextState = new Interpreter.State(left, state.scope);
      nextState.components = true;
      return nextState;
    }
  }
  if (!state.variable_) {
    state.variable_ = state.value;
  }
  // Fifth, set the variable.
  if (!state.doneSetter_) {
    state.doneSetter_ = true;
    var value = state.name_;
    var setter = this.setValue(state.variable_, value);
    if (setter) {
      return this.createSetter_(setter, state.variable_, value);
    }
  }
  // Next step will be step three.
  state.name_ = undefined;
  // Reevaluate the variable since it could be a setter on the global object.
  state.doneVariable_ = false;
  state.doneSetter_ = false;
  // Sixth and finally, execute the body if there was one.  this.
  if (node['body']) {
    return new Interpreter.State(node['body'], state.scope);
  }
};

Interpreter.prototype['stepForStatement'] = function(stack, state, node) {
  var mode = state.mode_ || 0;
  if (mode === 0) {
    state.mode_ = 1;
    if (node['init']) {
      return new Interpreter.State(node['init'], state.scope);
    }
  } else if (mode === 1) {
    state.mode_ = 2;
    if (node['test']) {
      return new Interpreter.State(node['test'], state.scope);
    }
  } else if (mode === 2) {
    state.mode_ = 3;
    if (node['test'] && !state.value) {
      // Done, exit loop.
      stack.pop();
    } else {  // Execute the body.
      state.isLoop = true;
      return new Interpreter.State(node['body'], state.scope);
    }
  } else if (mode === 3) {
    state.mode_ = 1;
    if (node['update']) {
      return new Interpreter.State(node['update'], state.scope);
    }
  }
};

Interpreter.prototype['stepFunctionDeclaration'] =
    function(stack, state, node) {
  // This was found and handled when the scope was populated.
  stack.pop();
};

Interpreter.prototype['stepFunctionExpression'] = function(stack, state, node) {
  stack.pop();
  stack[stack.length - 1].value = this.createFunction(node, state.scope);
};

Interpreter.prototype['stepIdentifier'] = function(stack, state, node) {
  stack.pop();
  if (state.components) {
    stack[stack.length - 1].value = [Interpreter.SCOPE_REFERENCE, node['name']];
    return;
  }
  var value = this.getValueFromScope(node['name']);
  // An identifier could be a getter if it's a property on the global object.
  if (value && typeof value === 'object' && value.isGetter) {
    // Clear the getter flag and call the getter function.
    value.isGetter = false;
    var scope = state.scope;
    while (!this.hasProperty(scope, node['name'])) {
      scope = scope.parentScope;
    }
    var func = /** @type {!Interpreter.Object} */ (value);
    return this.createGetter_(func, this.global);
  }
  stack[stack.length - 1].value = value;
};

Interpreter.prototype['stepIfStatement'] =
    Interpreter.prototype['stepConditionalExpression'];

Interpreter.prototype['stepLabeledStatement'] = function(stack, state, node) {
  // No need to hit this node again on the way back up the stack.
  stack.pop();
  // Note that a statement might have multiple labels.
  var labels = state.labels || [];
  labels.push(node['label']['name']);
  var nextState = new Interpreter.State(node['body'], state.scope);
  nextState.labels = labels;
  return nextState;
};

Interpreter.prototype['stepLiteral'] = function(stack, state, node) {
  stack.pop();
  var value = node['value'];
  if (value instanceof RegExp) {
    var pseudoRegexp = this.createObjectProto(this.REGEXP_PROTO);
    this.populateRegExp(pseudoRegexp, value);
    value = pseudoRegexp;
  }
  stack[stack.length - 1].value = value;
};

Interpreter.prototype['stepLogicalExpression'] = function(stack, state, node) {
  if (node['operator'] !== '&&' && node['operator'] !== '||') {
    throw SyntaxError('Unknown logical operator: ' + node['operator']);
  }
  if (!state.doneLeft_) {
    state.doneLeft_ = true;
    return new Interpreter.State(node['left'], state.scope);
  }
  if (!state.doneRight_) {
    if ((node['operator'] === '&&' && !state.value) ||
        (node['operator'] === '||' && state.value)) {
      // Shortcut evaluation.
      stack.pop();
      stack[stack.length - 1].value = state.value;
    } else {
      state.doneRight_ = true;
      return new Interpreter.State(node['right'], state.scope);
    }
  } else {
    stack.pop();
    stack[stack.length - 1].value = state.value;
  }
};

Interpreter.prototype['stepMemberExpression'] = function(stack, state, node) {
  if (!state.doneObject_) {
    state.doneObject_ = true;
    return new Interpreter.State(node['object'], state.scope);
  }
  var propName;
  if (!node['computed']) {
    state.object_ = state.value;
    // obj.foo -- Just access 'foo' directly.
    propName = node['property']['name'];
  } else if (!state.doneProperty_) {
    state.object_ = state.value;
    // obj[foo] -- Compute value of 'foo'.
    state.doneProperty_ = true;
    return new Interpreter.State(node['property'], state.scope);
  } else {
    propName = state.value;
  }
  stack.pop();
  if (state.components) {
    stack[stack.length - 1].value = [state.object_, propName];
  } else {
    var value = this.getProperty(state.object_, propName);
    if (value && typeof value === 'object' && value.isGetter) {
      // Clear the getter flag and call the getter function.
      value.isGetter = false;
      var func = /** @type {!Interpreter.Object} */ (value);
      return this.createGetter_(func, state.object_);
    }
    stack[stack.length - 1].value = value;
  }
};

Interpreter.prototype['stepNewExpression'] =
    Interpreter.prototype['stepCallExpression'];

Interpreter.prototype['stepObjectExpression'] = function(stack, state, node) {
  var n = state.n_ || 0;
  var property = node['properties'][n];
  if (!state.object_) {
    // First execution.
    state.object_ = this.createObjectProto(this.OBJECT_PROTO);
    state.properties_ = Object.create(null);
  } else {
    // Determine property name.
    var key = property['key'];
    if (key['type'] === 'Identifier') {
      var propName = key['name'];
    } else if (key['type'] === 'Literal') {
      var propName = key['value'];
    } else {
      throw SyntaxError('Unknown object structure: ' + key['type']);
    }
    // Set the property computed in the previous execution.
    if (!state.properties_[propName]) {
      // Create temp object to collect value, getter, and/or setter.
      state.properties_[propName] = {};
    }
    state.properties_[propName][property['kind']] = state.value;
    state.n_ = ++n;
    property = node['properties'][n];
  }
  if (property) {
    return new Interpreter.State(property['value'], state.scope);
  }
  for (var key in state.properties_) {
    var kinds = state.properties_[key];
    if ('get' in kinds || 'set' in kinds) {
      // Set a property with a getter or setter.
      var descriptor = {
        configurable: true,
        enumerable: true,
        get: kinds['get'],
        set: kinds['set']
      };
      this.setProperty(state.object_, key, null, descriptor);
    } else {
      // Set a normal property with a value.
      this.setProperty(state.object_, key, kinds['init']);
    }
  }
  stack.pop();
  stack[stack.length - 1].value = state.object_;
};

Interpreter.prototype['stepProgram'] = function(stack, state, node) {
  var expression = node['body'].shift();
  if (expression) {
    state.done = false;
    return new Interpreter.State(expression, state.scope);
  }
  state.done = true;
  // Don't pop the stateStack.
  // Leave the root scope on the tree in case the program is appended to.
};

Interpreter.prototype['stepReturnStatement'] = function(stack, state, node) {
  if (node['argument'] && !state.done_) {
    state.done_ = true;
    return new Interpreter.State(node['argument'], state.scope);
  }
  this.unwind(Interpreter.Completion.RETURN, state.value, undefined);
};

Interpreter.prototype['stepSequenceExpression'] = function(stack, state, node) {
  var n = state.n_ || 0;
  var expression = node['expressions'][n];
  if (expression) {
    state.n_ = n + 1;
    return new Interpreter.State(expression, state.scope);
  }
  stack.pop();
  stack[stack.length - 1].value = state.value;
};

Interpreter.prototype['stepSwitchStatement'] = function(stack, state, node) {
  if (!state.test_) {
    state.test_ = 1;
    return new Interpreter.State(node['discriminant'], state.scope);
  }
  if (state.test_ === 1) {
    state.test_ = 2;
    // Preserve switch value between case tests.
    state.switchValue_ = state.value;
    state.defaultCase_ = -1;
  }

  while (true) {
    var index = state.index_ || 0;
    var switchCase = node['cases'][index];
    if (!state.matched_ && switchCase && !switchCase['test']) {
      // Test on the default case is null.
      // Bypass (but store) the default case, and get back to it later.
      state.defaultCase_ = index;
      state.index_ = index + 1;
      continue;
    }
    if (!switchCase && !state.matched_ && state.defaultCase_ !== -1) {
      // Ran through all cases, no match.  Jump to the default.
      state.matched_ = true;
      state.index_ = state.defaultCase_;
      continue;
    }
    if (switchCase) {
      if (!state.matched_ && !state.tested_ && switchCase['test']) {
        state.tested_ = true;
        return new Interpreter.State(switchCase['test'], state.scope);
      }
      if (state.matched_ || state.value === state.switchValue_) {
        state.matched_ = true;
        var n = state.n_ || 0;
        if (switchCase['consequent'][n]) {
          state.isSwitch = true;
          state.n_ = n + 1;
          return new Interpreter.State(switchCase['consequent'][n],
                                       state.scope);
        }
      }
      // Move on to next case.
      state.tested_ = false;
      state.n_ = 0;
      state.index_ = index + 1;
    } else {
      stack.pop();
      return;
    }
  }
};

Interpreter.prototype['stepThisExpression'] = function(stack, state, node) {
  stack.pop();
  stack[stack.length - 1].value = this.getValueFromScope('this');
};

Interpreter.prototype['stepThrowStatement'] = function(stack, state, node) {
  if (!state.done_) {
    state.done_ = true;
    return new Interpreter.State(node['argument'], state.scope);
  } else {
    this.throwException(state.value);
  }
};

Interpreter.prototype['stepTryStatement'] = function(stack, state, node) {
  if (!state.doneBlock_) {
    state.doneBlock_ = true;
    return new Interpreter.State(node['block'], state.scope);
  }
  if (state.cv && state.cv.type === Interpreter.Completion.THROW &&
      !state.doneHandler_ && node['handler']) {
    state.doneHandler_ = true;
    var nextState = new Interpreter.State(node['handler'], state.scope);
    nextState.throwValue = state.cv.value;
    state.cv = undefined;  // This error has been handled, don't rethrow.
    return nextState;
  }
  if (!state.doneFinalizer_ && node['finalizer']) {
    state.doneFinalizer_ = true;
    return new Interpreter.State(node['finalizer'], state.scope);
  }
  stack.pop();
  if (state.cv) {
    // There was no catch handler, or the catch/finally threw an error.
    // Throw the error up to a higher try.
    this.unwind(state.cv.type, state.cv.value, state.cv.label);
  }
};

Interpreter.prototype['stepUnaryExpression'] = function(stack, state, node) {
  if (!state.done_) {
    state.done_ = true;
    var nextState = new Interpreter.State(node['argument'], state.scope);
    nextState.components = node['operator'] === 'delete';
    return nextState;
  }
  stack.pop();
  var value = state.value;
  if (node['operator'] === '-') {
    value = -value;
  } else if (node['operator'] === '+') {
    value = +value;
  } else if (node['operator'] === '!') {
    value = !value;
  } else if (node['operator'] === '~') {
    value = ~value;
  } else if (node['operator'] === 'delete') {
    var result = true;
    // If value is not an array, then it is a primitive, or some other value.
    // If so, skip the delete and return true.
    if (Array.isArray(value)) {
      var obj = value[0];
      if (obj === Interpreter.SCOPE_REFERENCE) {
        // 'delete foo;' is the same as 'delete window.foo'.
        obj = state.scope;
      }
      var name = String(value[1]);
      try {
        delete obj.properties[name];
      } catch (e) {
        if (state.scope.strict) {
          this.throwException(this.TYPE_ERROR, "Cannot delete property '" +
                              name + "' of '" + obj + "'");
        } else {
          result = false;
        }
      }
    }
    value = result;
  } else if (node['operator'] === 'typeof') {
    value = (value && value.class === 'Function') ? 'function' : typeof value;
  } else if (node['operator'] === 'void') {
    value = undefined;
  } else {
    throw SyntaxError('Unknown unary operator: ' + node['operator']);
  }
  stack[stack.length - 1].value = value;
};

Interpreter.prototype['stepUpdateExpression'] = function(stack, state, node) {
  if (!state.doneLeft_) {
    state.doneLeft_ = true;
    var nextState = new Interpreter.State(node['argument'], state.scope);
    nextState.components = true;
    return nextState;
  }
  if (!state.leftSide_) {
    state.leftSide_ = state.value;
  }
  if (state.doneGetter_) {
    state.leftValue_ = state.value;
  }
  if (!state.doneGetter_) {
    var leftValue = this.getValue(state.leftSide_);
    state.leftValue_ = leftValue;
    if (leftValue && typeof leftValue === 'object' && leftValue.isGetter) {
      // Clear the getter flag and call the getter function.
      leftValue.isGetter = false;
      state.doneGetter_ = true;
      var func = /** @type {!Interpreter.Object} */ (leftValue);
      return this.createGetter_(func, state.leftSide_);
    }
  }
  if (state.doneSetter_) {
    // Return if setter function.
    // Setter method on property has completed.
    // Ignore its return value, and use the original set value instead.
    stack.pop();
    stack[stack.length - 1].value = state.setterValue_;
    return;
  }
  var leftValue = Number(state.leftValue_);
  var changeValue;
  if (node['operator'] === '++') {
    changeValue = leftValue + 1;
  } else if (node['operator'] === '--') {
    changeValue = leftValue - 1;
  } else {
    throw SyntaxError('Unknown update expression: ' + node['operator']);
  }
  var returnValue = node['prefix'] ? changeValue : leftValue;
  var setter = this.setValue(state.leftSide_, changeValue);
  if (setter) {
    state.doneSetter_ = true;
    state.setterValue_ = returnValue;
    return this.createSetter_(setter, state.leftSide_, changeValue);
  }
  // Return if no setter function.
  stack.pop();
  stack[stack.length - 1].value = returnValue;
};

Interpreter.prototype['stepVariableDeclaration'] = function(stack, state, node) {
  var declarations = node['declarations'];
  var n = state.n_ || 0;
  var declarationNode = declarations[n];
  if (state.init_ && declarationNode) {
    // This setValue call never needs to deal with calling a setter function.
    // Note that this is setting the init value, not defining the variable.
    // Variable definition is done when scope is populated.
    this.setValueToScope(declarationNode['id']['name'], state.value);
    state.init_ = false;
    declarationNode = declarations[++n];
  }
  while (declarationNode) {
    // Skip any declarations that are not initialized.  They have already
    // been defined as undefined in populateScope_.
    if (declarationNode['init']) {
      state.n_ = n;
      state.init_ = true;
      return new Interpreter.State(declarationNode['init'], state.scope);
    }
    declarationNode = declarations[++n];
  }
  stack.pop();
};

Interpreter.prototype['stepWithStatement'] = function(stack, state, node) {
  if (!state.doneObject_) {
    state.doneObject_ = true;
    return new Interpreter.State(node['object'], state.scope);
  } else if (!state.doneBody_) {
    state.doneBody_ = true;
    var scope = this.createSpecialScope(state.scope, state.value);
    return new Interpreter.State(node['body'], scope);
  } else {
    stack.pop();
  }
};

Interpreter.prototype['stepWhileStatement'] =
    Interpreter.prototype['stepDoWhileStatement'];

// Preserve top-level API functions from being pruned/renamed by JS compilers.
// Add others as needed.
// The global object ('window' in a browser, 'global' in node.js) is 'this'.
this['Interpreter'] = Interpreter;
Interpreter.prototype['step'] = Interpreter.prototype.step;
Interpreter.prototype['run'] = Interpreter.prototype.run;
Interpreter.prototype['appendCode'] = Interpreter.prototype.appendCode;
Interpreter.prototype['createObject'] = Interpreter.prototype.createObject;
Interpreter.prototype['createObjectProto'] =
    Interpreter.prototype.createObjectProto;
Interpreter.prototype['createAsyncFunction'] =
    Interpreter.prototype.createAsyncFunction;
Interpreter.prototype['createNativeFunction'] =
    Interpreter.prototype.createNativeFunction;
Interpreter.prototype['getProperty'] = Interpreter.prototype.getProperty;
Interpreter.prototype['setProperty'] = Interpreter.prototype.setProperty;
Interpreter.prototype['nativeToPseudo'] = Interpreter.prototype.nativeToPseudo;
Interpreter.prototype['pseudoToNative'] = Interpreter.prototype.pseudoToNative;
// Obsolete.  Do not use.
Interpreter.prototype['createPrimitive'] = function(x) {return x;};