handle errors consistently

Currently, document.createElement is used in an attempt to generate an
error similar to those thrown by native implementations of window.btoa
and window.atob, throwing a ReferenceError in non-browser environments.

This commit adds and makes use of an InvalidCharacterError constructor
for environment-independent handling of errors.

Author: davidchambers

README.md

@@ -1,6 +1,6 @@
 # Base64.js
 
-< 500 byte* polyfill for browsers which don't provide [`window.btoa`][1] and
+≈ 500 byte* polyfill for browsers which don't provide [`window.btoa`][1] and
 [`window.atob`][2].
 
 Although the script does no harm in browsers which do provide these functions,

base64.js

@@ -1,12 +1,13 @@
 ;(function () {
 
-  var
-    object = typeof exports != 'undefined' ? exports : this, // #8: web workers
-    chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=',
-    INVALID_CHARACTER_ERR = (function () {
-      // fabricate a suitable error object
-      try { document.createElement('$'); }
-      catch (error) { return error; }}());
+  var object = typeof exports != 'undefined' ? exports : this; // #8: web workers
+  var chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=';
+
+  function InvalidCharacterError(message) {
+    this.message = message;
+  }
+  InvalidCharacterError.prototype = new Error;
+  InvalidCharacterError.prototype.name = 'InvalidCharacterError';
 
   // encoder
   // [https://gist.github.com/999166] by [https://github.com/nignag]
@@ -23,7 +24,9 @@
       output += map.charAt(63 & block >> 8 - idx % 1 * 8)
     ) {
       charCode = input.charCodeAt(idx += 3/4);
-      if (charCode > 0xFF) throw INVALID_CHARACTER_ERR;
+      if (charCode > 0xFF) {
+        throw new InvalidCharacterError("'btoa' failed: The string to be encoded contains characters outside of the Latin1 range.");
+      }
       block = block << 8 | charCode;
     }
     return output;
@@ -34,7 +37,9 @@
   object.atob || (
   object.atob = function (input) {
     input = input.replace(/=+$/, '')
-    if (input.length % 4 == 1) throw INVALID_CHARACTER_ERR;
+    if (input.length % 4 == 1) {
+      throw new InvalidCharacterError("'atob' failed: The string to be decoded is not correctly encoded.");
+    }
     for (
       // initialize result and counters
       var bc = 0, bs, buffer, idx = 0, output = '';

base64.min.js

@@ -23,7 +23,10 @@ describe 'Base64.js', ->
     assert.strictEqual btoa('qrstuvwxyz{|}~'), 'cXJzdHV2d3h5ent8fX4='
 
   it 'cannot encode non-ASCII input', ->
-    assert.throws -> btoa '✈'
+    assert.throws (-> btoa '✈'), (err) ->
+      err instanceof Error and
+      err.name is 'InvalidCharacterError' and
+      err.message is "'btoa' failed: The string to be encoded contains characters outside of the Latin1 range."
 
   it 'can decode Base64-encoded input', ->
     assert.strictEqual atob(''), ''
@@ -43,4 +46,7 @@ describe 'Base64.js', ->
     assert.strictEqual atob('cXJzdHV2d3h5ent8fX4='), 'qrstuvwxyz{|}~'
 
   it 'cannot decode invalid input', ->
-    assert.throws -> atob 'a'
+    assert.throws (-> atob 'a'), (err) ->
+      err instanceof Error and
+      err.name is 'InvalidCharacterError' and
+      err.message is "'atob' failed: The string to be decoded is not correctly encoded."