Add support for ESM plugins/presets

ESM modules can use `export default` to expose their plugin or preset.

This does not add support for config files in ESM just yet.

* Add support for plugins in ESM format w/ an `.mjs` extension
* Add support for plugins in ESM format w/ a `.js` extension if the nearest
  `package.json` has a `type: 'module'`
* Add support for interop bundles (CJS w/ `__esModule: true` field)

Author: wooorm

lib/configuration.js

@@ -74,11 +74,13 @@ function load(filePath, callback) {
       return callback(error, file)
     }
 
-    callback(null, self.create())
+    self.create().then(function (result) {
+      callback(null, result)
+    }, callback)
   }
 }
 
-function create(buf, filePath) {
+async function create(buf, filePath) {
   var self = this
   var fn = (filePath && loaders[path.extname(filePath)]) || defaultLoader
   var options = {prefix: self.pluginPrefix, cwd: self.cwd}
@@ -100,21 +102,21 @@ function create(buf, filePath) {
 
   if (contents === undefined) {
     if (self.defaultConfig) {
-      merge(
+      await merge(
         result,
         self.defaultConfig,
         Object.assign({}, options, {root: self.cwd})
       )
     }
   } else {
-    merge(
+    await merge(
       result,
       contents,
       Object.assign({}, options, {root: path.dirname(filePath)})
     )
   }
 
-  merge(result, self.given, Object.assign({}, options, {root: self.cwd}))
+  await merge(result, self.given, Object.assign({}, options, {root: self.cwd}))
 
   return result
 }
@@ -149,26 +151,22 @@ function loadJson(buf, filePath) {
   return result
 }
 
-function merge(target, raw, options) {
+async function merge(target, raw, options) {
   if (typeof raw === 'object' && raw !== null) {
-    addPreset(raw)
+    await addPreset(raw)
   } else {
     throw new Error('Expected preset, not `' + raw + '`')
   }
 
   return target
 
-  function addPreset(result) {
+  async function addPreset(result) {
     var plugins = result.plugins
 
     if (plugins === null || plugins === undefined) {
       // Empty.
     } else if (typeof plugins === 'object' && plugins !== null) {
-      if ('length' in plugins) {
-        addEach(plugins)
-      } else {
-        addIn(plugins)
-      }
+      await ('length' in plugins ? addEach(plugins) : addIn(plugins))
     } else {
       throw new Error(
         'Expected a list or object of plugins, not `' + plugins + '`'
@@ -178,59 +176,80 @@ function merge(target, raw, options) {
     target.settings = Object.assign({}, target.settings, result.settings)
   }
 
-  function addEach(result) {
+  async function addEach(result) {
     var index = -1
     var value
 
     while (++index < result.length) {
       value = result[index]
 
-      if (value !== null && typeof value === 'object' && 'length' in value) {
-        use.apply(null, value)
-      } else {
-        use(value)
-      }
+      // Keep order sequential instead of parallel.
+      // eslint-disable-next-line no-await-in-loop
+      await (value !== null && typeof value === 'object' && 'length' in value
+        ? use.apply(null, value)
+        : use(value))
     }
   }
 
-  function addIn(result) {
+  async function addIn(result) {
     var key
 
     for (key in result) {
-      use(key, result[key])
+      // Keep order sequential instead of parallel.
+      // eslint-disable-next-line no-await-in-loop
+      await use(key, result[key])
     }
   }
 
-  function use(usable, value) {
+  async function use(usable, value) {
     if (typeof usable === 'string') {
-      addModule(usable, value)
+      await addModule(usable, value)
     } else if (typeof usable === 'function') {
       addPlugin(usable, value)
     } else {
-      merge(target, usable, options)
+      await merge(target, usable, options)
     }
   }
 
-  function addModule(id, value) {
+  async function addModule(id, value) {
     var fp = loadPlugin.resolve(id, {cwd: options.root, prefix: options.prefix})
+    var ext
     var result
 
     if (fp) {
-      try {
-        result = require(fp)
-      } catch (error) {
-        throw fault(
-          'Cannot parse script `%s`\n%s',
-          path.relative(options.root, fp),
-          error.stack
-        )
+      ext = path.extname(fp)
+
+      /* istanbul ignore next - To do next major: Tests don’t run on Node 10 */
+      if (ext !== '.mjs') {
+        try {
+          result = require(fp)
+        } catch (error) {
+          if (ext !== '.cjs' && error.code === 'ERR_REQUIRE_ESM') {
+            ext = '.mjs'
+          } else {
+            throw fault(
+              'Cannot parse script `%s`\n%s',
+              path.relative(options.root, fp),
+              error.stack
+            )
+          }
+        }
+
+        if (result && typeof result === 'object' && result.__esModule) {
+          result = result.default
+        }
+      }
+
+      /* istanbul ignore next - To do next major: Tests don’t run on Node 10 */
+      if (ext === '.mjs') {
+        result = (await import(fp)).default
       }
 
       try {
         if (typeof result === 'function') {
           addPlugin(result, value)
         } else {
-          merge(
+          await merge(
             target,
             result,
             Object.assign({}, options, {root: path.dirname(fp)})

lib/find-up.js

@@ -4,6 +4,7 @@ var fs = require('fs')
 var path = require('path')
 var fault = require('fault')
 var debug = require('debug')('unified-engine:find-up')
+var wrap = require('trough/wrap')
 
 module.exports = FindUp
 
@@ -68,23 +69,32 @@ function load(filePath, callback) {
       result.code = 'ENOENT'
       result.path = error.path
       result.syscall = error.syscall
+      loaded(result)
     } else {
-      try {
-        result = self.create(buf, self.givenFilePath)
-        debug('Read given file `%s`', self.givenFilePath)
-      } catch (error_) {
-        result = fault(
-          'Cannot parse given file `%s`\n%s',
-          path.relative(self.cwd, self.givenFilePath),
-          error_.stack
+      wrap(self.create, onparse)(buf, self.givenFilePath)
+    }
+
+    function onparse(error, result) {
+      if (error) {
+        debug(error.message)
+        loaded(
+          fault(
+            'Cannot parse given file `%s`\n%s',
+            path.relative(self.cwd, self.givenFilePath),
+            error.stack
+          )
         )
-        debug(error_.message)
+      } else {
+        debug('Read given file `%s`', self.givenFilePath)
+        loaded(result)
       }
     }
 
-    givenFile = result
-    self.givenFile = result
-    applyAll(cbs, result)
+    function loaded(result) {
+      givenFile = result
+      self.givenFile = result
+      applyAll(cbs, result)
+    }
   }
 
   function find(directory) {
@@ -117,41 +127,40 @@ function load(filePath, callback) {
 
     function done(error, buf) {
       var fp = path.join(directory, self.names[index])
-      var contents
 
       /* istanbul ignore if - Hard to test. */
       if (error) {
         if (error.code === 'ENOENT') {
           return next()
         }
 
-        error = fault(
-          'Cannot read file `%s`\n%s',
-          path.relative(self.cwd, fp),
-          error.message
-        )
         debug(error.message)
-        return found(error)
-      }
-
-      try {
-        contents = self.create(buf, fp)
-      } catch (error_) {
         return found(
           fault(
-            'Cannot parse file `%s`\n%s',
+            'Cannot read file `%s`\n%s',
             path.relative(self.cwd, fp),
-            error_.message
+            error.message
           )
         )
       }
 
-      /* istanbul ignore else - maybe used in the future. */
-      if (contents) {
-        debug('Read file `%s`', fp)
-        found(null, contents)
-      } else {
-        next()
+      wrap(self.create, onparse)(buf, fp)
+
+      function onparse(error, result) {
+        if (error) {
+          found(
+            fault(
+              'Cannot parse file `%s`\n%s',
+              path.relative(self.cwd, fp),
+              error.message
+            )
+          )
+        } else if (result) {
+          debug('Read file `%s`', fp)
+          found(null, result)
+        } else {
+          next()
+        }
       }
     }
 

package.json

@@ -54,6 +54,7 @@
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
     "remark-toc": "^7.0.0",
+    "semver": "^6.0.0",
     "strip-ansi": "^6.0.0",
     "tape": "^5.0.0",
     "unified": "^9.0.0",