Closure Compiler recognizes and several JS module systems, including:

• ES6 Modules
• Common JS
• Closure-Library (goog.module)

During compilation, the compiler normalizes and inlines all of these disparate module by rewriting them into a form where the remaining passes can understand and optimize them fully. You can use multiple versions is possible to work with multiple module systems in a single compilation.

CommonJS and ES6 Modules are file based and a module is imported by it's path. Paths must be absolute or relative and omit the file extension:

• import foo from './folder/source'
• import foo from '/folder/source'
• require('./folder/source')
• require('/folder/source')

The root folder is the current working directory of the compiler when executed.

Each module must only use a single type of import or export. Unless otherwise noted, a module may not mix import or export methods within the same file.

ES6 Module support is enabled when the --language_in flag is used to specify an input language of ECMASCRIPT6 or higher.

ES6 modules use import and export statements to include and expose symbols.

import MySymbol from 'path/to/module';
export MySymbol;

// CommonJS modules have a single export and are therefore imported as namespaces.
// In this case we assume that the commonjs module has a
// "module.exports = { MySymbol: foo }" statement
import * as myModule from 'path/to/commonjs/module';
console.log(myModule.MySymbol);

// The compiler recognizes a special syntax to identify goog.module paths
import Event from 'goog:goog.events';

CommonJS module support is enabled in the compiler when the --process_common_js_modules flag is specified.

// ES6 modules have have multiple export properties. You must reference the desired property.
// In this case we assume that the ES6 module has a "export default MySymbol;" statement
var MySymbol = require('path/to/es6/module').default;
console.log(MySymbol);

// For goog.module, fall back go using goog.require
// ??

goog.module support is always enabled in the compiler.

How do we import other modules within a Closure-Library style file? Must we resort to using the normalized module identifier? If so, we should find a better way to do this.

The compiler can utilize import and export statements from any of the module systems to sort files and drop unreferenced files. See Managing Dependencies.