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CommonJS -> ES Deferred Execution Wrapper
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Babel CommonJS -> ES Deferred Execution Wrapper Format

This plugin implements a deferred execution wodule format that allows supporting CommonJS execution semantics through an ES modules protocol.

Modules converted in this way can only import from other modules converted to this format.

This project aims for accuracy and reliability transforming a tree of CommonJS modules into a spec-compliant tree of ES Modules.

require('babel-core').transform('<source>', {
  parserOpts: {
    allowReturnOutsideFunction: true
  plugins: [
    ['transform-cjs-dew', {
      filename: '"custom-filename.js"',
      dirname: '"/dirname"',
      define: {
        'process.env.NODE_ENV': '"development"'
      resolve (name, { wildcard?, optional?, browserResolve? }) {
        if (name === 'process')
          return 'process-path';
        if (name === './x')
          return './x.js';
      // for wildcard requires, return a string or array or null to indicate unknown resolution
      // '@empty' indicates an empty object resolution, where all named exports are undefined
      // by default, a null resolve embeds a Node require through a import 'module' path
      // browserOnly just inlines the module not found error directly
      browserOnly: false,
      // when a wildcard ends in *, permit automatic extension variations
      // in the conditional expression output
      wildcardExtensions: ['.js'],
      // optional support for ESM dependencies with default export
      esmDependencies (resolved) {
        return resolved.endsWith('.mjs');


import { dew as _depDew } from './dep.dew.js';
import depB from './dep.mjs';
var exports = {};
var module = {
  get exports () {
    return exports;
  set exports (_exports) {
    exports = _exports;
export function dew () {
  if (executed)
    return module.exports;
  executed = true;
  __dew__ = null;
  module.exports = function () {};
  exports.blah = 'hi';
  var a = _depDew().y;
  var b = depB;
  return module.exports;

To import a CommonJS module tree converted via the above into an ES module, the following execution wrapper is required:


import { dew } from './x.dew.js';
export default dew();

As well as execution wrapping, the following code transformations are handled:

  • Simple 'use strict' code conversion if not already strict.
  • Defines the exports and module variables in module scope.
  • Any use of global or GLOBAL defines global in the module scope.
  • Top-level this is replaced with an exports reference.
  • Top-level return is adjusted to always ensure a falsy return value.
  • Internal this references that are not direct calls, fallback to _global.
  • Implicit globals of the form globalName = ... are rescoped for a simple strict module conversion.
  • Use of Buffer and process is transformed into an import of buffer or process. This module name can be customized by the map configuration option.

The remaining strict conversion cases that don't convert are then just the edge cases of loose -> strict mode conversion:

  • Any use of with statements will throw
  • Multiple duplicate function parameters
  • Relying on dynamic arguments
  • Assigning to reserved names
  • Using eval to define variables
  • Expecting this being the global as the default context for function calls
  • delete statements of local variables are removed

The above should comprehensively cover the failure cases.



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