Convert CommonJS module into ES module


Convert CommonJS module into ES module. Powered by cjs-es.


npm install -D rollup-plugin-cjs-es


  • Convert CommonJS modules into ES modules so they can be processed by Rollup.
  • Emit warnings for unconverted requires.
  • Use a cache file to solve export type conflicts instead of using proxy modules, so it works better with tree-shaking.
  • Transform dynamic imports: Promise.resolve(require("...")).


import cjs from "rollup-plugin-cjs-es";

export default {
  input: ["entry.js"],
  output: {
    dir: "dist",
    format: "cjs"
  plugins: [
      nested: true

requireis reassigned

If require, exports, or moduleare dynamically assigned, the transformer can't find them and it will emit a warning e.g.

(function(r) {
const r = require;

These patterns are common in module loaders like UMD. I suggest using other plugins to unwrap the module back to the normal CJS pattern.

Lazy load and code splitting

To lazy load an ES module, we can use the import()function:


export const foo = () => {
  return import("./bar");


export const bar = "bar";

After rolluped into CommonJS format, the dist folder would contain two entries:


'use strict';

const foo = () => {
  return Promise.resolve(require("./bar.js"));
}; = foo;


'use strict';

const bar = "bar"; = bar;

So that bar.jsis not loaded until require("foo").foo()is called.

With this plugin, you can use the same feature using CommonJS syntax, by writing the require statement inside a promise:

module.exports = {
  foo: () => {
    return Promise.resolve(require("./bar.js"));

The plugin would transform it into a dynamic import().

Named import/export v.s. default import/export

Missing exports: foo is not exported by foo.js

In the following example, you would get a warning:


const foo = require("./foo");;


const myObject = {
  foo: () => console.log("foo");
// assign the entire object so that cjs-es won't convert the object pattern into named exports.
module.exports = myObject;
(!) Missing exports
foo is not exported by foo.js
1: import * as foo from "./foo";

That is because cjs-es tends to import named exports by default.

To solve this problem, the plugin generates a .cjsescachefile when a build is finished (whether it succeeded or not), which record the export type of each imported module. In the next build, it will read the cache file and determine the export type according to the cache.

You can also use exportTypeoption to tell the plugin that foo.jsexports default member manually:

  plugins: [
      exportType: {
        // the path would be resolved with the current directory.
        "foo.js": "default"

Dynamic import() problem with default export

In the following example, you would get an error under ES enviroment:


  .then(foo => foo());


module.exports = function() {

After rolluped into ES format and renamed them into .mjs:


  .then(foo => foo());


function foo() {

export default foo;
(node:9996) ExperimentalWarning: The ESM module loader is experimental.
(node:9996) UnhandledPromiseRejectionWarning: TypeError: foo is not a function
    at (file:///D:/Dev/node-test/dist/entry.mjs:2:16)
    at <anonymous>

To correctly call the default member, entry.jshas to be modified:

  .then(foo => foo.default());

However, this would break other enviroments like CommonJS:

(node:9432) UnhandledPromiseRejectionWarning: TypeError: foo.default is not a fu
    at (D:\Dev\node-test\dist\entry.js:4:27)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:118:7)
    at Function.Module.runMain (module.js:705:11)
    at startup (bootstrap_node.js:193:16)
    at bootstrap_node.js:660:3

Avoid default export if you want to use dynamic import()+ CommonJS in the same time.

Comparing to rollup-plugin-commonjs

rollup-plugin-commonjsuses a smart method to determine whether to use named or default import. It creates a proxy loader when a module is imported:


const foo = require("foo");;


import "foo";
import foo from "commonjs-proxy:foo";;

With this technic, it can first look into the "foo"module to check its export type, then generate the proxy module which maps named exports into a default export. However, if the required module "foo"uses named exports, it has to be converted into a single object:


import * as _ from "foo";
export default _;

As a result, all named exports are included in the bundle even that only baris used.

The same problem applies to cjs-es as well if you force a module to use default export:


const foo = require("./foo");;


module.exports = {
  foo: () => console.log("foo"),
  bar: () => console.log("bar")


const _export_foo_ = () => console.log("foo");


bundled with exportType: "default"

var foo = {
  foo: () => console.log("foo"),
  bar: () => console.log("bar")

Note that this won't be true after rollup supports tree-shaking for object literal. See


This module exports a single function.


cjsEsFactory(options?:Object) => rollupPlugin

optionshas following optional properties:

  • include: Array<string>. A list of minimatch pattern. Only matched files would be transformed. Match all files by default.

  • exclude: Array<string>. A list of minimatch pattern. Override options.include. Default: [].

  • cache: String. Path to the cache file. falseto disable the cache. Default: ".cjsescache".

  • sourceMap: boolean. If true then generate the source map. Default: true.

  • nested: boolean. If true then analyze the AST recursively, otherwise only top-level nodes are analyzed. Default: false.

  • exportType: null|string|object|function. Tell the plugin how to determine the export type. Valid export types are "named", "default".

    If exportTypeis a function, it has following signature:

    (moduleId) => exportType:String|null|Promise<String|null>

    The return value should be the export type of moduleId.

    If exportTypeis an object, it is a "path/to/file.js": typemap.

    Default: null.


  • 0.9.0 (Jun 14, 2019)

    • Breaking: cache format is chanaged. You may have to delete the old one.
    • Breaking: bump node version to use async/await.
    • Add: analyze missing exports and fallback to default object.
    • Fix: don't warn unmatched export type if nothing is imported.
  • 0.8.0 (Jun 5, 2019)

  • 0.7.0 (Sep 19, 2018)

    • Enhance: don't emit export type unmatch warning when the module is not loaded.
    • Update cjs-es to 0.6.1. Now module.exportsand exportsare bound to a single reference.
  • 0.6.0 (Jul 19, 2018)

    • Fix: destructuring error when initializing the plugin.
    • Update dependencies. Nested named exports won't be merged into a single namespace anymore.
  • 0.5.1 (Jun 30, 2018)

    • Fix: unexpected warning when importee exports both names and default.
  • 0.5.0 (Jun 29, 2018)

    • Add: options.cacheis a file path now.
    • Change: the format of the cache file is changed. The cache file only records modules that export default.
    • Fix: correctly record external modules in the cache.
    • Drop: code splitting with require()had been split out as a new plugin.
  • 0.4.0 (Jun 16, 2018)

    • Update rollup to 0.60.
    • Add: options.cache. Now the plugin would generate a cache file by default.
  • 0.3.2 (May 4, 2018)

    • Fix: TypeError: cannot access property 'name' on undefinedin unwrapImport.
  • 0.3.1 (May 1, 2018)

    • Fix: scope is not correctly analyzed in splitCodeTransformer.
  • 0.3.0 (May 1, 2018)

    • Update cjs-es to 0.4.4.
    • Add: warn users for unconverted require.
    • Add: options.nested.
    • Drop: options.hoistand options.dynamicImport.
  • 0.2.1 (Apr 28, 2018)

    • Update cjs-es to 0.3.2.
    • Add: include pos while reporting errors.
  • 0.2.0 (Apr 28, 2018)

    • Add: exportTypeoption.
    • Drop: importStyle, exportStyleoption.
  • 0.1.0 (Apr 27, 2018)

    • Initial releast.






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