gem-hunter

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: [
    cjs({
      nested: true
    })
  ]
};

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

(function(r) {
  r("foo");
})(require);

const r = require;
r("foo");

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

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

foo.js

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

bar.js

export const bar = "bar";

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

foo.js

'use strict';

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

exports.foo = foo;

bar.js

'use strict';

const bar = "bar";

exports.bar = bar;

So that bar.js is 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().

In the following example, you would get a warning:

entry.js

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

foo.js

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
https://github.com/rollup/rollup/wiki/Troubleshooting#name-is-not-exported-by-mo
dule
entry.js
foo is not exported by foo.js
1: import * as foo from "./foo";
2: foo.foo();
       ^

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

To solve this problem, the plugin generates a .cjsescache file 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 exportType option to tell the plugin that foo.js exports default member manually:

{
  plugins: [
    cjsEs({
      exportType: {
        // the path would be resolved with the current directory.
        "foo.js": "default"
      }
    })
  ]
}

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

entry.js

Promise.resolve(require("./foo"))
  .then(foo => foo());

foo.js

module.exports = function() {
  console.log("foo");
};

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

entry.mjs

import("./foo.mjs")
  .then(foo => foo());

foo.mjs

function foo() {
  console.log("foo");
}

export default foo;

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

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

Promise.resolve(require("./foo"))
  .then(foo => foo.default());

However, this would break other enviroments like CommonJS:

(node:9432) UnhandledPromiseRejectionWarning: TypeError: foo.default is not a fu
nction
    at Promise.resolve.then.foo (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.

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

source

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

transformed

import "foo";
import foo from "commonjs-proxy:foo";
foo.bar();

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:

commonjs-proxy:foo

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

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

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

entry.js

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

foo.js

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

bundled

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

_export_foo_();

bundled with exportType: "default"

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

foo.foo();

Note that this won't be true after rollup supports tree-shaking for object literal. See rollup/rollup#2201

This module exports a single function.

cjsEsFactory(options?:Object) => rollupPlugin

options has 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. false to 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 exportType is a function, it has following signature:

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

  The return value should be the export type of moduleId.

  If exportType is an object, it is a "path/to/file.js": type map.

  Default: null.

• 3.0.0 (Oct 25, 2023)

  • Breaking: bump rollup to 4.0

• 2.0.0 (Oct 17, 2022)

  • Breaking: bump rollup to 3.0

• 1.1.0 (Aug 8, 2022)

  • Bump dependencies.
  • Fix: variable is used before defined.

• 1.0.1 (Jul 27, 2020)

  • Bump dependencies.
  • Fix: drop this.isExternal.

• 1.0.0 (Mar 13, 2020)

  • Breaking: bump rollup to 2.0

• 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)

  • Bump dependencies. Update to rollup@1.13.1.

• 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.exports and exports are 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.cache is 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 undefined in 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.hoist and 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: exportType option.
  • Drop: importStyle, exportStyle option.

• 0.1.0 (Apr 27, 2018)

  • Initial releast.