Some packages we install from npm support both commonjs and es modules,
These packages can be imported as follows:
import express from 'express'
// or
const express = require('express')
I have created a package which I already published to npm using es modules.
and I since my another project which I'm working on is built with commonjs, I realized that I can not require it using the following syntax:
const stackPlayer = require('stack-player')
How can I support the two module systems in my package stack-player so that everyone around the world can use it?
Is there another method other than converting all of my project to es modules (which would be too complex since the project is 1 year old and is big enough to refuse the idea). ?
require() Usage
require() can, by default, only be used in CommonJS Modules. The built in method to import ECMAScript modules into CommonJS is using import(pathToFile).then(module => { }).
Support for require()
If you want to support require() for your package, you must provide a CommonJS module.
Here's a functioning example that demonstrates when and how to utilize require() or import(). There are some small differences how import() of a CommonJS module works compared to a ECMAScript Module. Especially that only the default property on the module object is available, when import() is used on a CommonJS file that exported something with module.exports.
index.js which imports different module types (from the demo above):(In case the stackblitz demo will be deleted:)
// executed as CommonJS module
console.time('');
import('./lib/example.cjs').then(({ default: example }) => {
console.timeLog('', 'import cjs', example() == 'Foo'); // true
});
import('./lib/index.mjs').then(({ example }) => {
console.timeLog('', 'import mjs', example() == 'Foo'); // true
});
try {
const example = require('./lib/example.cjs');
console.timeLog('', 'require cjs', example() == 'Foo'); // true
} catch (e) {
console.timeLog('', 'require cjs', '\n' + e.message);
}
try {
const example = require('./lib/index.mjs');
console.timeLog('', 'require mjs', example() == 'Foo');
} catch (e) {
console.timeLog('', 'require mjs', '\n' + e.message); // Error [ERR_REQUIRE_ESM]: require() of ES Module /path/to/lib/index.mjs not supported.
}
lib/example.cjs
module.exports = function example() {
return 'Foo';
};
lib/index.mjs
import example from './example.cjs';
export { example };
export default example;
Conditional Export for Packages
A conditional export can be supplied for packages to support require(), for example in a case where the CommonJS require() is no longer supported by your package. Refer to this link for more information.
The "exports" field allows defining the entry points of a package when imported by name loaded either via a node_modules lookup or a self-reference to its own name. It is supported in Node.js 12+ as an alternative to the "main" that can support defining subpath exports and conditional exports while encapsulating internal unexported modules.
package.json (example from the nodejs docs)
{
"exports": {
"import": "./index-import.js",
"require": "./index-require.cjs"
},
"type": "module"
}
If so, you have to provide two scripts: one for the CommonJS ("require": "filename") and one for the ECMAScript module ("import": "filename").
While index-require.js must provide the script via exports = ... or module.exports = ..., index-import.js must provide the script with export default.
Keyword Usage
You can only use specific keywords depending on the files module type.
CommonJS Modules
module.exports is used to define the values that a module exports and makes available for other modules to require. It can be set to any value, including an object, function, or a simple data type like a string or number.
exports, module
If you use them inside an ECMAScript module you'll get an undefined Error.
require()
require() inside ECMAScript modules is possible, but you have to use a workaround as mentioned in this answer or take a look at the docs for module.createRequire(fileName):
import { createRequire } from 'node:module';
const require = createRequire(import.meta.url);
// sibling-module.js is a CommonJS module.
const siblingModule = require('./sibling-module');
If you call require() from within a CommonJS on an ECMAScript module, it throws a not supported Error:
Error [ERR_REQUIRE_ESM]: require() of ES Module /path/to/script.mjs not supported.
With a more detailed error message depending on the situation:
Instead change the require of script.mjs in /path/to/app.js to a dynamic
import() which is available in all CommonJS modules.
Or:
/path/to/script.js is treated as an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which declares all .js files in that package scope as ES modules.
Instead rename /path/to/script.js to end in .cjs, change the requiring code to use dynamic import() which is available in all CommonJS modules, or change "type": "module" to "type": "commonjs" in /path/to/package.json to treat all .js files as CommonJS (using .mjs for all ES modules instead).
ECMAScript Moduls (ESM)
export default is used to export a single value as the default export of a module. This allows for a more concise way to import values, as the import statement can omit the curly braces when importing the default export.
Named exports, on the other hand, allow multiple values to be exported from a module. Named exports use the export keyword followed by an identifier and a value. (export const foo = "bar")
import ... from ...
It can handle CommonJS files and interprets them as if you would've used require().
Example based on express:
import express, { Route, Router } from 'express'; // EJS
// is similar to:
var express = require("express"), { Route, Router } = express; // CJS
Both CommonJS and ECMAScript modules support the import() function, but the returned object can have more properties on ESM files.
Summary:
CJS modules don't need to be converted to ESM, as they can be imported into ESM using the import ... from ... syntax without any modifications to the CJS module. However, it's advisable to write new modules using ECMAScript Module syntax, as it is the standard for both web and server-side applications and enables seamless use of the same code on both sides the browser/client-side and node/server-side.
Specifications
Additionally, I find this article on CommonJS vs. ES modules in Node.js from logrocket.com to be very informative. It delves into the pros and cons of ECMAScript compared to CommonJS in more depth.
Links:
MDN: import()
NodeJS.org: Difference between ECMAScript modules and CommonJS modules
There are two main scenarios:
1. Your package is written using CommonJS (CJS) module loading
This means your package uses require() to load dependencies. For this kind of package no special work is needed to support loading the package in both ES and CJS modules. ES modules are able to load CJS modules via the import statement, with the minor caveat that only default-import syntax is supported. And CJS modules are able to load other CJS modules via the require() function. So both ES modules and CJS modules are able to load CJS modules.
2. Your package is written using ES module loading
This means your package uses import to load dependencies. But don't be fooled - sometimes, especially when using TypeScript, you may be writing import in your code, but it's getting compiled to require() behind the scenes.
Unfortunately, CommonJS modules do not support loading ES modules except (in Node.js) by using the import() function (which is a bit painful and not a great solution).
In order to support CommonJS in this case, your best bet is to transpile your package into a CommonJS module, and ship both CommonJS and ESM versions of your package.
I do this in a number of my own packages mostly by using Rollup, which makes it relatively easy.
The basic concept is this:
Write your package as an ES module.
Install rollup: npm i -D rollup
Run npx rollup index.js --file index.cjs --format cjs to convert your code into a CJS module.
Export both from your package.json:
{
"name": "my-package",
"version": "1.0.0",
"main": "index.js",
"type": "module",
"exports": {
"import": "./index.js",
"require": "./index.cjs"
}
}
This way, the CJS module loader knows to load your index.cjs file, while the ESM loader knows to load your index.js file, and both are happy.
My application depends on library which uses ecma6 syntax for modules:
require lib = 'lib.js' // myapp.js
import Foo from 'bar' // lib.js
...
I want to run node myapp.js, but it results in syntax error, since node does not support ecma6 modules (and --experimental-modules would require lib.js to be named lib.mjs).
Is there any way to run myapp.js without changing the source code of lib.js?
look at https://babeljs.io/, you can compile your code (with that lib) to usual format
This answer:
How can I use ES6 in webpack.config.js?
seems to imply a transpilation step.
Can ES6 be used natively? How?
For example I tried to convert the simple example here from require to import and receive the following error
(function (exports, require, module, __filename, __dirname) { import path from 'path'
^^^^
SyntaxError: Unexpected identifier
You should use webpack-cli --config-register (-r) to allow that.
To use that flag, you must have babel-register installed. webpack --config-register babel-register --config ....
Another option is esm.
This question already has answers here:
ES2015 "import" not working in node v6.0.0 with with --harmony_modules option
(4 answers)
Closed 6 years ago.
I'm wanting to start using this ES6 on Node.js but I can't make it work. I'm adding the node test.js --harmony flag but I'm still getting bad syntax errors.
It doesn't even pass the first lines of code:
import env from 'node-env-file'
import api from '../src'
I'm getting this
(function (exports, require, module, __filename, __dirname) { import env from 'node-env-file'
^^^^^^
SyntaxError: Unexpected token import
How can I make my Node.js work with this type of syntax.
import keyword is not yet supported by any version of node because there is no JavaScript engine yet that natively supports ES6 modules.
You have to use somekind of ES6 to ES5 transpiler like Babel or stick to require.
I'm starting a React Native project, and I'd like to use ECMAScript 2015 for my code, and mocha for my unit tests. I've installed babel-register, babel-preset-es2015, and babel-preset-stage-2, and added this to my project's package.json file:
"babel": {
"presets": ["es2015", "stage-2"]
}
But when I run mocha --compilers js:babel-register and try to test a module that imports a React Native API...
import React from 'react-native';
let {Dimensions} = React;
...I get an error about the spread operator used by React:
node_modules/react-native/Libraries/react-native/react-native.js:107
...require('React'),
^^^
SyntaxError: Unexpected token ...
Is this a babel bug—shouldn't the stage-2 preset add support for the spread operator? Or am I missing something else?
I just started seeing this too. Seems like a bug. AFAIK babel doesn't compile files under node_modules because they should always be precompiled.
Also this is a pretty recent addition
https://github.com/facebook/react-native/commit/f9b744d50137de25357994fe2e829f98104e2242