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.
I have a error on importing esm:
requiere = require(module);
module.exports = require('./bot'); // this 2 line inside index.js
This is try open my bot.js; but I got an error:
ReferenceError: require is not defined in ES module scope, you can use import instead
This file is being treated as an ES module because it has a '.js' file extension and '/Users/vladimirlevcenko/MavenBot/package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.
I have written a TS file, that loads in a 3rd party package, using import XXX { YYY, ABC, 123 }from 'XXX';
It will compile to CommonJS no issue, and thats OK. But I'd like to compile it to an ESModule. I changed the target and module settings to esnext in my TS config file, and it does compile, however, when I run the file, it errors out saying:
SyntaxError: Named export 'ABC' not found. The requested module 'XXX' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:
import pkg from 'XXX';
const { ABC } = pkg;
Is there any settings I can use to tell Typescript to convert the imports to the import tye shown in the error message?
You may want to try nodenext instead of esnext for the module setting. This is currently experimental but it seems to address your need with CommonJS interop.
I haven't tried it myself so I cannot promise it will work.
I am using Egg framework for my NodeJs(v14.15.4) application
I want to use latest version of p-debounce library, the package is now pure ESM, Instead of const pDebounce = require('p-debounce') I must use import pDebounce from 'p-debounce' that not works on EggJs
If I use import
(node:10636) Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
(Use `node --trace-warnings ...` to show where the warning was created)
test.js:5
import pDebounce from 'p-debounce'
^^^^^^
SyntaxError: Cannot use import statement outside a module
If I use require
internal/modules/cjs/loader.js:1080
throw new ERR_REQUIRE_ESM(filename, parentPath, packageJsonPath);
^
Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: .\node_modules\p-debounce\index.js
require() of ES modules is not supported.
require() of .\node_modules\p-debounce\index.js from .\test.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename index.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from .\node_modules\p-debounce\package.json.
If I add "type": "module" in the package.json
const path = require('path');
^
ReferenceError: require is not defined
I have many require in my application and not want to change all to import at the moment
What is "type": "module" in the package.json ?
How can I fix the error?
You seem to be overlooking a particular point in the changelog you linked:
If you cannot move to ESM yet, don't upgrade to this version.
As you've pointed out, your application has already progressed significantly, and you have many require that you do not want to change at the moment. There is no need to upgrade to the latest version of p-debounce at this time, according to your needs.
When you do decide to migrate to ESM format, in order to upgrade individual files one at a time, you can use the .mjs extension to treat the code as an ECMAScript module.
ES modules are the future.
There is a way around in ES modules to require a common.js module. Using createRequire
WORKING EXEMPLE
axios-curlirize is ES module and axios support both.
import { createRequire } from 'module';
const require = createRequire(import.meta.url);
const axios = require('axios');
import curlirize from 'axios-curlirize';
curlirize(axios);
const { data } = await axios.get('https://jsonplaceholder.typicode.com/todos/1');
console.log(data)
Using ES modules you can use top-level await without function.
I have a bunch of files that are configuring my Jest tests. I need them to specify environment for Puppeteer browser and setup/teardown behaviours.
module.exports = {
globalSetup: './jest-config/setup.js',
globalTeardown: './jest-config/teardown.js',
testEnvironment: './jest-config/puppeteerEnv.js',
testResultsProcessor: './jest-config/testResultsProcessor.js',
};
Right now i have a .babelrc file inside my repository with "env" preset and it transpiles my tests files and allows to use imports inside my code. Although jest-config files are not affected by transpilation and i can't use there preferred syntax.
/jest-config/teardown.js:1
(function (exports, require, module, __filename, __dirname) { import
TestResultManager from '../helpers/testResultsManager';
SyntaxError: Unexpected token import
Is there any possibility that I can use the import syntax in jest-config files such as teardown/setup or reporters?
Thanks.