ECMAScript Modules (ESM) is a specification for using Modules in the Web. It's supported by all modern browsers and the recommended way of writing modular code for the Web.

Webpack supports processing ECMAScript Modules to optimize them.

Exporting

The export keyword allows to expose things from an ESM to other modules:

export const CONSTANT = 42;

export let variable = 42;
// only reading is exposed
// it's not possible to modify the variable from outside

export function fun() {
  console.log('fun');
}

export class C extends Super {
  method() {
    console.log('method');
  }
}

let a, b, other;
export { a, b, other as c };

export default 1 + 2 + 3 + more();

Importing

The import keyword allows to get references to things from other modules into an ESM:

import { CONSTANT, variable } from './module.js';
// import "bindings" to exports from another module
// these bindings are live. The values are not copied,
// instead accessing "variable" will get the current value
// in the imported module

import * as module from './module.js';
module.fun();
// import the "namespace object" which contains all exports

import theDefaultValue from './module.js';
// shortcut to import the "default" export

Flagging modules as ESM

By default webpack will automatically detect whether a file is an ESM or a different module system.

Node.js established a way of explicitly setting the module type of files by using a property in the package.json. Setting "type": "module" in a package.json does force all files below this package.json to be ECMAScript Modules. Setting "type": "commonjs" will instead force them to be CommonJS Modules.

{
  "type": "module"
}

In addition to that, files can set the module type by using .mjs or .cjs extension. .mjs will force them to be ESM, .cjs force them to be CommonJs.

In DataURIs using the text/javascript or application/javascript mime type will also force module type to ESM.

In addition to the module format, flagging modules as ESM also affect the resolving logic, interop logic and the available symbols in modules.

Imports in ESM are resolved more strictly. Relative requests must include a filename and file extension (e.g. *.js or *.mjs) unless you have the behaviour disabled with fullySpecified=false.

Only the "default" export can be imported from non-ESM. Named exports are not available.

CommonJs Syntax is not available: require, module, exports, __filename, __dirname.