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@babel/plugin-transform-modules-commonjs

History

VersionChanges
v7.14.0Implemented the importInterop option

NOTE: This plugin is included in @babel/preset-env under the modules option

This plugin transforms ECMAScript modules to CommonJS. Note that only the syntax of import/export statements (import "./mod.js") and import expressions (import('./mod.js')) is transformed, as Babel is unaware of different resolution algorithms between implementations of ECMAScript modules and CommonJS.

Example

In

export default 42;

Out

Object.defineProperty(exports, "__esModule", {
  value: true,
});

exports.default = 42;

Installation

npm install --save-dev @babel/plugin-transform-modules-commonjs

Usage

With a configuration file (Recommended)

// without options
{
  "plugins": ["@babel/plugin-transform-modules-commonjs"]
}

// with options
{
  "plugins": [
    ["@babel/plugin-transform-modules-commonjs", {
      "allowTopLevelThis": true
    }]
  ]
}

Via CLI

babel --plugins @babel/plugin-transform-modules-commonjs script.js

Via Node API

require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-transform-modules-commonjs"],
});

Options

importInterop

"babel" | "node" | "none", or (specifier: string) => "babel" | "node" | "none". Defaults to "babel".

CommonJS modules and ECMAScript modules are not fully compatible. However, compilers, bundlers and JavaScript runtimes developed different strategies to make them work together as well as possible.

This option specify which interop strategy Babel should use. When it's a function, Babel calls this function passing the import specifier (for example, @babel/core in import { transform } from "@babel/core"), and the function should return the interop to use for that specific import.

"babel"

When using exports with babel a non-enumerable __esModule property is exported. This property is then used to determine if the import is the default export or if it contains the default export.

import foo from "foo";
import { bar } from "bar";
foo;
bar;

// Is compiled to ...

"use strict";

function _interopRequireDefault(obj) {
  return obj && obj.__esModule ? obj : { default: obj };
}

var _foo = _interopRequireDefault(require("foo"));
var _bar = require("bar");

_foo.default;
_bar.bar;

When this import interop is used, if both the imported and the importer module are compiled with Babel they behave as if none of them was copiled.

This is the default behavior.

"node"

When importing CommonJS files (either directly written in CommonJS, or generated with a compiler) Node.js always binds the default export to the value of module.exports.

import foo from "foo";
import { bar } from "bar";
foo;
bar;

// Is compiled to ...

"use strict";

var _foo = require("foo");
var _bar = require("bar");

_foo;
_bar.bar;

This is not exactly the same as what Node.js does since Babel allows accessing any property of module.exports as a named export, while Node.js only allows importing statically analyzable properties of module.exports. However, any import working in Node.js will also work when compiled with Babel using importInterop: "node".

"none"

If you know that the imported file has been transformed with a compiler that stores the default export on exports.default (such as Babel), you can safely omit the _interopRequireDefault helper.

import foo from "foo";
import { bar } from "bar";
foo;
bar;

// Is compiled to ...

"use strict";

var _foo = require("foo");
var _bar = require("bar");

_foo.default;
_bar.bar;

loose

boolean, defaults to false.

By default, when using exports with babel a non-enumerable __esModule property is exported.

var foo = (exports.foo = 5);

Object.defineProperty(exports, "__esModule", {
  value: true,
});

In environments that don't support this you can enable loose mode on @babel/plugin-transform-modules-commonjs and instead of using Object.defineProperty an assignment will be used instead.

var foo = (exports.foo = 5);
exports.__esModule = true;

strict

boolean, defaults to false

By default, when using exports with babel a non-enumerable __esModule property is exported. In some cases this property is used to determine if the import is the default export or if it contains the default export.

var foo = (exports.foo = 5);

Object.defineProperty(exports, "__esModule", {
  value: true,
});

In order to prevent the __esModule property from being exported, you can set the strict option to true.

lazy

boolean, Array<string>, or (string) => boolean, defaults to false

Changes Babel's compiled import statements to be lazily evaluated when their imported bindings are used for the first time.

This can improve initial load time of your module because evaluating dependencies up front is sometimes entirely un-necessary. This is especially the case when implementing a library module.

The value of lazy has a few possible effects:

  • false - No lazy initialization of any imported module.

  • true - Do not lazy-initialize local ./foo imports, but lazy-init foo dependencies.

    Local paths are much more likely to have circular dependencies, which may break if loaded lazily, so they are not lazy by default, whereas dependencies between independent modules are rarely cyclical.

  • Array<string> - Lazy-initialize all imports with source matching one of the given strings.

  • (string) => boolean - Pass a callback that will be called to decide if a given source string should be lazy-loaded.

The two cases where imports can never be lazy are:

  • import "foo";

    Side-effect imports are automatically non-lazy since their very existence means that there is no binding to later kick off initialization.

  • export * from "foo"

    Re-exporting all names requires up-front execution because otherwise there is no way to know what names need to be exported.

You can read more about configuring plugin options here

noInterop

boolean, defaults to false

⚠️ Deptecated: Use the importInterop option instead.

When set to true, this option has the same behavior as setting importInterop: "none".

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