[SWC plugin] mutable CJS exports

This is a SWC plugin to emit mutable CJS exports.

This SWC plugin has only been tested for compatibility with jest. It should be used with @swc/jest.

This project was previously called jest_workaround

plugin version

https://swc.rs/docs/plugin/selecting-swc-core

usage

install

1npm i -D jest @swc/core @swc/jest swc_mut_cjs_exports

1// jest.config.js
2const fs = require("node:fs");
3
4const swcrc = JSON.parse(fs.readFileSync(".swcrc", "utf8"));
5
6// If you have other plugins, change this line.
7((swcrc.jsc ??= {}).experimental ??= {}).plugins = [
8  ["swc_mut_cjs_exports", {}],
9];
10
11module.exports = {
12  transform: {
13    "^.+\\.(t|j)sx?$": ["@swc/jest", swcrc],
14  },
15};

Alternative implementation without .swcrc file

1// jest.config.js
2
3module.exports = {
4  transform: {
5    "^.+\\.(t|j)sx?$": [
6      "@swc/jest",
7      {
8        jsc: {
9          experimental: {
10            plugins: [["swc_mut_cjs_exports", {}]],
11          },
12        },
13      },
14    ],
15  },
16};

Make sure that module.type is commonjs in your .swcrc since this plugin does not touch non-workaround parts, such as import statements.

FAQ

1. When do I need this?

If you're using the swc compiler to transform your code to comply with the ESM specification, but you're also using Jest to test it in a CJS environment, you may encounter issues due to the immutable issue of exports.

This plugin can help by transforming the export statements into mutable exports.

2. Do I have a better choice?

You may have other options depending on your specific needs:

• If you're able to run Jest in an ESM environment, you can use swc to transpile TypeScript/JSX syntax or downgrade JavaScript syntax without module conversion. Simply set the value of module.type to es6 to achieve this.

• It's possible that some issues related to running Jest in an ESM environment will be resolved over time. Keep an eye on facebook/jest#9430 for updates.

• If you don't need the behavior of ESM specifically, you can stick with the CJS syntax to get the CJS behavior of exports.

These options may be worth considering before using this plugin.

CJS syntax

1exports.foo = function foo() {
2  return 42;
3};

CTS(CJS in TypeScript) syntax

1export = {
2  foo: function foo() {
3    return 42;
4  },
5};

Notes:

• ESM style export means immutable exports when transformed into CJS
• ESM style import means hoisted require when transformed into CJS

3. After upgrading the plugin version, the changes have not taken effect.

This is a known issue. You could remove the Jest cache by running jest --clearCache as a workaround.