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Test TypeScript CommonJS

This guide sets up Jest for a TypeScript package without "type": "module". The ts-cjs preset transforms TypeScript with ts-jest and transforms ESM-only dependencies to CommonJS with jest-esm-transformer-2.

Steps

Section titled “Steps”

  1. Install Jest, the preset package, and the transformers:

    Terminal window
    pnpm add -D jest @repobuddy/jest ts-jest typescript jest-esm-transformer-2

  2. Add the watch plugins if you use jest --watch:

    Terminal window
    pnpm add -D jest-watch-suspend jest-watch-toggle-config jest-watch-typeahead

  3. Create jest.config.js:

    jest.config.js
    module.exports = {
      preset: '@repobuddy/jest/presets/ts-cjs-watch',
    }

    ts-watch also works: it picks ts-cjs-watch when package.json has no "type": "module".

  4. Make sure tsconfig.json compiles to CommonJS. ts-jest reads it.

  5. Put tests in the source folder and name them <name>.spec.ts or another test file name.

Finished config

Section titled “Finished config”
jest.config.js
module.exports = {
  preset: '@repobuddy/jest/presets/ts-cjs-watch',
}

Verify

Section titled “Verify”
Terminal window
pnpm exec jest

No NODE_OPTIONS flag is needed. The ts-cjs fixture in this repository runs 14 suites this way, including one that imports chalk 6, an ESM-only package.

With ts-jest 29.4.14 the run prints a warning that the ts-jest option isolatedModules is deprecated. The preset sets that option, so the warning appears even when tsconfig.json sets isolatedModules: true. The tests still run.

Section titled “Related”