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Version: 29.2

ESM Support

important

Jest will take into account of the following things when working with ESM:

  • ESM runtime
  • The value of module option in tsconfig file is either:
    • Node16/Node18/NodeNext: this MUST go together with type: "module" in package.json.
    • Otherwise, the value MUST BE one of the ES values, e.g. ES2015, ES2020 etc...

One can configure ts-jest to work with Jest in ESM mode by following the steps below.

tip

We have EXAMPLE APPS which contains some projects which have basic setup to work with ESM (next to CJS config).

Configure Jest runtime​

warning

Jest runtime currently has a few issues related to support ESM:

Overall progress and discussion can be found at https://github.com/jestjs/jest/issues/9430

info

If one is using Jest config in TypeScript, one should install ts-node as a dev dependency.


npm install -D ts-node

Execute Jest with with --experimental-vm-modules flag for NodeJs


node --experimental-vm-modules node_modules/jest/bin/jest.js

tip

Alternative way for Yarn users:


yarn node --experimental-vm-modules $(yarn bin jest)

This command will also work if you use Yarn Plug'n'Play.

Configure tsconfig​

One can choose EITHER ONE of the following options for tsconfig:

Using ES module values​

tip

See more details about ES module values

important

ts-jest recommends to use ES2022 or ESNext when using ES module values to achieve full support for all recent ESM features.

tsconfig.spec.json
{
  "compilerOptions": {
    "module": "ES2022", // or `ESNext`
    "target": "ESNext",
    "esModuleInterop": true
  }
}

Using hybrid module values​

tip

See more details about hybrid module

important

Currently, the code transpiler ONLY supports hybrid values with isolatedModules: true

tsconfig.spec.json
{
  "compilerOptions": {
    "module": "Node16", // or Node18/NodeNext
    "target": "ESNext",
    "esModuleInterop": true,
    "isolatedModules": true
  }
}

Configure Jest config​

tip

Jest will attempt to load ESM files from node_modules with default jest-resolve which usually works for most of the cases. However, there are cases like Angular libraries ESM built files or ESM files which are outside node_modules might not be loaded correctly.

To fix that, one can use moduleNameMapper in jest config to instruct Jest to load the correct ESM files or create a custom Jest resolver.

Using ESM presets​

tip

See available ESM preset creator functions HERE

jest.config.ts
import type { Config } from 'jest'
import { createDefaultEsmPreset } from 'ts-jest'

const presetConfig = createDefaultEsmPreset({
  //...options
})

export default {
  ...presetConfig,
} satisfies Config

NOT using ESM presets​

jest.config.ts
import type { Config } from 'jest'
import { TS_EXT_TO_TREAT_AS_ESM, ESM_TS_TRANSFORM_PATTERN } from 'ts-jest'

export default {
  extensionsToTreatAsEsm: [...TS_EXT_TO_TREAT_AS_ESM],
  transform: {
    [ESM_TS_TRANSFORM_PATTERN]: [
      'ts-jest',
      {
        //...other `ts-jest` options
        useESM: true,
      },
    ],
  },
} satisfies Config

Resolve .mjs/.mts extensions​

To work with .mts extension, besides the requirement to run Jest and ts-jest in ESM mode, there are a few extra requirements to be met:

  • package.json should contain "type": "module"
  • A custom Jest resolver to resolve .mjs extension, for example:
custom-resolver.ts
import type { SyncResolver } from 'jest-resolve'

const mjsResolver: SyncResolver = (path, options) => {
  const mjsExtRegex = /\.mjs$/i
  const resolver = options.defaultResolver
  if (mjsExtRegex.test(path)) {
    try {
      return resolver(path.replace(mjsExtRegex, '.mts'), options)
    } catch {
      // use default resolver
    }
  }

  return resolver(path, options)
}

export default mjsResolver

and then add this to Jest config:

jest.config.ts
import type { Config } from 'jest'

const config: Config = {
  //...other options
  resolver: '<rootDir>/path/to/custom-resolver.ts',
}