ts-jest

29.4.9 · active · verified Sat Apr 18

ts-jest is a Jest transformer with source map support that enables testing projects written in TypeScript using Jest. It supports all TypeScript features, including type-checking. The current stable version is 29.4.9. Note that ts-jest does not follow semantic versioning, which means major breaking changes can occur without a corresponding major version increment. Releases are frequent, primarily for patch updates.

Common errors

```
Jest: ts-jest preset was not applied. Ensure that you have configured jest.config.js/ts to use a preset.
```
cause The Jest configuration file (`jest.config.js` or `jest.config.ts`) does not include `preset: 'ts-jest'`.

fix Add `preset: 'ts-jest'` to your Jest configuration file, for example: `export default { preset: 'ts-jest' };`
```
Cannot find module 'typescript' or 'jest' or '@jest/types'
```
cause A required peer dependency of `ts-jest` (like `typescript`, `jest`, `@jest/types`, `babel-jest`, etc.) is not installed or its version is incompatible.

fix Install all necessary peer dependencies explicitly using `npm install -D jest typescript @types/jest ts-jest` (or `yarn add --dev ...`) and ensure their versions fall within the ranges specified by `ts-jest`.
```
Error: Jest: a transformer must at least provide a `process` function.
```
cause This usually indicates an incompatibility between `ts-jest` and the installed version of Jest or TypeScript, or a corrupted `node_modules`.

fix Verify that your `jest` and `typescript` versions satisfy `ts-jest`'s peer dependency requirements. Try clearing your `node_modules` and `npm cache clean --force` (or `yarn cache clean`), then reinstall dependencies.
```
TSxxxx: [TypeScript compilation error]
```
cause TypeScript compiler errors are being reported during the test run, indicating issues with your code's types or `tsconfig.json` configuration.

fix Address the specific TypeScript errors in your source or test files. Ensure your `tsconfig.json` (especially `compilerOptions` like `target`, `module`, `strict`) is correctly configured for your project and `ts-jest`.

Warnings

breaking ts-jest explicitly states it does not follow semantic versioning. This means breaking changes can occur in minor or even patch releases, making upgrades potentially risky.
Fix: Always review the CHANGELOG.md before upgrading ts-jest, especially across minor versions, and thoroughly test your application after any update.
gotcha ts-jest has strict peer dependency requirements for Jest and TypeScript versions. Incompatible versions can lead to unexpected errors or installation failures.
Fix: Ensure that your installed versions of `jest` (`^29.0.0 || ^30.0.0`) and `typescript` (`>=4.3 <7`) match the peer dependency range specified by ts-jest.
gotcha There's a fundamental difference between using `ts-jest` for TypeScript compilation and using Babel with `@babel/preset-typescript`. `ts-jest` handles type-checking and Jest integration, while Babel with `preset-typescript` only strips types, potentially missing type errors.
Fix: Choose the appropriate tool based on your project's needs. If you require full TypeScript type-checking during tests, `ts-jest` is the correct choice. If you only need fast transpilation without type checks, Babel might be preferred (though often used with other transformers).

Install

npm install ts-jest npm
yarn add ts-jest yarn
pnpm add ts-jest pnpm

Quickstart

This quickstart shows how to set up `ts-jest` for a basic TypeScript project. It includes the necessary `package.json` dependencies and `scripts`, a minimal `jest.config.ts` (generated via `npx ts-jest config:init`), a simple TypeScript function, and its corresponding test file. After installation and configuration, tests can be run using `npm test`.

{
  "devDependencies": {
    "jest": "^29.4.9",
    "typescript": "^5.0.0",
    "ts-jest": "^29.4.9",
    "@types/jest": "^29.4.9"
  },
  "scripts": {
    "test": "jest"
  }
}

// jest.config.ts (created by `npx ts-jest config:init`)
export default {
  preset: 'ts-jest',
  testEnvironment: 'node',
  roots: ['<rootDir>/src'],
};

// src/sum.ts
export function sum(a: number, b: number): number {
  return a + b;
}

// src/sum.test.ts
import { sum } from './sum';

describe('sum', () => {
  it('adds two numbers', () => {
    expect(sum(1, 2)).toBe(3);
  });
});

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