ts-jest

What it is

ts-jest is a Jest transformer that runs TypeScript tests with full type checking during the test run.

Configure Jest to use the ts-jest preset. It compiles each test file with tsc, reporting type errors as test failures.

Installation

npm install -D ts-jest @types/jest

Getting started

The smallest useful thing you can do with it, and what each part means.

Configuration

typescript

12345678910111213141516import type { Config } from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  transform: {
    '^.+\\.tsx?$': ['ts-jest', {
      // isolatedModules is much faster but skips type checking.
      isolatedModules: false,
      tsconfig: 'tsconfig.test.json',
    }],
  },
  moduleNameMapper: { '^@/(.*)$': '<rootDir>/src/$1' },
};

export default config;

moduleNameMapper must mirror the paths in tsconfig — Jest does not read them, so aliases that compile fine will fail to resolve at test time.

Advanced usage

Where the library earns its place over a simpler alternative.

Typed mocks

typescript

1234567891011121314import { mocked } from 'jest-mock';
import { fetchBook } from './api';

jest.mock('./api');

const mockFetch = mocked(fetchBook);  // keeps the original signature

test('returns the book', async () => {
  mockFetch.mockResolvedValue({ id: '42', title: 'Dune' });

  // A wrong shape here is a compile error, not a runtime surprise.
  await expect(getBookTitle('42')).resolves.toBe('Dune');
  expect(mockFetch).toHaveBeenCalledWith('42');
});

mocked preserves the real signature, so mock return values are checked against what the function actually promises — the main reason to use ts-jest over Babel.

Errors and fixes

The failures you are most likely to hit, and what actually resolves them.

Cannot find module '@/thing' from test: Jest does not read tsconfig paths. Add the equivalent entry to moduleNameMapper.
Tests are slow: Type checking each file is the cost. Enable isolatedModules for speed, accepting that types go unchecked in the run.

Best practices

Keep type checking on unless test runs become a bottleneck; it is the reason to use ts-jest at all.
Mirror tsconfig paths in moduleNameMapper.
Consider Vitest for new projects — it handles TypeScript natively and is faster.
Use a separate tsconfig.test.json so test-only types do not leak into the build.

Background

Why it exists, and what it was reacting to.

ts-jest exists because Jest's Babel transform strips types without checking them. ts-jest uses the real compiler, so a type error fails the test rather than passing silently.

Related libraries

Others in Testing for TypeScript.

typescript-eslint

tsd