ts-jest for TypeScript: Complete Guide with Examples

What is ts-jest and why is it needed?

ts-jest is a TypeScript preprocessor for the popular testing framework Jest. It allows you to run tests written in TypeScript without the need for preliminary compilation to JavaScript. The library automatically transforms TypeScript code into JavaScript during test execution, using the built-in TypeScript compiler.

The main goal of ts-jest is to provide seamless integration of TypeScript with Jest. Without it, you would have to manually configure TypeScript compilation before each test run or use additional tools like ts-node. ts-jest makes this process transparent and automatic.

Key advantages of using ts-jest:

• Automatic compilation — TypeScript files are compiled "on the fly" during testing
• Full TypeScript support — all language features (decorators, generics, types) work correctly
• Source maps — on errors, you see the original TypeScript code, not the compiled JavaScript
• Performance — ts-jest caches compilation results, speeding up subsequent runs
• Flexibility — you can configure TypeScript compiler options through Jest configuration

Installing ts-jest

To work with ts-jest, you will need Node.js (version 14 or higher) and a package manager npm or yarn. Installation is done with one command:

npm install --save-dev jest typescript ts-jest @types/jest

Or via yarn:

yarn add --dev jest typescript ts-jest @types/jest

Package explanation:

• jest — the testing framework itself
• typescript — the TypeScript compiler
• ts-jest — the preprocessor for integration
• @types/jest — TypeScript types for Jest (for autocompletion and type checking)

After installation, you need to create or update the jest.config.js file (or jest.config.ts):

// jest.config.jsmodule.exports = {  preset: 'ts-jest',  testEnvironment: 'node',  roots: ['<rootDir>/src'],  testMatch: ['**/__tests__/**/*.ts?(x)', '**/?(*.)+(spec|test).ts?(x)'],  moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json', 'node']};

Also, make sure the correct parameters are set in tsconfig.json:

{  "compilerOptions": {    "target": "ES2020",    "module": "commonjs",    "strict": true,    "esModuleInterop": true,    "skipLibCheck": true,    "forceConsistentCasingInFileNames": true,    "outDir": "./dist",    "rootDir": "./src",    "declaration": true,    "sourceMap": true  },  "include": ["src/**/*"],  "exclude": ["node_modules", "dist", "**/*.test.ts"]}

Quick start with a code example

Let's create a simple function and a test for it. Project structure:

my-project/├── src/│   ├── math.ts│   └── __tests__/│       └── math.test.ts├── jest.config.js├── tsconfig.json└── package.json

File: src/math.ts

export function add(a: number, b: number): number {  return a + b;}

export function subtract(a: number, b: number): number {  return a - b;}

export function multiply(a: number, b: number): number {  return a * b;}

export function divide(a: number, b: number): number {  if (b === 0) {    throw new Error('Division by zero');  }  return a / b;}

File: src/__tests__/math.test.ts

import { add, subtract, multiply, divide } from '../math';

describe('Math functions', () => {  test('adds 1 + 2 to equal 3', () => {    expect(add(1, 2)).toBe(3)