How to Test TypeScript with Jest?

Last Updated : 03 Apr, 2025

Jest, a powerful JavaScript testing framework, seamlessly integrates with TypeScript, providing all the tools needed to write and run tests for your TypeScript code, ensuring quality and reliability.

It handles TypeScript compilation automatically, offers features like mocking and code coverage, and includes a user-friendly test runner for efficient testing.

Here's a breakdown of how to test TypeScript with Jest:

1. Setting Up Jest with TypeScript

These steps will get Jest working with your TypeScript project:

Step 1: Initialize and Install

Get started with Node.js and install the necessary packages

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

Step 2: Create a jest.config.js file

module.exports = {
    preset: 'ts-jest',
    testEnvironment: 'node',
};

Step 3: Add a tsconfig.json file if you don't already have one

{
    "compilerOptions": {
        "target": "ES6",
        "module": "commonjs",
        "strict": true,
        "esModuleInterop": true,
        "skipLibCheck": true,
        "forceConsistentCasingInFileNames": true
    }
}

Step 4: Once the setup is complete, you can start writing tests. Jest allows you to write test cases using its built-in functions like describe, test, it, expect, etc.

Step 5: Create a TypeScript file, say sum.ts

export const sum = (a: number, b: number): number => {
  return a + b;
};

Step 6: Create a test file, sum.test.ts

import { sum } from './sum';

test('adds 1 + 2 to equal 3', () => {
    expect(sum(1, 2)).toBe(3);
});

Note: Running the tests is straightforward using the Jest command-line interface. Jest provides detailed feedback on your tests, showing which tests passed, failed, and why.

Step 7: Run your tests using the following command

npx jest

Output:

How to Test TypeScript with Jest — Output

Case 1: Testing Mock Functions using Jest

Jest provides powerful mocking capabilities, allowing you to mock functions, modules, and timers. This is useful for isolating the code under test and controlling its dependencies.

Example: This example shows the testing of function.

JavaScript

// sum.ts
export const fetchData = (callback: (data: string) => void) => {
    setTimeout(() => {
        callback('peanut butter');
    }, 1000);
};

JavaScript

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

test('fetchData calls callback with "peanut butter"', done => {
    function callback(data: string) {
        expect(data).toBe('peanut butter');
        done();
    }

    fetchData(callback);
});

Output:

Testing Mock Functions using Jest — Output

Case 2: Testing Asynchronous Code

Testing asynchronous code can be challenging, but Jest makes it easier with support for async functions, promises, and callback-based code.

Example: This example shows the testing of Asynchronous function.

JavaScript

// sum.ts
export const fetchDataPromise = (): Promise<string> => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('peanut butter');
    }, 1000);
  });
};

JavaScript

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

test('the data is peanut butter', async () => {
  const data = await fetchDataPromise();
  expect(data).toBe('peanut butter');
});