Testing in TypeScript with Jest

What is testing in TypeScript?

Testing in TypeScript is the process of verifying the correctness of code written in the TypeScript language using specialized libraries and frameworks. TypeScript is a superset of JavaScript that adds static typing, making testing particularly effective: types help identify errors at the compilation stage, while tests check the business logic and behavior of the application. The main library for testing TypeScript is Jest, which supports TypeScript out of the box or through additional configurations.

Why is testing needed in TypeScript?

Testing is necessary to ensure code reliability, prevent regressions, speed up development, and document function behavior. In TypeScript, tests are especially useful because they check not only logic but also type compliance, which reduces the number of runtime errors. Without tests, it is difficult to maintain large projects where changes in one module can break other parts of the system.

How to install Jest for TypeScript?

To install Jest with TypeScript support, run the following commands in the terminal:

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

Then create a configuration file jest.config.js:

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

Or use the initialization command: npx ts-jest config:init. After that, you can run tests with the command npx jest.

Main features of Jest in TypeScript

• describe and it — grouping and describing test cases.
• expect — assertions for checking values.
• matchers — built-in functions for comparison (toBe, toEqual, toBeTruthy, etc.).
• mock functions — simulating dependencies and modules.
• async/await — support for asynchronous code.
• coverage — a built-in utility for measuring test code coverage.

Example code in TypeScript

Let's look at a simple function and its test:

// math.tsexport function add(a: number, b: number): number {  return a + b;}

// math.test.tsimport { add } from './math';

describe('add function', () => {  it('should add two numbers correctly', () => {    expect(add(2, 3)).toBe(5);  });

  it('should handle negative numbers', () => {    expect(add(-1, 1)).toBe(0);  });

  it('should return NaN for invalid inputs', () => {    // TypeScript won't allow passing a string, but if you bypass types:    expect(add(NaN, 5)).toBeNaN();  });});

Run the test with the command npx jest. You will see a report on test execution and coverage.

When to use Jest for TypeScript?

Jest is recommended for use in the following cases:

• Node.js applications and API servers in TypeScript.
• Frontend projects with React, Angular, or Vue (Jest works with DOM via jsdom).
• Libraries and utilities where type safety is important.
• Projects with CI/CD that require automated test execution.
• Microservice architecture for isolated module testing.

Avoid Jest if you only need to test types (use tsc --noEmit) or if the project uses a different testing framework (e.g., Mocha or Vitest).