TypeScript Testing: A Step-by-Step Guide for Beginners

Why Testing TypeScript Isn't Scary, But Beneficial

You wrote a cool function in TypeScript, ran the project — everything works. A week later you added a new feature, and the old logic broke. Sound familiar? If so, you urgently need to become friends with testing. And don't be afraid: tests are not a boring obligation, but your personal "bug detector" that works 24/7.

In this article, we'll break down where to start testing TypeScript as a beginner. You'll learn how to set up the environment, write your first unit test, and not get confused with types. No fluff — only practice and real code examples.

Why test TypeScript at all if it's already safe?

TypeScript indeed catches many errors at compile time: incorrect types, missing fields, wrong arguments. But static typing doesn't save you from logical errors. Your function can compile perfectly but produce an incorrect result. For example:

function sum(a: number, b: number): number {\n  return a - b; // Error! Subtraction instead of addition\n}

TypeScript won't complain — the types are correct. But a unit test will immediately show that sum(2, 2) does not equal 4. Therefore, testing TypeScript is not a whim, but a necessity for any serious project.

Step 1. Choosing a Tool: Why Jest is a Beginner's Best Friend

There are several popular testing frameworks on the market: Mocha, Jasmine, Vitest, Jest. For beginners, I strongly recommend Jest. Why?

\n
• Works "out of the box" — no lengthy setup required.
\n
• Built-in mocking mechanism and code coverage.
\n
• Excellent TypeScript support via ts-jest.
\n
• Clear error messages.
\n

Let's install Jest in your project. I assume you already have TypeScript. If not, create a new project:

mkdir my-ts-project\ncd my-ts-project\nnpm init -y\nnpm install typescript jest @types/jest ts-jest --save-dev

Now create a jest.config.js file in the project root:

module.exports = {\n  preset: 'ts-jest',\n  testEnvironment: 'node',\n  roots: ['/src'],\n  testMatch: ['**/__tests__/**/*.ts', '**/?(*.)+(spec|test).ts'],\n};

And add a script to package.json:

"scripts": {\n  "test": "jest",\n  "test:watch": "jest --watch"\n}

Done! Now you can write tests.

Step 2. Writing Your First Test in TypeScript

Let's create a simple function in the file src/math.ts:

export function add(a: number, b: number): number {\n  return a + b;\n}\n\nexport function divide(a: number, b: number): number {\n  if (b === 0) {\n    throw new Error('Division by zero is not allowed');\n  }\n  return a / b;\n}

Now let's write a test for it. Create a folder src/__tests__/ and a file math.test.ts:

import { add, divide } from '../math';\n\ndescribe('Math functions', () => {\n  test('adds two numbers correctly', () => {\n    expect(add(2, 3)).toBe(5);\n    expect(add(-1, 1)).toBe(0);\n    expect(add(0, 0)).toBe(0);\n  });\n\n  test('divides two numbers', () => {\n    expect(divide(10, 2)).toBe(5);\n    expect(divide(9, 3)).toBe(3);\n  });\n\n  test('throws error when dividing by zero', () => {\n    expect(() => divide(5, 0)).toThrow('Division by zero is not allowed');\n  });\n});

Run npm test and see the green report. Congratulations — you've just written your first unit tests in TypeScript!

What happened here?

\n
• describe — groups tests into a logical block.
\n
• test or it — an individual test case.
\n
• expect — an assertion that checks