Testing with Jest & `ts-jest`

Install Jest for TypeScript

npm i -D jest ts-jest @types/jest
npx ts-jest config:init

Updates jest.config.js to use ts-jest transformer.

Simple test

// sum.ts
export const sum = (a: number, b: number) => a + b;

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

test('adds two numbers', () => {
  expect(sum(2, 3)).toBe(5);
});

Run with npm test.

Mocking modules

jest.mock('axios');
import axios from 'axios';
(axios.get as jest.Mock).mockResolvedValue({ data: { id: 1 } });

Using expect.assertions

Ensures async assertions fire.

test('async fetch', async () => {
  expect.assertions(1);
  const data = await fetchData();
  expect(data).toBe('done');
});