Using Jest with TypeScript

Pro egghead lesson on Jest / TypeScript

No testing solution out there is perfect. That said, jest is an excellent unit testing option which provides great TypeScript support.

Note: We assume you start off with a simple node package.json setup. Also all TypeScript files should be in a src folder which is always recommended (even without Jest) for a clean project setup.

Step 1: Install

Install the following using npm:

npm i jest @types/jest ts-jest -D

Explanation:

Install jest framwork (jest)
Install the types for jest (@types/jest)
Install the TypeScript preprocessor for jest (ts-jest) which allows jest to transpile TypeScript on the fly and have source-map support built in.
Save all of these to your dev dependencies (testing is almost always a npm dev-dependency)

Step 2: Configure Jest

Add the following jest.config.js file to the root of your project:

module.exports = {
  "roots": [
    "<rootDir>/src"
  ],
  "transform": {
    "^.+\\.tsx?$": "ts-jest"
  },
  "testRegex": "(/__tests__/.*|(\\.|/)(test|spec))\\.(jsx?|tsx?)$",
  "moduleFileExtensions": [
    "ts",
    "tsx",
    "js",
    "jsx",
    "json",
    "node"
  ]
}

Explanation:

We always recommend having all TypeScript files in a src folder in your project. We assume this is true and specify this using roots option.
The transform config just tells jest to use ts-jest for ts / tsx files.
The testRegex tells Jest to look for tests in any __tests__ folder AND also any files anywhere that use the (.test|.spec).(js|jsx|ts|tsx) extension e.g. asdf.test.tsx etc.
The moduleFileExtensions tells jest to our file extensions. This is needed as we add ts/tsx into the defaults (js|jsx|json|node).

Step 3: Run tests

Run npx jest from your project root and jest will execute any tests you have.

Optional: Add script target for npm scripts

Add package.json:

{
  "test": "jest"
}

This allows you to run the tests with a simple npm t.
And even in watch mode with npm t -- --watch.

Optional: Run jest in watch mode

npx jest -w

Example

For a file foo.ts:

export const sum
= (...a: number[]) =>
  a.reduce((acc, val) => acc + val, 0);

A simple foo.test.ts:

import { sum } from '../';

test('basic', () => {
  expect(sum()).toBe(0);
});

test('basic again', () => {
  expect(sum(1, 2)).toBe(3);
});

Notes:

Jest provides the global test function.
Jest comes prebuilt with assertions in the form of the global expect.

Example async

Jest has built-in async/await support. e.g.

test('basic',async () => {
  expect(sum()).toBe(0);
});

test('basic again', async () => {
  expect(sum(1, 2)).toBe(3);
}, 1000 /* optional timeout */);

Reasons why we like jest

For details on these features see jest website

Built-in assertion library.
Great TypeScript support.
Very reliable test watcher.
Snapshot testing.
Built-in coverage reports.
Built-in async/await support.

Jest