Unit Testing a TypeScript Library

July 2017
Unit tests are the cornerstone of a bug free application. As your codebase grows, you can't always test every feature manually. But you can test every feature using unit tests. And it's not even hard to set up. Here's how to set up unit tests in Typescript!

Step 1: Install packages for unit testing

Unit tests assert some properties of the code. For example a method add(x,y) should correctly add x and y. With a unit test you can then test some sample cases like assert(add(3,4)).equals(7). To get this assert method and to execute tests in the first place, we need some testing software. Here we'll choose Mocha and Chai. To install mocha and chai, you can run:

yarn add mocha @types/mocha chai @types/chai ts-node typescript --dev

You will need to have yarn installed for this to work, see install manual.

Step 2: Writing your first unit test

Let's say you have the following unit:

export function add(x: number, y: number) {
  return x + y;

Then a first unit test could look like:

import { add } from './math';

import * as mocha from 'mocha';
import * as chai from 'chai';

const expect = chai.expect;
describe('My math library', () => {

  it('should be able to add things correctly' , () => {


Step 3: Run the unit test(s)

You can run the test with the following command:

mocha --reporter spec --compilers ts:ts-node/register src/**/*.test.ts

And basically that's it. You should see an output in the console, that looks like this:

You can put this long command into package.json into "scripts: { "test": "..."}" and then run the tests with npm test. So the complete package.json at this point would look like:

  "devDependencies": {
    "@types/chai": "^4.0.0",
    "@types/mocha": "^2.2.41",
    "chai": "^4.0.2",
    "mocha": "^3.4.2",
    "ts-node": "^3.1.0"
  "scripts": {
    "test": "mocha --reporter spec --compilers ts:ts-node/register 'src/**/*.test.ts'"

If you want to run a single test, run mocha --reporter spec --compilers ts:ts-node/register --grep "TestName" 'src/**/*.test.ts' where "TestName" matches any describe or it. What's also quite cool, with IntelliJ you can setup your unit tests such that you can execute single tests right from the code! Here is the manual.

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