Hello Typescript and Mocha!

Posted on Mon 27 July 2015 in TypeScript

Let's see what we can do with TypeScript then. First things first we need to create a local project to hack on:

$ npm init
name: (hello-ts-mocha)
version: (1.0.0)
description: Getting started with TypeScript and mocha
entry point: (index.js)
test command: mocha
git repository:
license: (ISC)

Now let's grab the TypeScript compiler so we can build the project:

npm install typescript --save

Now let's grab Mocha for our tests

npm install mocha --save-dev

Create a Subject to Test

Time to create our subject that we're going to test, let's do some TDD (: Create a new file, index.ts in the project root

export default class Calculator {
    add(x : number, y : number) : number {
        return 0;

And now to compile this down to ES5 so we can execute it under node:

./node_modules/typescript/bin/tsc index.ts --module commonjs

We can simplify the compilation step by adding a prepublish npm script to the package.json file:

    "scripts": {
        "prepublish": "tsc index.ts --module commonjs ",
        "test": "mocha"

We can now compile with npm install.

Adding a Test

Let's create our first test; following convention lets create our test-case in test/CalculatorTest.

Before writing the test-case we need to grab the Mocha Type Definitions (requried for the TypeScript compiler (tsc) to work). Easiest way to get these it to use the DefinitelyTyped TypeScript Definition manager (tsd).

npm install tsd -g

Once installed we can use it to grab mocha's type definitions:

tsd install mocha --save

This will add a typings/ folder to our project. Here's what our source tree looks like so far:

├── index.ts
├── node_modules
│   └── typescript
├── package.json
├── test
│   └── CalculatorTest.js
├── tsd.json
└── typings
    ├── mocha
    └── tsd.d.ts

Now we can reference the mocha type definition using a /// <reference /> comment:

/// <reference path="../typings/mocha/mocha.d.ts" />
import Calculator from '../index';

describe('Calculator', () => {
    var subject : Calculator;

    beforeEach(function () {
        subject = new Calculator();

    describe('#add', () => {
        it('should add two numbers together', () => {
            var result : number = subject.add(2, 3);
            if (result !== 5) {
                throw new Error('Expected 2 + 3 = 5 but was ' + result);

Before we can run the tests we need to compile them, best way to do this is to add a preTest NPM script to invoke the typscript compiler before running mocha. Open up the package.json file again:

    "scripts": {
        "pretest": "tsc test/*Test.ts --module commonjs",
        "test": "mocha"

We can now run our test-case with npm test:

$ npm test
> [email protected] pretest /private/tmp/foo
> tsc test/*Test.ts --module commonjs

> [email protected] test /private/tmp/foo
> mocha

      1) should add two numbers together

  0 passing (19ms)
  1 failing

  1) Calculator #add should add two numbers together:
     Error: Expected 2 + 3 = 5 but was 0
      at Context.<anonymous> (test/CalculatorTest.js:12:23)

Have fun fixing the test-case :) Full source over over at github.com/jonnyreeves/hello-ts-mocha