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Step by step guide

Let's setup mocha with earl and write our first assertion. I will assume in this guide that you use TypeScript but earl works with JavaScript as well.

First install dependencies:

npm install --save-dev typescript ts-node mocha @types/mocha earljs

Note: it's earljs not earl.

Setup TypeScript

Here's a minimal tsconfig.json:

"compilerOptions": {
"lib": ["ES2018"],
"module": "CommonJS",
"moduleResolution": "node",
"strict": true,
"target": "ES2015",
"esModuleInterop": true,
"sourceMap": true
"include": ["src", "test"]

Function under tests

Lets create simple function that we are going to test:


export function sum(a: number, b: number): number {
return a + b

Configure mocha

First, we should deal with mocha setup. Create config file in the root of the project:


module.exports = {
require: [
'ts-node/register/transpile-only', // required to run TypeScript code
extension: ['ts'],
watchExtensions: ['ts'],
spec: ['test/**/*.test.ts'],

Write our first test case

Lets create our first test suite. I like to mirror directory structure of src/ inside my test/ directory so that's what we are going to do:


import { expect } from 'earljs'
import { sum } from '../src/sum'

describe('sum', () => {
it('sums numbers', () => {
const actual = sum(2, 2)


Last thing is to create package.json test script:

// ...
scripts: {
"test": "mocha"

Run npm test and voilà! You just wrote your first test case using earl!

Make it fail by change expected value to 5. You should see error message clearly explaining problem: Error: 4 not equal to 5.

Now it's time for some fun.

Sometimes you might want to match not exact number but rather whole range of values - that's what matchers are for. Rewrite your assertion to:

expect(actual).toEqual(expect.numberCloseTo(4, 2))

This assertion will match every number "close to" 4 in radius of 2. In this case it doesn't really make sense since addition will always give us accurate results but earl comes with many different matchers which can be very helpful.

Another thing that you might want to try is changing asserted value to a value of a different type, for example:


Right away, you will get a compile time error saying that you want to assert values of a different type (which for sure will be different).

Now, as you gained a basic knowledge of earl, let's dive deeper.