Overview

Let's write JavaScript tests using Jest.

Preparation

Since we want to use jest and ESModules, we need to install babel-preset-2015. (babel-jest is provided with jest.)

npm install --save-dev jest babel-preset-2015

The contents of .babelrc look like this.

{
  "presets": ["es2015"]
}

The package.json looks like this.

{
  "scripts": {
    "test": "jest"
  },
  "devDependencies": {
    "babel-preset-es2015": "^6.24.1",
    "jest": "^23.6.0"
  }
}

The directory structure looks like this. tree -a -I "node_modules"

.
├── .babelrc
├── package-lock.json
├── package.json
├── src
│   ├── esmodules
│   │   └── calc.js
│   └── native
│       └── calc.js
└── test
    ├── esmodules
    │   └── calc.test.js
    └── native
        └── calc.test.js

6 directories, 7 files

There are two patterns for creating test files.

Files located under a directory named __tests__ are treated as test files.
Files with the extensions *.spec.js or *.test.js are treated as test files.

This time, we will use the latter format and place the test files in the test directory.

Writing Tests for Native JavaScript

We will implement a function that performs addition and subtraction.

./src/native/calc.js

const counter = 1

const add = function add(num) {
  return counter + num
}

const subtract = function subtract(num) {
  return counter - num
}

module.exports = {
  add, subtract
}

We will test whether each function returns the correct calculation result.

./test/native/calc.test.js

const calc = require("../../src/native/calc")

describe('Calc - native', () => {
  test('add', () => {
    expect(calc.add(1)).toBe(2)
  })

  test('subtract', () => {
    expect(calc.subtract(1)).toBe(0)
  })
})

describe(name, fn) creates a block that groups multiple tests into a test suite.

Run the tests. npm test ./test/native/calc.test.js

Test results.

> @ test /Users/k.takeuchi/localdev/project/til/javascript/test/jest
> jest "./test/native/calc.test.js"

 PASS  test/native/calc.test.js
  Calc - native
    ✓ add (2ms)
    ✓ subtract (1ms)

Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        0.887s, estimated 1s
Ran all test suites matching /.\/test\/native\/calc.test.js/i.

Writing Tests using ESModules

We will create a class that implements methods for addition and subtraction.

./src/esmodules/calc.js

export default class Calc {
  constructor(counter) {
    this.counter = counter
  }

  add(num) {
    this.counter += num

    return this.counter
  }

  subtract(num) {
    this.counter -= num

    return this.counter
  }
}

We will test whether each method returns the correct calculation result.

./test/esmodules/calc.test.js

import Calc from "../../src/esmodules/calc"

describe('Calc - esmodules', () => {
  test('add', () => {
    const calc = new Calc(1)
    expect(calc.add(1)).toBe(2)
  })

  test('subtract', () => {
    const calc = new Calc(1)
    expect(calc.subtract(1)).toBe(0)
  })
})

Matchers

Refer to Jest - Expect.

Thoughts

I think the Jest API is well organized to be easy to understand even for beginners. The documentation was also easy to read. I found it surprisingly easy to get started with testing, so I want to actively write tests for JavaScript.