How - To Use Jest To Test Vanilla JavaScript Files Without A Framework

## The Issue

I'm building a plain old html and vanilla javascript site. I decided I wanted to setup a test suite and picked Jest since it was the first search result. The Jest page says "It works with projects using : Babel, TypeScript, Node, React, Angular, Vue, and more!" It took a little hunting to figure out how to run it without using any of those frameworks. Here's what I ended up with. It provides [TODO: Code shorthand span ] on the command line for vanilla javascript testing.

## The Process

1. Install jest globally

bash

npm install -g jest

2. Create a jest.config.js file in your working directory with :

javascript

const config = {
  verbose: true,
  testEnvironment: 'jsdom',
}

module.exports = config

3. Create a [TODO: Code shorthand span ] directory in your working directory

bash

mkdir tests

4. Create a test file in your [TODO: Code shorthand span ] directory (e.g. ` tests/functions.test.js ` )

javascript

const functions = require('../functions')

test('addition works', () => {
  expect(functions.sum(1, 2)).toBe(3)
})

4. Create the file to be tested in your working directory (e.g. ` functions.js ` )

javascript

function sum(a, b) {
  return a + b + a
}

if (typeof module !== 'undefined') {
  module.exports.sum = sum
}

5. Run jest from the command line with :

bash

jest --watchAll

When you fire that off, you'll get the output from jest that looks something like this :

bash

PASS  ./functions.test.js
  ✓ addition works (2 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.593 s
Ran all test suites.

The process stays open and runs tests as you make changes. Press [TODO: Code shorthand span ] to exit it.

- You only need to do the first step (where you install jest globally) once

- The [TODO: Code shorthand span ] command hangs if you try to run it without a configuration (e.g. ` jest.config.js ` ) file

- Jest uses a Node environment by default. The [TODO: Code shorthand span ] line in the [TODO: Code shorthand span ] file switches to JSDOM which allows you to use things like ` document.addEventListener('DOMContentLoaded', function () {} `

- More details on the configuration options are here

- The [TODO: Code shorthand span ] check is to prevent a [TODO: Code shorthand span ] error when the file is used in the browser

- Use [TODO: Code shorthand span ] instead of [TODO: Code shorthand span ] to do a single run instead of watching files

- If you're in a git repo, you can also do [TODO: Code shorthand span ] instead of [TODO: Code shorthand span ] to focus on files that are in the repo

- The examples on the jest site only export one function per file. With the code above, you can test multiple functions in a single file (e.g. you could add a function called [TODO: Code shorthand span ] , export it with [TODO: Code shorthand span ] , and test it with [TODO: Code shorthand span ]