Rahul Banerjee
Rahul Banerjee

Rahul Banerjee

Automate JavaScript Code Linting and Formatting in 5 simple Steps

Automate JavaScript Code Linting and Formatting in 5 simple  Steps
Rahul Banerjee

Published on Aug 18, 2021

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What are Git Hooks?

Git hooks are basically scripts fired before an important action occurs, eg: before a commit is made, before code is pushed to a repo after a commit is made, etc. You can learn more about Git Hooks and the different kinds of hooks over here

A pre-commit hook is a hook that is run before you make a commit.


  1. Install Mocha and Chai for testing
  2. Install Eslint for Linting
  3. Install Pretty and Pretty-quick for formatting
  4. Install Husky to setup pre-commit hook
  5. Test the pre-commit hook

Current Project Setup

I have two files, the first file is called 'utils.js'. It has 4 basic arithmetic functions.

const add = 
(a, b) => a + b

const subtract = 
(a, b) => a - b

const multiply = 
(a, b) => a * b

const divide = 
(a, b) => a / b

module.exports = {

As you can see it is weirdly formatted and is missing semi-colons. This is done intentionally.

The second file is index.js. It simply imports the functions from utils.js and exports them.

const {add, 
    multiply} = require('./utils')
module.exports =  {add,subtract, divide, multiply}

This is also intentionally formatted in a weird manner.

The project also has a basic package.json file generated using npm init

Step 1 Setup Testing

We will use mocha and chai for testing. We will write a single test case for each of our functions. To learn more about mocha and chai in-depth, refer to this article.

First, let's install mocha and chai

npm install --save-dev mocha

Next, let's install chai

npm install --save-dev chai

Now, we will create a file 'tester.js', and add a few tests to it.

/* eslint-disable import/no-extraneous-dependencies */
/* eslint-disable no-undef */
const {
  add, subtract, divide, multiply,
} = require('.');
assert = require('chai').assert;

describe('Sum', () => {
  context('Adding 1 and 1', () => {
    it('should return 2', () => {
      assert(add(1, 1) === 2);

describe('Subtract', () => {
  context('Subtracting 1 from 1', () => {
    it('should return 0', () => {
      assert(subtract(1, 1) === 0);

I have not included the entire tester file, there are a couple more test cases for multiply and divide.

Inside your package.json, add the following under scripts

"test": "mocha --timeout 2000 tester.js"

If you do not have 'scripts' in your package.json, create one. It should look like this

"scripts": {
  "test": "mocha --timeout 2000 tester.js"

Now you can go to the terminal and run the following command

npm test

Step 2 Setup Linter

We will use the package eslint. First, let's install the package

npm install eslint --save-dev

Now we will need to initialize our linter

./node_modules/.bin/eslint --init

You will get a bunch of questions, answer them based on your project.
Finally, we will add a new command inside 'scripts' in 'package.json' You can add this under the 'test' command we added in the previous section.

"lint": "eslint --fix *.js"

This will run the linter on all your javascript files and fix the linting errors wherever it can.
You can also disable some es-lint checks by adding comments to the top of the file or above certain lines. For example, I disabled a couple of checks in the 'tester.js' file

/* eslint-disable import/no-extraneous-dependencies */
/* eslint-disable no-undef */

Step 3 Setup Prettier

We will have to install a couple of packages prettier and pretty-quick to format the code.

Install prettier using the following command

 npm install prettier -save-dev

Install pretty-quick using the following command

npm install pretty-quick --save-dev

Now we will add another command to our 'scripts' section in 'package.json'

  "pretty-quick": "pretty-quick"

Do not run the command right now. Let's set up the pre-commit hook and run the command automatically.

Step 4 Setup Husky

We will use husky to set up our pre-commit hook.
Install the package

npm install husky@4 --save-dev

If you install V5 of husky, you might need to do some additional work to set up the pre-commit hook.

After installation add the following inside your 'package.json'

"husky": {
    "hooks": {
      "pre-commit": "pretty-quick --staged && npm run lint && npm test"

Basically, we tell husky to run the above commands (pretty-quick, lint, and the tests) before our files can be committed.

The --staged runs the formatter only on the staged files.

Step 5 Test Pre-commit hook

Now we can finally test our pre-commit hook.
First, add your files

git add .

Type the following command to commit your files

git commit -m "Test commit"

You should see husky running the prettier, linter and testing scripts. Below is a screenshot of the terminal.

Screen Shot 2021-08-18 at 12.59.18 AM

If the linter returns an error or one of the test cases fails, the commit will be unsuccessful and your files will remain in the staging area and not be committed.

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