Get hooked on Git hooks

If you’re like me, you’re crazy over automating boring stuff. One of the things I got hooked on (pun intended) during the last year, and which helps in that automation process, is Git Hooks. If you haven’t heard of Git Hooks and want to see some cool ways of improving your daily git workflow stay tuned!

What are Git Hooks? 🎣

This page from Git documentation sums it up pretty well but in general Git Hooks are Gits answer on firing custom event when some Git related action occurs. We will focus on client-side pre-commit and commit-msg hooks today but following options are available:

Client-Side Hooks

• pre-commit - runs before we even type the commit message.
• prepare-commit-msg - runs before the commit message editor is opened up but after the default message is created.
• commit-msg - good place to validate project state or the commit message before allowing commit to proceed further.
• post-commit - runs after the entire commit process is completed, used mostly for notifications.
• pre-rebase - runs before the rebase.
• post-merge - runs after the successful merge.
• pre-push - runs during the Git push.
• pre-auto-gc - runs before Git triggers a garbage collector.

Server-Side Hooks

• pre-receive- the first script that is run on the client-side push, if it exits non-zero,the push is not accepted.
• update - pretty similar to the pre-receive excepts it runs once for every branch that the client-side wants to update. For example, if we’re pushing to five branches at the same time, pre-receive will run once, update will run five times.
• post-receive - similar to the client-side post-commit just on the server-side.

Talk is cheap, show me the code

Since Git hooks don’t have the best out of the box experience, we’ll use the Husky library to make stuff easier:

yarn add husky --dev

You’re now able to include hook definition inside package.json like this:

// package.json
{
  // ...
  "husky": {
    "hooks": {
      "pre-commit": "<cool-script>",
      "commit-msg": "<even-cooler-script>"
    }
  }
  // ...
}

pre-commit

In most of cases we want to run the pre-commit hook only on staged files, lint-staged library helps us with that:

yarn add lint-staged --dev

After we’ve added the lint-staged we’re able to do something like this inside a package.json:

// package.json
{
  "husky": {
    "hooks": {
      "pre-commit": "lint-staged" }
  },
  "lint-staged": { "*.{js,md,css,scss,html}": ["<yet-another-cool-command-1>", "<yet-another-cool-command-2>"] }}

Now when we know the basics, it’s time to start adding scripts that will help our repository become better place ✨.

First let’s add prettier - hope you’ve heard of it since it’s the best thing that happened to code formatting in a while.

yarn add prettier --dev

We can pass arguments to the prettier script directly but I’m fan of config files, so we’ll create a .prettierrc file in the project root directory:

// .prettierrc
{
  "singleQuote": true,
  "tabWidth": 2,
  "trailingComma": "es5"
  // other available options: https://prettier.io/docs/en/options.html
}

Prettier will format all staged files on the commit so they follow a code convention defined inside the .prettierrc.

// package.json
{
  // ...
  "lint-staged": {
    "*.{js,md,css,scss,html}": ["prettier --write"]
  }
}

Time to lint our .js files, we can easily do that with eslint.

yarn add eslint --dev

We will define a config file again, this time the eslintrc.json:

// eslintrc.json
{
  "extends": "eslint:recommended",
  "env": {
    "browser": true,
    "commonjs": true,
    "node": true,
    "es6": true
  },
  "parserOptions": {
    "ecmaVersion": 2018
  },
  "rules": {
    "no-console": 2, // using console.log() throws error
    "curly": "warn" // enforce usage of curly braces, if(foo) foo++ will throw warning
  }
}

We need to define a special rule that will be triggered for .js files only. eslint will prevent committing if error is thrown.

// package.json
{
  // ...
  "lint-staged": {
    "*.{js,md,css,scss,html}": ["prettier --write"],
    "*.js": ["eslint --fix"] }
}

As the final step I’ll show you how to run relevant unit tests (relevant to committed files) and prevent commit if some of them are failing.

yarn add jest --dev
yarn add eslint-plugin-jest --dev

We should add previously installed jest plugin to our eslint config file so we eliminate eslint errors on .spec.js files.

// eslintrc.json
{
  "extends": ["eslint:recommended", "plugin:jest/recommended"], "env": {
    "browser": true,
    "commonjs": true,
    "node": true,
    "es6": true
  },
  "parserOptions": {
    "ecmaVersion": 2018
  },
  "rules": {
    "no-console": 2,
    "curly": "warn"
  },
  "plugins": ["jest"]}

Now extend lint-staged script:

// package.json
{
  // ...
  "lint-staged": {
    "*.{js,md,css,scss,html}": ["prettier --write"],
    "*.js": ["eslint --fix", "jest --bail --findRelatedTests"] }
}

--bail will skip execution of other tests when first test fails and --findRelatedTests is pretty self-explanatory 😁.

To demonstrate how this works we can create two files test-file.js and test-file.spec.js

// test-file.js
function sumTwoNumbers(a, b) {
  return a + b
}

module.exports = sumTwoNumbers

We’re intentionally making the unit test fail so we can see commit failing:

// test-file.spec.js
const sumTwoNumbers = require('./test-file')

it('should sum two numbers incorrectly', () => {
  const result = sumTwoNumbers(2, 3)

  expect(result).toBe(6)
})

commit-msg

There are only two hard things in Computer Science: cache invalidation and naming things

This rule applies to commit messages also, we’ve all seen or written commits like this in past:

git log --oneline

7c1f5c5 final fix
93393a0 aaaaa
3626b1d TEST WIP
45bc996 small css fix
29b2993 css final final fix
a2f6e18 lol
3ae828c UNIT TESTS ADDED WOO

This is an extreme example but it perfectly shows how we can’t make a clear conclusion about what is going on in a particular commit.

If we check history of commit messages created during previous examples:

git log --oneline

2c1f5c5 feat: add jest testing
85bc9g6 refactor: reformat html file

Much cleaner right? This commits follow Conventional Commit convention created by Angular team.

In general the pattern that commit message should follow mostly look like this:

type(scope?): subject #scope is optional

Some of common types are:

• feat - commit adds a new feature.
• fix - commit fixes a bug.
• docs - commit introduces documentation changes.
• style - commit introduces code style change (indentation, format, etc.).
• refactor - commit introduces code refactoring.
• perf - commit introduces code performances.
• test - commit adds test to an existing feature.
• chore - commit updates something without impacting the user (ex: bump a dependency in package.json)

---

So, now when we know this, it’s perfect time to introduce commit-msg hook where we’ll check if commit message respect this rules before we commit.

First we want to install commitlint, something like eslint just for commit messages.

# install commitlint cli and conventional config
yarn add --dev @commitlint/{config-conventional,cli}

Of course we need to create another config file, .commitlintrc.json, the last one I promise! 🤞

// .commitlintrc.json
{
  // Extend previously installed config
  "extends": ["@commitlint/config-conventional"]
}

Now we can extend hooks property inside the package.json:

// package.json
    // ...
    "husky": {
        "hooks": {
            "pre-commit": "lint-staged",
            "commit-msg": "commitlint -E HUSKY_GIT_PARAMS" }
    }
    // ...

---

Quick recap of what we learned today:

lint-staged inside the pre-commit hook will take care of:

• formatting all staged files via Prettier.
• check all staged .js files for syntax errors via Eslint
• check if relevant .spec.js unit test files are failing before we commit via Jest

commitlint inside the commit-msg hook will take care of:

• enforce commit message to follow Conventional Commit rules via Commitlint.

---

See also

• cz-cli - The commitizen command line utility.
• husky-sandbox - Code samples from this post.