JavaScript style guide

We use Airbnb’s JavaScript Style Guide and its accompanying linter to manage most of our JavaScript style guidelines.

In addition to the style guidelines set by Airbnb, we also have a few specific rules listed below.

noteYou can run ESLint locally by running yarn run lint:eslint:all or yarn run lint:eslint $PATH_TO_FILE.

Avoid forEach

Avoid forEach when mutating data. Use map, reduce or filter instead of forEach when mutating data. This minimizes mutations in functions, which aligns with Airbnb’s style guide. 

// bad
users.forEach((user, index) => {
  user.id = index;
});

// good
const usersWithId = users.map((user, index) => {
  return Object.assign({}, user, { id: index });
});

Limit number of parameters

If your function or method has more than 3 parameters, use an object as a parameter instead. 

// bad
function a(p1, p2, p3) {
  // ...
};

// good
function a(p) {
  // ...
};

Avoid classes to handle DOM events

If the only purpose of the class is to bind a DOM event and handle the callback, prefer using a function. 

// bad
class myClass {
  constructor(config) {
    this.config = config;
  }

  init() {
    document.addEventListener('click', () => {});
  }
}

// good

const myFunction = () => {
  document.addEventListener('click', () => {
    // handle callback here
  });
}

Pass element container to constructor

When your class manipulates the DOM, receive the element container as a parameter. This is more maintainable and performant. 

// bad
class a {
  constructor() {
    document.querySelector('.b');
  }
}

// good
class a {
  constructor(options) {
    options.container.querySelector('.b');
  }
}

Converting Strings to Integers

When converting strings to integers, parseInt has a slight performance advantage over Number, but Number is semantic and can be more readable. Prefer parseInt, but do not discourage Number if it significantly helps readability.

WARNING: parseInt must include the radix argument. 

// bad
parseInt('10');

// bad
things.map(parseInt)

// ok
Number("106")

// good
things.map(Number)

// good
parseInt("106", 10)

CSS Selectors - Use js- prefix

If a CSS class is only being used in JavaScript as a reference to the element, prefix the class name with js-. 

// bad
<button class="add-user"></button>

// good
<button class="js-add-user"></button>

ES Module Syntax

Use ES module syntax to import modules: 

// bad
const SomeClass = require('some_class');

// good
import SomeClass from 'some_class';

// bad
module.exports = SomeClass;

// good
export default SomeClass;

We still use require in scripts/ and config/ files.

Absolute vs relative paths for modules

Use relative paths if the module you are importing is less than two levels up. 

// bad
import GitLabStyleGuide from '~/guides/GitLabStyleGuide';

// good
import GitLabStyleGuide from '../GitLabStyleGuide';

If the module you are importing is two or more levels up, use an absolute path instead: 

// bad
import GitLabStyleGuide from '../../../guides/GitLabStyleGuide';

// good
import GitLabStyleGuide from '~/GitLabStyleGuide';

Additionally, do not add to global namespace.

Do not use DOMContentLoaded in non-page modules

Imported modules should act the same each time they are loaded. DOMContentLoaded events are only allowed on modules loaded in the /pages/* directory because those are loaded dynamically with webpack.

Avoid XSS

Do not use innerHTML, append() or html() to set content. It opens up too many vulnerabilities.

Avoid single-line conditional statements

Indentation is important when scanning code as it gives a quick indication of the existence of branches, loops, and return points. This can help to quickly understand the control flow. 

// bad
if (isThingNull) return '';

if (isThingNull)
  return '';

// good
if (isThingNull) {
  return '';
}

ESLint

ESLint behavior can be found in our tooling guide.

IIFEs

Avoid using IIFEs (Immediately-Invoked Function Expressions). Although we have a lot of examples of files which wrap their contents in IIFEs, this is no longer necessary after the transition from Sprockets to webpack. Do not use them anymore and feel free to remove them when refactoring legacy code.

Global namespace

Avoid adding to the global namespace. 

// bad
window.MyClass = class { /* ... */ };

// good
export default class MyClass { /* ... */ }

Side effects

Top-level side effects

Top-level side effects are forbidden in any script which contains export: 

// bad
export default class MyClass { /* ... */ }

document.addEventListener("DOMContentLoaded", function(event) {
  new MyClass();
}

Avoid side effects in constructors

Avoid making asynchronous calls, API requests or DOM manipulations in the constructor. Move them into separate functions instead. This makes tests easier to write and avoids violating the Single Responsibility Principle. 

// bad
class myClass {
  constructor(config) {
    this.config = config;
    axios.get(this.config.endpoint)
  }
}

// good
class myClass {
  constructor(config) {
    this.config = config;
  }

  makeRequest() {
    axios.get(this.config.endpoint)
  }
}
const instance = new myClass();
instance.makeRequest();

Pure Functions and Data Mutation

Strive to write many small pure functions and minimize where mutations occur 

  // bad
  const values = {foo: 1};

  function impureFunction(items) {
    const bar = 1;

    items.foo = items.a * bar + 2;

    return items.a;
  }

  const c = impureFunction(values);

  // good
  var values = {foo: 1};

  function pureFunction (foo) {
    var bar = 1;

    foo = foo * bar + 2;

    return foo;
  }

  var c = pureFunction(values.foo);

Export constants as primitives

Prefer exporting constant primitives with a common namespace over exporting objects. This allows for better compile-time reference checks and helps to avoid accidental undefineds at runtime. In addition, it helps in reducing bundle sizes.

Only export the constants as a collection (array, or object) when there is a need to iterate over them, for instance, for a prop validator. 

  // bad
  export const VARIANT = {
    WARNING: 'warning',
    ERROR: 'error',
  };

  // good
  export const VARIANT_WARNING = 'warning';
  export const VARIANT_ERROR = 'error';

  // good, if the constants need to be iterated over
  export const VARIANTS = [VARIANT_WARNING, VARIANT_ERROR];