gitlab-org--gitlab-foss/doc/development/fe_guide/style_guide_js.md

Style guides and linting

See the relevant style guides for our guidelines and for information on linting:

JavaScript

We defer to Airbnb on most style-related conventions and enforce them with eslint.

See our current .eslintrc for specific rules and patterns.

Common

ESlint

• Never disable eslint rules unless you have a good reason. You may see a lot of legacy files with /* eslint-disable some-rule, some-other-rule */ at the top, but legacy files are a special case. Any time you develop a new feature or refactor an existing one, you should abide by the eslint rules.

• Never Ever EVER disable eslint globally for a file

  // bad
  /* eslint-disable */
  
  // better
  /* eslint-disable some-rule, some-other-rule */
  
  // best
  // nothing :)
  

• If you do need to disable a rule for a single violation, try to do it as locally as possible

  // bad
  /* eslint-disable no-new */
  
  import Foo from 'foo';
  
  new Foo();
  
  // better
  import Foo from 'foo';
  
  // eslint-disable-next-line no-new
  new Foo();
  

• When they are needed always place ESlint directive comment blocks on the first line of a script, followed by any global declarations, then a blank newline prior to any imports or code.

  // bad
  /* global Foo */
  /* eslint-disable no-new */
  import Bar from './bar';
  
  // good
  /* eslint-disable no-new */
  /* global Foo */
  
  import Bar from './bar';
  

• Never disable the no-undef rule. Declare globals with /* global Foo */ instead.

• When declaring multiple globals, always use one /* global [name] */ line per variable.

  // bad
  /* globals Flash, Cookies, jQuery */
  
  // good
  /* global Flash */
  /* global Cookies */
  /* global jQuery */
  

• Use up to 3 parameters for a function or class. If you need more accept an Object instead.

  // bad
  fn(p1, p2, p3, p4) {}
  
  // good
  fn(options) {}
  

Modules, Imports, and Exports

• Use ES module syntax to import modules

  // bad
  require('foo');
  
  // good
  import Foo from 'foo';
  
  // bad
  module.exports = Foo;
  
  // good
  export default Foo;
  

• Relative paths

  Unless you are writing a test, always reference other scripts using relative paths instead of ~

  In app/assets/javascripts:

  // bad
  import Foo from '~/foo'
  
  // good
  import Foo from '../foo';
  

  In spec/javascripts:

  // bad
  import Foo from '../../app/assets/javascripts/foo'
  
  // good
  import Foo from '~/foo';
  

• Avoid using IIFE. Although we have a lot of examples of files which wrap their contents in IIFEs (immediately-invoked function expressions), 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.

• Avoid adding to the global namespace.

  // bad
  window.MyClass = class { /* ... */ };
  
  // good
  export default class MyClass { /* ... */ }
  

• Side effects are forbidden in any script which contains exports

  // bad
  export default class MyClass { /* ... */ }
  
  document.addEventListener("DOMContentLoaded", function(event) {
    new MyClass();
  }
  

Data Mutation and Pure functions

• 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);
  

• Avoid constructors with side-effects

• Prefer .map, .reduce or .filter over .forEach A forEach will cause side effects, it will be mutating the array being iterated. Prefer using .map, .reduce or .filter

  const users = [ { name: 'Foo' }, { name: 'Bar' } ];
  
  // bad
  users.forEach((user, index) => {
    user.id = index;
  });
  
  // good
  const usersWithId = users.map((user, index) => {
    return Object.assign({}, user, { id: index });
  });
  

Parse Strings into Numbers

• parseInt() is preferable over Number() or +

  // bad
  +'10' // 10
  
  // good
  Number('10') // 10
  
  // better
  parseInt('10', 10);
  

CSS classes used for JavaScript

• If the class is being used in Javascript it needs to be prepend with js-

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

Vue.js

Basic Rules

• Only include one Vue.js component per file.

• Export components as plain objects:

  export default {
    template: `<h1>I'm a component</h1>
  }
  

Naming

• Extensions: Use .vue extension for Vue components.

• Reference Naming: Use PascalCase for Vue components and camelCase for their instances:

  // bad
  import cardBoard from 'cardBoard';
  
  // good
  import CardBoard from 'cardBoard'
  
  // bad
  components: {
    CardBoard: CardBoard
  };
  
  // good
  components: {
    cardBoard: CardBoard
  };
  

• Props Naming:

• Avoid using DOM component prop names.

• Use kebab-case instead of camelCase to provide props in templates.

  // bad
  <component class="btn">
  
  // good
  <component css-class="btn">
  
  // bad
  <component myProp="prop" />
  
  // good
  <component my-prop="prop" />
  

#### Alignment
- Follow these alignment styles for the template method:

  ```javascript
  // bad
  <component v-if="bar"
      param="baz" />

  <button class="btn">Click me</button>

  // good
  <component
    v-if="bar"
    param="baz"
  />

  <button class="btn">
    Click me
  </button>

  // if props fit in one line then keep it on the same line
  <component bar="bar" />

Quotes

• Always use double quotes " inside templates and single quotes ' for all other JS.

  // bad
  template: `
    <button :class='style'>Button</button>
  `
  
  // good
  template: `
    <button :class="style">Button</button>
  `
  

Props

• Props should be declared as an object

  // bad
  props: ['foo']
  
  // good
  props: {
    foo: {
      type: String,
      required: false,
      default: 'bar'
    }
  }
  

• Required key should always be provided when declaring a prop

  // bad
  props: {
    foo: {
      type: String,
    }
  }
  
  // good
  props: {
    foo: {
      type: String,
      required: false,
      default: 'bar'
    }
  }
  

• Default key should always be provided if the prop is not required:

  // bad
  props: {
    foo: {
      type: String,
      required: false,
    }
  }
  
  // good
  props: {
    foo: {
      type: String,
      required: false,
      default: 'bar'
    }
  }
  
  // good
  props: {
    foo: {
      type: String,
      required: true
    }
  }
  

Data

• data method should always be a function

  // bad
  data: {
    foo: 'foo'
  }
  
  // good
  data() {
    return {
      foo: 'foo'
    };
  }
  

Directives

• Shorthand @ is preferable over v-on

  // bad
  <component v-on:click="eventHandler"/>
  
  
  // good
  <component @click="eventHandler"/>
  

• Shorthand : is preferable over v-bind

  // bad
  <component v-bind:class="btn"/>
  
  
  // good
  <component :class="btn"/>
  

Closing tags

• Prefer self closing component tags

  // bad
  <component></component>
  
  // good
  <component />
  

Ordering

• Order for a Vue Component:
  1. name
  2. props
  3. data
  4. components
  5. computedProps
  6. methods
  7. lifecycle methods
  8. beforeCreate
  9. created
  10. beforeMount
  11. mounted
  12. beforeUpdate
  13. updated
  14. activated
  15. deactivated
  16. beforeDestroy
  17. destroyed
  18. template

SCSS

• SCSS