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Natalia Tepluhina ac02e49631 Added a note about v-slot directive

2019-05-28 14:03:27 +00:00

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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();

There are few rules that we need to disable due to technical debt. Which are:
1. no-new
2. class-methods-use-this

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
  const SomeClass = require('some_class');

  // good
  import SomeClass from 'some_class';

  // bad
  module.exports = SomeClass;

  // good
  export default SomeClass;

Import statements are following usual naming guidelines, for example object literals use camel case:

  // some_object file
  export default {
    key: 'value',
  };

  // bad
  import ObjectLiteral from 'some_object';

  // good
  import objectLiteral from 'some_object';

Relative paths: when importing a module in the same directory, a child directory, or an immediate parent directory prefer relative paths. When importing a module which is two or more levels up, prefer either ~/ or ee/.

In app/assets/javascripts/my-feature/subdir:

// bad
import Foo from '~/my-feature/foo';
import Bar from '~/my-feature/subdir/bar';
import Bin from '~/my-feature/subdir/lib/bin';

// good
import Foo from '../foo';
import Bar from './bar';
import Bin from './lib/bin';

In spec/javascripts:

// bad
import Foo from '../../app/assets/javascripts/my-feature/foo';

// good
import Foo from '~/my-feature/foo';

When referencing an EE component:

// bad
import Foo from '../../../../../ee/app/assets/javascripts/my-feature/ee-foo';

// good
import Foo from 'ee/my-feature/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. Although we aim for code without side-effects we need some side-effects for our code to run.

If the class won't do anything if we only instantiate it, it's ok to add side effects into the constructor (Note: The following is just an example. If the only purpose of the class is to add an event listener and handle the callback a function will be more suitable.)
```
// Bad
export class Foo {
  constructor() {
    this.init();
  }
  init() {
    document.addEventListener('click', this.handleCallback)
  },
  handleCallback() {

  }
}

// Good
export class Foo {
  constructor() {
    document.addEventListener()
  }
  handleCallback() {
  }
}
```
On the other hand, if a class only needs to extend a third party/add event listeners in some specific cases, they should be initialized outside of the constructor.

Prefer .map, .reduce or .filter over .forEach A forEach will most likely 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

`eslint-vue-plugin`

We default to eslint-vue-plugin, with the plugin:vue/recommended. Please check this rules for more documentation.

Basic Rules

The service has it's own file
The store has it's own file

Use a function in the bundle file to instantiate the Vue component:

  // bad
  class {
    init() {
      new Component({})
    }
  }

  // good
  document.addEventListener('DOMContentLoaded', () => new Vue({
    el: '#element',
    components: {
      componentName
    },
    render: createElement => createElement('component-name'),
  }));

Do not use a singleton for the service or the store

  // bad
  class Store {
    constructor() {
      if (!this.prototype.singleton) {
        // do something
      }
    }
  }

  // good
  class Store {
    constructor() {
      // do something
    }
  }

Use .vue for Vue templates. Do not use %template in HAML.

Naming

Extensions: Use .vue extension for Vue components. Do not use .js as file extension (#34371).

Reference Naming: Use PascalCase for their instances:

  // bad
  import cardBoard from 'cardBoard.vue'

  components: {
    cardBoard,
  };

  // good
  import CardBoard from 'cardBoard.vue'

  components: {
    CardBoard,
  };

Props Naming: Avoid using DOM component prop names.

Props Naming: 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:

With more than one attribute, all attributes should be on a new line:

  // 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>

The tag can be inline if there is only one attribute:

  // good
    <component bar="bar" />

  // good
    <component
      bar="bar"
      />

  // bad
   <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 be provided if the prop is not required. Note: There are some scenarios where we need to check for the existence of the property. On those a default key should not be provided.

  // good
  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"/>

Shorthand # is preferable over v-slot

  // bad
  <template v-slot:header></template>

  // good
  <template #header></template>

Closing tags

Prefer self closing component tags

  // bad
  <component></component>

  // good
  <component />

Ordering

Tag order in .vue file

<script>
  // ...
</script>

<template>
  // ...
</template>

// We don't use scoped styles but there are few instances of this
<style>
  // ...
</style>

Properties in a Vue Component: Check order of properties in components rule.

`:key`

When using v-for you need to provide a unique :key attribute for each item.

If the elements of the array being iterated have an unique id it is advised to use it:

  <div
    v-for="item in items"
    :key="item.id"
  >
    <!-- content -->
  </div>

When the elements being iterated don't have a unique id, you can use the array index as the :key attribute

  <div
    v-for="(item, index) in items"
    :key="index"
  >
    <!-- content -->
  </div>

When using v-for with template and there is more than one child element, the :key values must be unique. It's advised to use kebab-case namespaces.

  <template v-for="(item, index) in items">
    <span :key="`span-${index}`"></span>
    <button :key="`button-${index}`"></button>
  </template>

When dealing with nested v-for use the same guidelines as above.

  <div
    v-for="item in items"
    :key="item.id"
  >
    <span
      v-for="element in array"
      :key="element.id"
    >
      <!-- content -->
    </span>
  </div>

Useful links:

Vue and Bootstrap

Tooltips: Do not rely on has-tooltip class name for Vue components

  // bad
  <span
    class="has-tooltip"
    title="Some tooltip text">
    Text
  </span>

  // good
  <span
    v-tooltip
    title="Some tooltip text">
    Text
  </span>

Tooltips: When using a tooltip, include the tooltip directive, ./app/assets/javascripts/vue_shared/directives/tooltip.js

Don't change data-original-title.

  // bad
  <span data-original-title="tooltip text">Foo</span>

  // good
  <span title="tooltip text">Foo</span>

  $('span').tooltip('_fixTitle');

The Javascript/Vue Accord

The goal of this accord is to make sure we are all on the same page.

When writing Vue, you may not use jQuery in your application.
1. If you need to grab data from the DOM, you may query the DOM 1 time while bootstrapping your application to grab data attributes using dataset. You can do this without jQuery.
2. You may use a jQuery dependency in Vue.js following this example from the docs.
3. If an outside jQuery Event needs to be listen to inside the Vue application, you may use jQuery event listeners.
4. We will avoid adding new jQuery events when they are not required. Instead of adding new jQuery events take a look at different methods to do the same task.
You may query the window object 1 time, while bootstrapping your application for application specific data (e.g. scrollTo is ok to access anytime). Do this access during the bootstrapping of your application.
You may have a temporary but immediate need to create technical debt by writing code that does not follow our standards, to be refactored later. Maintainers need to be ok with the tech debt in the first place. An issue should be created for that tech debt to evaluate it further and discuss. In the coming months you should fix that tech debt, with it's priority to be determined by maintainers.
When creating tech debt you must write the tests for that code before hand and those tests may not be rewritten. e.g. jQuery tests rewritten to Vue tests.
You may choose to use VueX as a centralized state management. If you choose not to use VueX, you must use the store pattern which can be found in the Vue.js documentation.
Once you have chosen a centralized state management solution you must use it for your entire application. i.e. Don't mix and match your state management solutions.

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