We use [Airbnb's JavaScript Style Guide][airbnb-style-guide] and it's accompanying linter to manage most of our JavaScript style guidelines.
We use [Airbnb's JavaScript Style Guide](https://github.com/airbnb/javascript) and it's 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.
In addition to the style guidelines set by Airbnb, we also have a few specific rules
listed below.
> **Tip:**
You can run eslint locally by running `yarn eslint`
## Arrays
## Avoid forEach
Avoid forEach when mutating data. Use `map`, `reduce` or `filter` instead of `forEach`
when mutating data. This will minimize mutations in functions,
which aligns with [Airbnb's style guide](https://github.com/airbnb/javascript#testing--for-real).
```javascript
// bad
users.forEach((user,index)=>{
user.id=index;
});
// good
constusersWithId=users.map((user,index)=>{
returnObject.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.
```javascript
// bad
functiona(p1,p2,p3){
// ...
};
// good
functiona(p){
// ...
};
```
## Avoid side effects in constructors
Avoid making asynchronous calls, API requests or DOM manipulations in the `constructor`.
Move them into separate functions instead. This will make tests easier to write and
code easier to maintain.
<aname="avoid-foreach"></a><aname="1.1"></a>
```javascript
// bad
classmyClass{
constructor(config){
this.config=config;
axios.get(this.config.endpoint)
}
}
-[1.1](#avoid-foreach)**Avoid ForEach when mutating data** Use `map`, `reduce` or `filter` instead of `forEach` when mutating data. This will minimize mutations in functions ([which is aligned with Airbnb's style guide][airbnb-minimize-mutations])
// good
classmyClass{
constructor(config){
this.config=config;
}
```
// bad
users.forEach((user, index) => {
user.id = index;
});
makeRequest(){
axios.get(this.config.endpoint)
}
}
constinstance=newmyClass();
instance.makeRequest();
// good
const usersWithId = users.map((user, index) => {
return Object.assign({}, user, { id: index });
});
```
```
## Functions
## Avoid classes to handle DOM events
<aname="limit-params"></a><aname="2.1"></a>
If the only purpose of the class is to bind a DOM event and handle the callback, prefer
using a function.
-[2.1](#limit-params)**Limit number of parameters** If your function or method has more than 3 parameters, use an object as a parameter instead.
-[3.1](#avoid-constructor-side-effects)**Avoid side effects in constructors** Avoid making some operations in the `constructor`, such as asynchronous calls, API requests and DOM manipulations. Prefer moving them into separate functions. This will make tests easier to write and code easier to maintain.
When your class manipulates the DOM, receive the element container as a parameter.
This is more maintainable and performant.
```javascript
// bad
class myClass {
constructor(config) {
this.config = config;
axios.get(this.config.endpoint)
}
}
```javascript
// bad
classa{
constructor(){
document.querySelector('.b');
}
}
// good
class myClass {
constructor(config) {
this.config = config;
}
// good
classa{
constructor(options){
options.container.querySelector('.b');
}
}
```
makeRequest() {
axios.get(this.config.endpoint)
}
}
const instance = new myClass();
instance.makeRequest();
## Use ParseInt
```
Use `ParseInt` when converting a numeric string into a number.
-[3.2](#avoid-classes-to-handle-dom-events)**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.
// good
parseInt('10',10);
```
```
// bad
class myClass {
constructor(config) {
this.config = config;
}
## CSS Selectors - Use `js-` prefix
init() {
document.addEventListener('click', () => {});
}
}
If a CSS class is only being used in JavaScript as a reference to the element, prefix
the class name with `js-`.
// good
```html
// bad
<buttonclass="add-user"></button>
const myFunction = () => {
document.addEventListener('click', () => {
// handle callback here
});
}
```
// good
<buttonclass="js-add-user"></button>
```
<aname="element-container"></a><aname="3.3"></a>
## Absolute vs relative paths for modules
-[3.3](#element-container)**Pass element container to constructor** When your class manipulates the DOM, receive the element container as a parameter.
This is more maintainable and performant.
Use relative paths if the module you are importing is less than two levels up.
-[4.1](#use-parseint)**Use ParseInt** Use `ParseInt` when converting a numeric string into a number.
// good
importGitLabStyleGuidefrom'~/GitLabStyleGuide';
```
```
// bad
Number('10')
Additionally, **do not add to global namespace**.
// good
parseInt('10', 10);
```
## Do not use `DOMContentLoaded` in non-page modules
## CSS Selectors
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.
<aname="use-js-prefix"></a><aname="5.1"></a>
## Avoid XSS
-[5.1](#use-js-prefix)**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-`
Do not use `innerHTML`, `append()` or `html()` to set content. It opens up too many
vulnerabilities.
```
// bad
<button class="add-user"></button>
## Disabling ESLint in new files
// good
<button class="js-add-user"></button>
```
Do not disable ESLint when creating new files. Existing files may have existing rules
disabled due to legacy compatibility reasons but they are in the process of being refactored.
## Modules
Do not disable specific ESLint rules. Due to technical debt, you may disable the following
rules only if you are invoking/instantiating existing code modules.
-[6.1](#use-absolute-paths)**Use absolute paths for nearby modules** Use absolute paths if the module you are importing is less than two levels up.
```
// bad
import GitLabStyleGuide from '~/guides/GitLabStyleGuide';
// good
import GitLabStyleGuide from '../GitLabStyleGuide';
```
<aname="use-relative-paths"></a><aname="6.2"></a>
-[6.2](#use-relative-paths)**Use relative paths for distant modules** If the module you are importing is two or more levels up, use a relative path instead of an absolute path.
```
// bad
import GitLabStyleGuide from '../../../guides/GitLabStyleGuide';
// good
import GitLabStyleGuide from '~/GitLabStyleGuide';
```
<aname="global-namespace"></a><aname="6.3"></a>
-[6.3](#global-namespace)**Do not add to global namespace**
<aname="domcontentloaded"></a><aname="6.4"></a>
-[6.4](#domcontentloaded)**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.
## Security
<aname="avoid-xss"></a><aname="7.1"></a>
-[7.1](#avoid-xss)**Avoid XSS** Do not use `innerHTML`, `append()` or `html()` to set content. It opens up too many vulnerabilities.
-[8.1](#disable-eslint-file)**Disabling ESLint in new files** Do not disable ESLint when creating new files. Existing files may have existing rules disabled due to legacy compatibility reasons but they are in the process of being refactored.
-[8.2](#disable-eslint-rule)**Disabling ESLint rule** Do not disable specific ESLint rules. Due to technical debt, you may disable the following rules only if you are invoking/instantiating existing code modules
-[no-new][no-new]
-[class-method-use-this][class-method-use-this]
> Note: Disable these rules on a per line basis. This makes it easier to refactor in the future. E.g. use `eslint-disable-next-line` or `eslint-disable-line`
