README.md

Airbnb: Don’t use iterators like for-in or for-of

When using airbnb, I noticed that for-in and for-of are restricted. This topic is to try to understand why and also try to find the best practice.

ESLint

ESLint provides a rule: no-restricted-syntax. With this rule, Airbnb disallows certain syntax forms:

'no-restricted-syntax': [
  'error',
  {
    selector: 'ForInStatement',
    message: 'for..in loops iterate over the entire prototype chain, which is virtually never what you want. Use Object.{keys,values,entries}, and iterate over the resulting array.',
  },
  {
    selector: 'ForOfStatement',
    message: 'iterators/generators require regenerator-runtime, which is too heavyweight for this guide to allow them. Separately, loops should be avoided in favor of array iterations.',
  },
  {
    selector: 'LabeledStatement',
    message: 'Labels are a form of GOTO; using them makes code confusing and hard to maintain and understand.',
  },
  {
    selector: 'WithStatement',
    message: '`with` is disallowed in strict mode because it makes code impossible to predict and optimize.',
  },
],

Arrays built in methods

I summarized all the loop/iterators related APIs here:

ES5

• classic for loop
• for...in
• Array.prototype.forEach()
  为 Array 对象引入了 Array.forEach 方法以代替 for 循环，Array.forEach 方法的特点是自带闭包，以解决因为缺乏块级作用域导致需要使用取巧的方法来解决 var 的作用域问题。
• Array.prototype.map()
• Array.prototype.filter()
• Array.prototype.reduce()
• Array.prototype.every()
• Array.prototype.some()
• Object.keys()

ES6/ES2015

• for...of
  ECMAScript 引入了一种新的循环语句 for...of，主要的用途是代替 for...in 循环语句, see difference.
• Array.prototype.keys()
• Array.prototype.values()
• Array.prototype.entries()
  在 ES2015 标准中，数组类型再次被赋予了一个名为 entries 的方法，它可以返回对应的数组中每一个元素与其下标配对的一个新数组。
• Array.prototype.find()
• Array.prototype.findIndex()
• Array.from()

ES2016

• Array.prototype.includes()
  indexOf 通过返回值是否等于-1 来获得查询对象是否被该数组包含。而 includes 则是通过返回 true 或者 false 来得出结果，对于只是查询是否包含，语义显得更清晰一些。

ES2017

• Object.entries()
• Object.values()

Best Practice

See TestLoop.js "Best Choice"

My Understanding

I think Airbnb team do this for the following purpose:

1. The polyfill of for..of doesn't work well and Airbnb still supports IE 11. They take it very seriously.
2. They encourage to use function in a "functional" way

Related Topic

Airbnb JavaScript Style Guide
Using 'ForOfStatement' is not allowed (no-restricted-syntax) Array Iteration Why and when to use forEach, map, filter, reduce, and find in JavaScript.