$ es-checker

ECMAScript  Feature Detection (v1.4.1)

Variables

  √ let and const

  √ TDZ error for too-early access of let or const declarations

  √ Redefinition of const declarations not allowed

  √ destructuring assignments/declarations for arrays and objects

  √ ... operator

Data Types

  √ For...of loop

  √ Map, Set, WeakMap, WeakSet

  √ Symbol

  √ Symbols cannot be implicitly coerced

Number

  √ Octal (e.g. 0o1 ) and binary (e.g. 0b10 ) literal forms

  √ Old octal literal invalid now (e.g.  )

  √ Static functions added to Math (e.g. Math.hypot(), Math.acosh(), Math.imul() )

  √ Static functions added to Number (Number.isNaN(), Number.isInteger() )

String

  √ Methods added to String.prototype (String.prototype.includes(), String.prototype.repeat() )

  √ Unicode code-point escape form in string literals (e.g. \u{20BB7} )

  √ Unicode code-point escape form in identifier names (e.g. var \u{20BB7} = ; )

  √ Unicode code-point escape form in regular expressions (e.g. var regexp = /\u{20BB7}/u; )

  √ y flag for sticky regular expressions (e.g. /b/y )

  √ Template String Literals

Function

  √ arrow function

  √ default function parameter values

  √ destructuring for function parameters

  √ Inferences for function name property for anonymous functions

  × Tail-call optimization for function calls and recursion

Array

  × Methods added to Array.prototype ([].fill(), [].find(), [].findIndex(), [].entries(), [].keys(), [].values() )

  √ Static functions added to Array (Array.from(), Array.of() )

  √ TypedArrays like Uint8Array, ArrayBuffer, Int8Array(), Int32Array(), Float64Array()

  √ Some Array methods (e.g. Int8Array.prototype.slice(), Int8Array.prototype.join(), Int8Array.prototype.forEach() ) added to the TypedArray prototypes

  √ Some Array statics (e.g. Uint32Array.from(), Uint32Array.of() ) added to the TypedArray constructors

Object

  √ __proto__ in object literal definition sets [[Prototype]] link

  √ Static functions added to Object (Object.getOwnPropertySymbols(), Object.assign() )

  √ Object Literal Computed Property

  √ Object Literal Property Shorthands

  √ Proxies

  √ Reflect

Generator and Promise

  √ Generator function

  √ Promises

Class

  √ Class

  √ super allowed in object methods

  √ class ABC extends Array { .. }

Module

  × Module export command

  × Module import command

=========================================

Passes  feature Detections

Your runtime supports % of ECMAScript

=========================================

let arr = ['react', 'angular', 'vue']

// WRONG

if (arr.indexOf('react')) { // 0 -> evaluates to false, definitely as we expected

  console.log('Can use React') // this line would never be executed

}

// Correct

if (arr.indexOf('react') !== -1) {

  console.log('Can use React')

}

或者使用一点点hack 位运算符 ~ 使代码更加紧凑一些，因为~（位异或）对任何数字相当于-(a + 1):

let arr = ['react', 'angular', 'vue']

// Correct

if (~arr.indexOf('react')) {

  console.log('Can use React')

}

在ES7中使用includes代码如下:

let arr = ['react', 'angular', 'vue']

// Correct

if (arr.includes('react')) {

  console.log('Can use React')

}

还能在字符串中使用includes:

let str = 'React Quickly'

// Correct

if (str.toLowerCase().includes('react')) {  // true

  console.log('Found "react"')

}

除了增强了可读性语义化，实际上给开发者返回布尔值，而不是匹配的位置。

includes也可以在NaN(非数字)使用。最后 ，includes第二可选参数fromIndex，这对于优化是有好处的，因为它允许从特定位置开始寻找匹配。
更多例子：

console.log([1, 2, 3].includes(2)) // === true)

console.log([1, 2, 3].includes(4)) // === false)

console.log([1, 2, NaN].includes(NaN)) // === true)

console.log([1, 2, -0].includes(+0)) // === true)

console.log([1, 2, +0].includes(-0)) // === true)

console.log(['a', 'b', 'c'].includes('a')) // === true)

console.log(['a', 'b', 'c'].includes('a', 1)) // === false)