ES6 & Javascript

es 和 js 的关系： 前者是后者的规格，后者是前者的实现
通过 es-checker 检查当前 node 环境对于 es6 的支持情况

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npm install -g es-checker
es-checker
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λ es-checker

ECMAScript 6 Feature Detection (v1.4.2)

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. 01 )
  √ 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} = 42; )
  √ 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 39 feature Detections
Your runtime supports 92% of ECMAScript 6
=========================================

变量

es6 中我们可以通过 let 定义变量

不能重复声明

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let str = "";
let str = "123";

变量重复定义：
Identifier ‘str’ has already been declared

暂时性死区

在变量声明之前调用了该变量，就会形成这个变量的暂时性死区。

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console.log(str);
let str = "";

声明之前调的语法错误
Cannot access ‘str’ before initialization

变量提升

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// 通过var 定义了一个变量str并在声明之前调用
console.log(str);
var str = "";

// 变量提升在内存中的执行时，会将str前置
var str;
console.log(str);
str = "";

块及作用域

作用域

  • 全局作用域
  • 块级作用域

常量

es6 中我们可以通过 const 定义常量

不能重复声明

暂时性死区

没有变量提升

块级作用域

恒定的值

通过 const 定义的变量的值（数据类型和引用）不可以重新赋值。

  • 数据类型不可改
  • 不可以对 const 的值进行重新赋值的操作
  • 可以修改数据的项，不可以修改数据的指针
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const a = 123;
a = 456;
// 常量的值不可以被修改
// const a = 123;
// a = 456

// 重新赋值
// const arr = [1, 23, , 5, , 6, 6];
// arr = [1, 23, , 5, , 6, 6];

// 引用相同，重新赋值不可
// const temp = [1, 23, , 5, , 6, 6];
// const arr = temp;
// arr = temp;

// 数组特殊情况
// const temp = [1, 23, , 5, , 6, 6];
// temp[3] = 123;
// temp.push(132)

// 对象的特殊情况
var temp = { name: "zhangsan" };
temp.name = "123";
temp.age = 123;
console.log(temp);
// const obj = temp;
// obj = temp;

错误类型

  • syntax error 语法错误
  • Reference Error 引用错误
  • type error 类型错误