ES6 let和const定义变量与常量的应用实例分析

本文实例讲述了ES6 let和const定义变量与常量的应用。分享给大家供大家参考，具体如下：

关于 let

{  var a = 12;  let b = 15;  {   console.log(a); // 12   console.log(b); // 15  }}

console.log(a); // Uncaught ReferenceError: a is not defined，直接报错let a = 12; // let定义的变量不能进行变量的提升

console.log(b); // undefinedvar b = 12;// 上面的过程存在变量提升过程如下：/*var b;console.log(b);b = 12;*/

{ var a = 12; let b = 15; console.log(b); // 15}console.log(a); // 12console.log(b); // Uncaught ReferenceError: b is not defined

for(var i=0;i<5;i++) {}console.log(i); // 5 for(let m=0;m<5;m++){}console.log(m); // Uncaught ReferenceError: m is not defined

var a = 18;var b = 10;fun();function fun(){ console.log(b); // 10 console.log(a); // Uncaught ReferenceError: a is not defined let a;}

let a;var a; // Uncaught SyntaxError: Identifier 'a' has already been declared

var config = (function () { var res = []; res.push(1); res.push(2); res.push(3); return res;})();console.log(config); // [1,2,3]

let config;{ config = []; config.push(1); config.push(2); config.push(3);}console.log(config); // [1,2,3]

'use strict';function a(){console.log(1);}(function () { a(); // 去除'use strict' 之后 报错：a is not a function，加上'use strict'之后正常，输出1 if(false) {  function a(){console.log(2);} // 此处重新声明和赋值了一个a函数,上面输出1的时候表示没有被提升 }})();

var arr = [];for(var i=0;i<5;i++) { arr.push(function () {  console.log(i); })}arr[1](); // 5 

var arr = [];for(var i=0;i<5;i++) { arr.push((function (i) {  return function () {   console.log(i);  } })(i));}arr[1](); // 1

var arr = [];for(let i=0;i<5;i++) { arr.push(function () {  console.log(i); })}arr[1](); // 1