ES6 let和const定义变量与常量的应用实例分析

{
  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); // undefined
var 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); // 12
console.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

const a = {
 name:'J'
};
a.name = 'Z';
a.age = 10;
console.log(a); // {name: "Z", age: 10}
a = {}; // 尝试修改引用的地址
console.log(a); // Uncaught TypeError: Assignment to constant variable.

const a = {
 name:'K'
}
a.age = 13;
console.log(a); // {name: "K", age: 13}
Object.freeze(a);
a.age = 14;
console.log(a); // Uncaught TypeError: Cannot assign to read only property 'age' of object '#<Object>'