因此，即使在块级作用域内定义的变量，在块级作用域外仍然能被访问。
这源于通过var声明的变量实际上总是在距离最近的函数内或全局作用域中注册的，
不关注块级作用域。

for( let i=0;i<10;i++){
}
console.log(i);//error, i is not defined

const FLAG= true;//使用const定义的全局静态变量，常使用大写表示
FLAG = false;
console.log(FLAG); // ture

const objConst =  {};
objConst.name = 'imaginecode';

#es5
function () {
}
#es6
()=>{
}

window.onload = function() {
  alert('abc);
};
#es6
window.onload = ()=> {
  alert('abc');
};

function show(a,b,...args) {
  alert(a);
  alert(b);
  alert(args);//args = [3,4,5,6]
}
show(1,2,3,4,5,6);

let arr=[1,2,3];
show(...arr); //==show(1,2,3)
function(a,b,c){
}

function show(a,b=1,c=2){//b=1,c=2默认参数
  console.log(a,b,c);
}
show(7,8,9);//result: 7,8,9

//数组
var [,value] = ["color","red"];
console.log(value);//"red"
//对象
var person = {
  name:"imaginecode",
  age: 17
}
var {name:personName,age:personAge} = person;
console.log(personName);//"imaginecode"
console.log(personAge);//17

let arr=[1,2,3];
#解构赋值
let [a,b,c]=[1,2,3];//左右两边必须一样

let {a,b,c}={a:1,b:2,c:3};//json
let [{a,b,c},[n1,n2,n3],num,str]=[{a:1,b:2},[1,2,3],1,'abc']; 
# 等同
let [json,arr,num,str][{a:1,b:2},[1,2,3],1,'abc'];
console.log(json,arr,num,str);

let arr = [1,2,3];
let result = arr.map(function(item){
  return item*2;
});
alert(result);
# 箭头函数改写
let result = arr.map(item=>item*2);

# 求和
let arr = [1,2,3,4,5,6];
let result = arr.reduce(function(tmp,item,index){
  return tmp+item;
  //tmp：中间结果
  //item:arr中的元素，index:下标
});
alert(result);

# 求平均数
let arr = [1,2,3,4,5,6];
let result = arr.reduce(function(tmp,item,index){
  if(index!=arr.length-1){//不是最后一个
    return tmp+item;
  }else {//是最后一个数
    return (tmp+item)/arr.length;
  }
});
alert(result);

let arr = [1,2,3,4,5,6];
let result = filter(item=>item%3==0);
alert(result);

  let arr = [
    {title:'a',price:10002},
    {title:'b',price:999},
    {title:'c',price:789},
    {title:'d',price:10090},
    {title:'e',price:11122},
  ];
  let result=arr.filter(json=>json.price>=10000);
  console.log(result);

let arr = [1,2,3];
arr.forEach((item,index)=。{
  alert(index+':'+item);
})；

function sum(num1,num2,...nums){
    var result = num1 + num2;
    for(let i=0,len=nums.length;i<len;i++){
        console.log(nums[i]);
        result +=nums[i];
    }
}
var result = sum(1,2,3,4,5,6);

var result = sum(...[1,2,3,4,5,6]);

function sum(num1,num2=0){
}
sum(5);

let str= 'http://abc.com';
if(str.startsWith('http://')){
}else if(){
}

let str = 'a.txt';
if(str.endsWith('.txt'){
  alert('文本文件');
})

let a=12;
let str = `a${a}c`;//str = a12c
# 字符串拼接
let title = '标题';
let content = '内容';
let str = '<div>\
   <h1>'+title+'</h1>\
   <p>'+content+'</p>\
</div>';
#改写
let str2 = `<div>
    <h1>${title}</h1>
    <p>${content}</p>
</div>`;

class User{
  constructor(name,pass){
    this.name = name;
    this.pass = pass;
  }
  showName() {
    alert(this.name);
  }
  showPass() {
    alert(this.pass);
  }
}
var user = new User('blue','123');
user.showName();
user.showPass();

class vipUser extends User{
  constructor(name,pass,level){
    super(name,pass); //super关键字，超类(父类)
    this.level = level;
  }
  showLevel(){
    alert(this.level);
  }
}
var v1 = new vipUser('blue','123');
v1.showName();
v1.showPass();
v1.showLevel();

class Test extends React.Component {
            constructor(...args) {
                super(...args);
            }
            render(){
                return <span>123</span>;
            }
        }
        window.onload = function() {
            let oDiv = document.getElementById('div1');
            ReactDOM.render(
                <Test></Test>,
                oDiv
            );
        };

let str = '{"a":12,"b":5，"c":"abc"}';
let json = JSON.parse(str);

let a =1;
let b =5;
let json = {a:a,c:b};//等价于let json = {a,c:b};

let json = {
  a:1,
  show(){
    alert(this.a);
  }
}
json.show();