ES6常用特性与一些注意点

数组解构

const arr = [100, 200, 300]
let [foo, bar, baz] = arr    //100 200 300
let [, , baz] = arr    //300
let [foo, ...rest] = arr    //100,[200,300]
let [foo] = arr    //100
let [foo, bar, baz = 400, more = 123] = arr    //100 200
//解构时等号后为默认值
let [foo, bar, baz = 400, more = 123,and] = arr    //100 200 300 123 undefined  

对象解构

const obj = { name: 'zs', age: 18 }
const { name,age:newAge = '20' } = obj    //name值为'zs'，newAge为18，覆盖默认值20 

字符串扩展方法

const msg = 'Error: foo is not defined.'
//是否以某个字符串开始
console.log(msg.startsWith('Error'))        //true
//是否以某个字符串结尾
console.log(msg.endsWith('.'))        //true
//是否包含某个字符串
console.log(msg.includes('foo'))        //true

函数参数：默认值与剩余参数

function foo(a=100,...args){
  console.log(a,args)
}
foo()      //100 []
foo(101,10)    //101 [10]
foo(200,300,400)    //200 [300,400]

箭头函数

const person = {
    name:'tom',
    say1:()=>{ console.log(this.name) },
    say2:function(){ console.log(this.name) }
}
person.say1()    //'' this为window
person.say2()    //'tom' this为person

对象字面量增强

const bar = "bar"
const obj = {
  name: "tom",
  //属性名和变量名相同时可省略变量名
  bar,
}
console.log(obj)    //{name: "tom", bar: "bar"}

Object.assign

const obj = {a:1,b:2}
const obj1 = Object.assign({},obj)
obj1.a=11
console.log(obj)    //{a: 1, b: 2}
console.log(obj1)    //{a: 11, b: 2}

Object.is

0 == false // true
0 === false // false
+0 === -0 // true
NaN === NaN //false
Object.is(+0,-0) // false
Object.is(NaN,NaN) // true

Class 类

class Person {
  constructor (name, age) {
    this.name = name;
    this.age = age;
  }
  sayHi () {
    console.log(`hi,my name is ${this.name}`)
  } 
  //类方法，this指向的当前的类
  static create (name,age) {
    return new Person(name,age)
  }     
}
const p1 = new Person("tom",18)
//调用类方法直接创建实例对象
const p2 = Person.create("zs",19)
console.log(p1)    //Person {name: "tom", age: 18}
console.log(p2)    //Person {name: "zs", age: 19}
-------------------------------------------------------------------------------
//子类
class Student extends Person {
  constructor (name,age,number) {
    //调用父类构造方法
    super(name,age)
    this.number = number
  }
  hello () {
    //调用父类函数
    super.sayHi()
    console.log(`学号是 ${this.number}`)
  }
}
const s1 = new Student("tom",18,101)
s1.hello()    //hi,my name is tom       学号是101

set数据结构

// 创建数据结构
const s = new Set()
s.add(1).add(2).add(3).add(4).add(2)
console.log(s)     //Set(4) {1, 2, 3, 4}
//遍历
s.forEach(i => console.log(i))    //1 2 3 4
for (let i of s) {
   console.log(i)    //1 2 3 4
}
//获得集合的长度
console.log(s.size)    //4
//是否存在某个值
console.log(s.has(4))    //true
//删除某个值
console.log(s.delete(100))    //打印false， s不变
console.log(s.delete(3))    //打印true， Set(3) {1, 2, 4}
//清空集合
s.clear()    //Set(0) {}

Map 数据结构

const map = new Map()
const a = { a: 1}
const b = 123
map.set(a,100)
map.set(b,200)
//获取对应的值
console.log(map.get(a))  //100
//map.has() 是否包含某个数据
//map.delete() 删除
// map.clear() 清空
map.forEach((value,key) => {
  console.log(key,value)      //{a: 1} 100 ； 123 200
})

数组includes方法

const arr = [1,true,NaN,23,'hello']
console.log(arr.indexOf(NaN))    //-1 无法查找NaN的位置
console.log(arr.includes(NaN))    //ture

指数运算

console.log(2 ** 3)    //8  （2的3次方）