函数拓展

//形参默认值
function fn(a=1){
  console.log(a)
}
fn(2)

///与解构赋值一起用
function fn({name}){
  console.log(name)
}
fn({name:"小明"})

//和扩展预算符一起使用
function fn(a,...b){
  console.log(a)//1
  console.log(b)//[2, 3, 4, 5]
}
fn(1,2,3,4,5)

%accordion% 练习 %accordion%

//形参默认值
function fn(a,b=1){
  console.log(a+b)
}
fn(1) //2

%/accordion%

箭头函数

1、单行的时候可以不加大括号

let fn = ()=> console.log("hello world")
console.log()

2、单行使用的时候可以直接返回值，不需要加return

let fn = ()=> "hello world"
console.log(fn())

3、无参数写法

//没有参数的时候可以使用括号也可以使用下划线
let fn = _ => "hello world"
console.log(fn())

%accordion% 练习 %accordion%

//有参数
let fn = () => 1
console.log(fn()); //1

// 无参数
let fn = _ => 1
console.log(fn());//1

%/accordion%

4、绑定this指针

//使用箭头函数的时候，无法修改this指针
function fn(){
  console.log(this)//obj
}
let fn = ()=>{
  console.log(this)//window
}

let obj = {
  name:"小明"
}

fn.call(obj)

%accordion% 练习 %accordion%

let obj = { name: "张三" }
function fn() {
    console.log(this);
}
fn.call(obj)//{name: "张三"}


let obj = { name: "张三" }
let fn = () => {
    console.log(this);
    //Window {parent: Window, opener: null, top: Window, length: 0, frames: Window, …}
}
fn.call(obj)

%/accordion%

5、无法当成构造函数使用

let Person = ()=>{

}
new Person()//报错

6、箭头函数没有arguments

let Person = ()=>{
  console.log(arguments)//报错
}
Person()

%accordion% 练习 %accordion%

// 错误法
let fn = (name) => {
    this.name = name
}
let f = new fn("张三")
console.log(f);


let fn = () => {
    console.log(arguments);
}
fn(1, 2, 3)

%/accordion%