function foo() {
    console.log(this)
}
const obj = {
    a: 2,
    foo: function(){console.log(this)}
}
foo()  //this指向为window
obj.foo() //this指向为对象obj
let foo2 = obj.foo
foo2()  //this指向为window

let foo = ()=> {
    console.log(this)
}
const obj = {
    a: 2,
    foo: function () {
        return ()=>{console.log(this)}
    }
}
foo()  //this指向window
obj.foo()()  //this指向对象obj
let foo2 = obj.foo()
foo2()  //this指向对象obj
箭头函数是匿名函数，不能作为构造函数，不能使用new

function foo() {
    console.log(this)
}
new foo()  //window
let foo2 = ()=>{
    console.log(this)
}
new foo2() //报错 Uncaught TypeError: foo2 is not a constructor

function foo() {
    console.log(arguments)
}
foo(1,2,3)  //[1,2,3]
let foo2 = ()=>{
    console.log(arguments)
}
foo2(1,2,3)  // 报错 Uncaught ReferenceError: arguments is not defined

let foo2 = (...arguments)=>{
    console.log(arguments)
}
foo2(1,2,3) //[1,2,3]

var a = 1
var obj={
    a:2
}
function foo() {
    console.log(this.a)
}
foo()  //1
foo.call(obj) //2，输出obj.a
let foo2 = ()=>{
    console.log(this.a)
}
foo2()  //1
foo2.call(obj) //1，仍然输出window.a

var a = ()=>{  
    return 1;
}
function b(){  
    return 2;
}
console.log(a.prototype);//undefined
console.log(b.prototype);//object{...}