Function default parameters, arrow functions, and remaining parameters in ES6 - explanation

1. Function default parameters , When calling, the last parameter can be executed without passing the corresponding argument

function show(a=" ha-ha ",b=" Hello ") {
    console.log(a,b);
}
show(' La! ',);// La!   Hello 
function show2({x,y=0}) {
    console.log(x,y)
}
show2({x:1});//1,0

2. The parameter name of the function has been defined by default , No more let,const Repeat the definition

function show3(a=18) {
    let a=100;
    console.log(a)
}
show3();// Report errors Identifier 'a' has already been declared

3. Three points ... Extension operator 、rest Operator , Remainder operator , Can be applied to expand arrays Reset array Sort

let arr=['apple','banana','orange'];
    console.log(...arr);//apple banana orange
// Expand array 
function show4(...a) {
    console.log(a);
}
show4(1,2,3,4,5);//[1, 2, 3, 4, 5]
// Reset array 
function show4(...arr1) {
    //let arr1=Array.prototype.slice.call(arguments);
    return arr1.sort();
}
console.log(show4(10,2,111,4,50));// Sort by the first number . Non size 
// Sort 
function show5(a,b,c) {
      console.log(a,b,c);
}
show5(...[1,9,8]);//1 9 8
function show6(a,b,...c) {
      console.log(a,b);
      console.log(c)
}
show6(1,2,3,4,5);//1 2  //[3,4,5]
// Receive the remaining parameters as an array 
let arr3=[1,2,3,4,5],
    arr4=[...arr3];
    // Equate to 
let arr5=Array.from(arr3);
console.log(arr3,arr4,arr5);
// Copy the array 
let str='a-b-c';
let arr6=Array.from(str);
console.log(arr6);//["a", "-", "b", "-", "c"];
// String into an array of single characters 

4. Arrow function

function show7() {
      return 1;
}
// Equate to 
let show8=()=>1;
console.log(show7(),show8()); //1 1
let show9=(d=8,f=12)=>{
     console.log(d,f);
     return d+f;
};
show9(5,12);
console.log(show9(5,12));//17

 Format 1：()=>return Things that are , Not very practical 
 Format 2：
()=>{
 sentence ;
    return;
}

var id=10;//var  Define a variable that belongs to window
let json={
    id:1,
    show:function() {
        setTimeout(()=>{
            alert(this.id);// Who is calling this,this Whose is it? . namely this To define the object where the function is located , Not the object at runtime .
        }//1 instead of 10
        ,2000);
        alert(this.id);
    }
};
json.show();

(1) When you use the arrow function ,this Does not belong to the current function , It belongs to the scope of the currently defined object .
(2) There is no arguments, use ... solve
(3) Arrow functions cannot be constructors

let show10=(...args)=>{
    console.log(args);
};
show10(1,2,3,4,5);
let show11=()=>{
    this.name='abc';
};
let s=new show11();
alert(s.name);// Report errors 
ajax(url,(res)=>{
    // The current function does not have this Scope 
})

es5：this Who is calling this Who belongs to , Arrowhead function this Is the object where the function is defined .