ES6 syntax extension

Writing time ：2022 year 7 month 27 Japan

                        ES6  Grammar extension 

The remaining parameters ：

Recognize the remaining parameters ：
const add=(x,y,z,…arg)=>{};
Essence of residual parameters ===> The remaining parameters are always an array , Even if there is no value, it is an empty array
const add = (x, y, …arg) => {
console.log(x, y, arg);
}
add(2, 3, 5, 6);// x=2 y=3 arg=[5,6]

The default values of the remaining parameters
1. The remaining parameters of the arrow function
The parameter part of the arrow function, even if there is only one remaining parameter , You can't omit parentheses ：
const add=(…arg)=>{}

2. Use the remaining parameters instead of arguments Get actual parameters :
const add = function () {
// arguments Is a class array object
console.log(arguments);
}
add(1, 2);
const add = (…arg) => {
// Not in arrow function arguments object
// You can use the remaining parameters arg Instead of arguments
console.log(arg);// arg Is an array
}
add(1, 2);

3. The location of the remaining parameters
The remaining parameter can only be the last parameter , After that, there can be no other parameters , Otherwise, an error will be reported :
const add=(x,y,z,…arg)=>{};

Application of remaining parameters :

1. complete add function
const add = (…arg) => {
let sum = 0;
for (let i = 0; i < array.length; i++) {
sum += arg[i];
}
return sum;
}
add(1, 2, 4, 4, 6, 7, 6)
2. In combination with deconstruction assignment
The remaining parameters do not have to be used as parameters of the function
// Must be the last parameter
const [num, …args] = [2, 3, 5, 6, 7];
//const […args,num ] = [2, 3, 5, 6, 7];// Report errors
console.log(num, args);//args = [3,5,6,7]
// The remaining elements z Combine deconstruction objects z It can also be an object
const { x, y, …z } = { x: 3, a: ‘a’, b: ‘b’, y: 2 };
console.log(z);//z={a: “a”,b: “b”}
// Deconstruction of function parameters
const func = ({ x, y, …z }) => {
console.log(z);//z={a: “a”,b: “b”}
}
func({ x: 3, a: ‘a’, b: ‘b’, y: 2 })