javascript function

//function of javascript

//Code 1:
'use strict'
function foo(x) {
    console.log('x= ' + x);
    for(var i = 0; i < arguments.length; i ++) {
        console.log('arguments[' + i + ']=' + arguments[i]);
    }
};
foo(1,2,3,4,5);
//Code 1 explanation:
//1. The parameters used by the method can be more or less than the specified number
//2. The 'arguments' keyword is similar to an array, used to display the input parameters

//Code 2:
'use strict'
function func1(a, b, c) {
    if (arguments.length === 2) {
        c = b;
        b = null;
    }
    console.log(a + c);
};
func1(1,2);
//Code 2 explanation:
//1. The 'arguments' keyword is used to determine the number of parameters passed in. The above //code turns parameter b into a variable parameter

//Code 3:
'use strict'
function func1(a, b, ...rest) {
    console.log(a);
    console.log(b);
    console.log(rest);
};
func1(1,2,3,4,5);
//1
//2
//[3,4,5]
func1(1);
//1
//undefined
//[]
//Code 3 explanation:
//1. The ES6 standard introduces the 'rest' keyword to get all the remaining parameters

//Code 4:
'use strict'
function sum(...rest) {
    var sum = 0;
    for (var i = 0; i < rest.length; i ++) {
        sum += rest[i];
    }
    return sum;
};
var result = sum(1,2,3,4,5);
console.log(result);
//15
//Code 4 explanation:
//1. The function of the above method sum(...rest) is to sum the input parameters

//Code 5:
'use strict'
function fun1() {
    where
    job = 'cook',
    desciption = ' cook delicious food';
    console.log(job + desciption);
};
fun1();
//Code 5 explanation:
//1. Due to the existence of variable promotion, it is best for js functions to declare all variables at the starting position

//Code 6:
where
a = 1,
b = 2;
[a, b] = [b, a];
console.log(a);
console.log(b);
//Application example 1: Swap the values ​​of variables a and b
var {hostname:domain, pathname:path} = location;
//Application example 2: Get the domain name and path of the current page
//Code 6 explanation:
//1. Application of destructuring assignment

//Code 7:
function getAge() {
    var year = new Date().getFullYear();
    return year - this.birthYear;
}
var li = {
    name: 'Li',
    birthYear: 1991,
    age: getAge
}
var ageLi = li.age ();
console.log (ageLi);
//27
ageLi = getAge ();
console.log (ageLi);
// NaN
ageLi = getAge.apply(li, []);
console.log (ageLi);
//27
//Code 7 explanation:
//1. Use the apply() method to change the point of this

//Code 8:
'use strict'
var count = 0;
var oldParseInt = parseInt;
window.parseInt = function() {
    count += 1;
    oldParseInt.apply(null, arguments);
}
for (var i = 0; i < 5; i ++) {
    parseInt('19');
}
console.log(count);
//Code 8 explanation:
//1. Replace the function 'parseInt' of the 'window' object and let it have a counting function