Quickly master javascript's apply() and call() methods

1. What is apply() and call()

    <1.apply() and call() are two functions that come with the function, and they have the same function

    <2. Function: Change this in the method. Example: say.call(little monster), point this in the method say() to the object 'little monster'

2. When to use apply() and call()

In js object-oriented programming, we often define it like this:

function cat() {
}
cat.prototype = {
    food : 'jerry',
    say  : function () {
        console.log('I like '+ this.food);
    }
};
let cat_tom = new cat();
cat_tom.say();             // I like jerry

If: there is an object mouse = {food:'tom'}
and we don't want to redefine the say method for its mouse , then we can use the say method of cat_tom through call or apply , as follows:

let mouse = {food:'tom'};
cat_tom.say.call(mouse);     //I like tom
cat_tom.say.apply(mouse);    //I like tom

So, it can be seen that call and apply are for Appears by dynamically changing this ;

When an object does not have a method, but other There are objects , we can use the methods of other objects to operate with call or apply.

3. The only difference between apply() and call()

• apply() packs the parameters into an Array and passes it in;
• call() passes parameters in order .

For example, to call Math.max(1, 2, 3), use apply() and call() respectively to achieve the following:

Math.max.apply(null, [1, 2, 3]);  // 3
Math.max.call(null, 1, 2, 3);     // 3