When learning JavaScript, it is always difficult to understand the usage of call() and apply(). I believe many friends have the same idea as me. Now share it with your friends.

call

Syntax: call([thisObj[,arg1[, arg2[, [,.argN]]]]])
Definition: Call an object Method to replace the current object with another object.

After reading the syntax and definition, it is still difficult to understand. The call function can accept several parameters. The first parameter is the object of the current operation, and the following parameters are the parameters of the called method. So It’s still hard to understand, so go straight to the code:

function Teacher(name, age) {  //定义一个构造函数Teacher
    this.name = name;
    this.age = age;
    this.intro = function() {
        alert("My name is " + this.name + "," + "I'm " + this.age)
    }
}

function Student(name, age) {  //定义一个构造函数Student
    Teacher.call(this, name, age);  //Student调用了Teacher的方法
}
var studentA = new Student("Lily", 19); //实例化一个studentA,传入参数
studentA.intro(); //调用studentA的intro方法 输出：My name is Lily,I'm 19

 In the above example, studentA is instantiated through the constructor Student. It should inherit all the inheritable methods of the Student constructor, but we see that the Student function does not have any method, but Student called it, which can be said to have inherited Teacher's method, so studentA also has the intro() method. The tenth line of the code plays a key role Teacher.call(this, name, age). The function of this sentence is to make the current object call the method of another object. The this keyword here refers to the Student() object. After instantiation this refers to the studentA object, and the following parameters are the parameters passed in by Teacher. Maybe the above example is not very common, but we often see code like this:

var str="你好美女";
Array.prototype.join.call(str,"!");    //你!好!美!女

The join method is a unique method for arrays. It is a method defined in the prototype of the Array() constructor, so we instantiate All arrays that have been transformed inherit this method, but the string str does not have a join method, but here we borrow the join method of the array to process the string. The expression on the left side of call is the method of the borrowed object, and the first parameter object in call() borrows the method of the expression object on the left side of call (I don’t know if I can understand this, so I’ll just make do with it).

apply

The apply() method has the same function as the call method, except that the parameters passed in are different.

Syntax: apply([thisObj[,argArray]])
Definition: Apply a method of a certain object and replace the current object with another object.

The first parameter of apply is the same as call, and the second parameter is an array object (array-like can also be used). It no longer passes parameters in the form of a single parameter list, but Put all parameters into an array and pass them in together. Here we have to talk about the arguments object, which exists in every function and is an array-like object composed of all the actual parameters of the function. So the second parameter of apply is passed in the arguments object. Look at the following example:

var arr=[2,3,12,5,234,199,21,1000,2432];
var max=Math.max.apply(Math,arr);
console.log(max); //2432

There is no method to find the maximum value in the array, but there is max in the Math object to find the maximum value, so we use the apply method to let the array inherit the max() method of Math to find the array. The maximum value of The beauty of these two functions goes far beyond that. Experts are welcome to add more.

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