The This usage in JavaScript

This keyword is very common and important in JavaScript. So what does this mean?

Summarize:

1.this represents an internal object that is automatically generated when the function is run, and can only be used inside the function;

2.this always points to the object that called the function;

Here are four scenarios to discuss the use of this in detail

One: purely function calls

This is the most commonly used method of a function, which is a global call, where this represents the Globals object window

    function Test () {        this. x =1;        Alert (this. x);        Console.log (this); //      }    Test ();

And here's the case:

        var x = 2;         function Test () {            this. x =1;            Alert (this. x);            Console.log (this); //          }        Test ();        Console.log (x); // 1    

You can prove that this in the function here refers to window.

Second: Invocation as an object method

A function can also act as a method call to an object, at which point this is the ancestor object.

        function Test () {            Console.log (this. x);   1            console.log (this);   o        }        var o = {};         = 1;         = test;        O.M ();    

Three: called as a constructor function

The so-called constructor is to generate a new object from this function. This will point to this new object

        function Test (name) {            this. Name= name;            Console.log (this); // Test {name: "hehe"}         }        varnew Test ("hehe");    

Four: Apply Call

Apply () is a method of a function object that changes the calling object of a function, and its first parameter is the object that is called after the function is changed.

This is worth the value of the first parameter, and if the argument is null, the default value is the global object

        varx = 0; functionTest () {Console.log ( This); Console.log ( This. x); }        varo = {}; O.x= 2; O.M=test; O.M ();//this.x-->2O.m.apply ();//this.x-->0        //Results:        //Object {X:2}        //2        //Window {...}        //0

　　

The This usage in JavaScript