Usage of this in JavaScript

This is a keyword in the JavaScript language.

It represents an internal object that is automatically generated when the function is run, and can only be used inside a function. Like what

function test () {
    
this.x = 1;
    
}

The value of this will change as the function is used differently. But there is a general principle, and that is this refers to the object that invokes the function.

Here are four different scenarios for a detailed discussion of this usage.

Case one: pure function call

This is the most common use of a function, which is a global call, so this represents the globally object.

Take a look at the code below and it runs 1.

function test () {
    
this.x = 1;
    
alert (this.x);
    
}
    
Test (); 1

To prove that this is the global object, I make some changes to the code:

var x = 1;
    
function test () {
    
alert (this.x);
    
}
    
Test (); 1

The results of the operation are still 1. Change again:

var x = 1;
    
function test () {
    
this.x = 0;
    
}
    
Test ();
    
alert (x); 0

Case two: Call as an object method

A function can also be invoked as a method of an object, at which point this is the ancestor object.

function test () {
    
alert (this.x);
    
}
    
var o = {};
    
o.x = 1;
    
O.M = test;
    
O.M (); 1

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