What is the usage of this in javascript

Usage of this: 1. In pure function calls, this is used to represent the global object; 2. When the function is called as an object method, this is used to represent the superior object; 3. When called as a constructor, this Used to represent new objects; 4. When apply is called, this is used to represent the first parameter of the apply() function.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

This is a keyword in the JavaScript language.

It is an object automatically generated inside the function body when the function is running, and can only be used inside the function body.

function test() {
　this.x = 1;
}

In the above code, when the function test is running, there will automatically be a this object available internally.

So, what is the value of this?

This has different values ​​in different usage scenarios of the function. In general, this is the environment object in which the function runs. The following is a detailed discussion of the usage of this in four situations.

Scenario 1: Pure function call

This is the most common usage of functions and is a global call, so this represents the global object. Please look at the following code, its running result is 1.

var x = 1;
function test() {
   console.log(this.x);
}
test();  // 1

Case 2: Calling as an object method

The function can also be called as a method of an object, in which case this refers to the superior object.

function test() {
  console.log(this.x);
}
var obj = {};
obj.x = 1;
obj.m = test;
obj.m(); // 1

Case 3 Calling as a constructor

The so-called constructor function is a function through which a new object can be generated. At this time, this refers to this new object.

function test() {
　this.x = 1;
}
var obj = new test();
obj.x // 1

The running result is 1. In order to show that this is not a global object at this time, we make some changes to the code:

var x = 2;
function test() {
  this.x = 1;
}
var obj = new test();
x  // 2

The running result is 2, indicating that the value of global variable x has not changed at all.

Case four apply call

apply() is a method of the function, and its function is to change the calling object of the function. Its first parameter represents the changed object that calls this function. Therefore, this refers to the first parameter at this time.

var x = 0;
function test() {
　console.log(this.x);
}
var obj = {};
obj.x = 1;
obj.m = test;
obj.m.apply() // 0

When the parameter of apply() is empty, the global object is called by default. Therefore, the running result at this time is 0, proving that this refers to the global object.

If you modify the last line of code to

obj.m.apply(obj); //1

, the running result becomes 1, which proves that this represents the object obj.

[Recommended learning: javascript advanced tutorial]

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