Analysis of common situations of JavaScript this keyword pointing

In the previous experience of writing code, I often try to write this, and I don't know where to point. Today, I looked at the javascript design pattern I explored, which specifically talked about the objects that this refers to in different situations, which is very interesting.

The situation referred to by this

this has the following four situations:

1. Object. Formal call of function: object. function ();

2. Formal call of ordinary function: function ();

3. Constructor call;

4. Function. prototype. call or Function. prototype. apply call;

1. Object. Formal call of function

Object. When called as a function, refers to this object.

var obj = {
  num:1,
  getNum:function(){
    alert(this.num);// Output 1
  }
}
obj.getNum();

2. Call in the form of ordinary function

When a normal function is called, this points to a global object.

window.num = 2;
var obj = {
  num:1,
  getNum:function(){
    alert(this.num);
  }
}
obj.getNum();// Eject 1
var fun1 = obj.getNum;
fun1();  // Eject 2

Amazing, call the function fun () directly; Calling functions xxx. fun () through objects; this refers to different objects.

3. Constructor call

In order to use the new keyword, the creation object of javascript can be defined as follows.

var person = function(){
this. name = 'Liu Bei';
}

In this case, this refers to the object being created.

var p = new person();
alert (p. name); //Eject Liu Bei

4. Function. prototype. call or Function. prototype. apply call

var obj1 = {
  name:' Guan Yu ',
  getName:function(){
    return this.name;
  }
}
var obj2 = {
  name:' Liu Bei '
}
alert(obj1.getName());  // Guan Yu 
alert(obj1.getName.call(obj2));  // Liu Bei