9 application scenarios and three composite application scenarios of this in JavaScript_javascript skills

【Scenario 1】this in the global environment points to the global object

this.a = 10;
alert(a);//10
b = 20;
alert(this.b);//20
var c = 30;
alert(this.c);//30

【Scenario 2】This of the object’s internal function points to the current object that calls the function

var a = 10;
var bar = {
 a: 20,
 test: function(){
  alert(this.a);
 }
}
bar.test();//20

[Scenario 3] This of the global environment function points to the global object

var a = 10;
function foo(){
 alert(this.a);
}
foo();//10

【Scenario 4】this in the anonymous function points to the global object

var a = 10;
var foo = {
 a: 20,
 fn: (function(){
  alert(this.a);
 })()
}
foo.fn//10

[Scenario 5] This in setInterval and setTimeout timers points to the global object

var a = 10;
var oTimer1 = setInterval(function(){
 var a = 20;
 alert(this.a);//10
 clearInterval(oTimer1);
},100);

[Scenario 6] this in eval points to this in the calling context

(function(){
 eval("alert(this)");//[object Window]
})();
function Foo(){
 this.bar = function(){
  eval("alert(this)");//[object Object]
 }
}
var foo = new Foo();
foo.bar();

[Scenario 7] This in the constructor points to the constructed new object

function Person(name,age){
 this.name = name;
 this.age = age;
 this.sayName = function(){
  alert(this.name);
 }
}
var p1 = new Person('lily','20');
p1.sayName();//'lily'

[Scenario 8] this in new Function points to the global object

(function(){
 var f = new Function("alert(this)");
 f();//[object Window]
})();
function Foo(){
 this.bar = function(){
  var f = new Function("alert(this)");
  f();//[object Window]
 }
}
var foo = new Foo();
foo.bar();

[Scenario 9] this in apply and call points to the object in the parameter

var a = 10;
var foo = {
 a: 20,
 fn: function(){
  alert(this.a);
 }
};
var bar ={
 a: 30
}
foo.fn.apply();//10(若参数为空，默认指向全局对象)
foo.fn.apply(foo);//20
foo.fn.apply(bar);//30

【Composite Scene 1】

var someone = {
 name: "Bob",
 showName: function(){
  alert(this.name);
 }
};
var other = {
 name: "Tom",
 showName: someone.showName
}
other.showName();　　//Tom

//以上函数相当于

var other = {
 name: "Tom",
 showName: function(){
  alert(this.name);
 }
}
other.showName();　　//Tom

【Composite Scene 2】

var name = 2;
var a = {
 name: 3,
 fn: (function(){
  alert(this.name);
 })(),
 fn1:function(){
  alert(this.name);
 }
}
a.fn;//2[匿名函数中的this指向全局对象]
a.fn1();//3[对象内部函数的this指向调用函数的当前对象]

【Composite Scene 3】

var name = "Bob"; 
var nameObj ={ 
 name : "Tom", 
 showName : function(){ 
 alert(this.name); 
}, 
 waitShowName : function(){
  var that = this;
  setTimeout(function(){
   that.showName();
  }, 1000);
 }
}; 
nameObj.waitShowName();//"Tom"[that=this改变this的指向，使this从指向全局变量变化到指向nameObj]

var name = "Bob"; 
var nameObj ={ 
 name : "Tom", 
 showName : function(){ 
  alert(this.name); 
 }, 
 waitShowName : function(){
  var that = this;//that指向nameObj
  setTimeout(function(){
   (function(){ 
    alert(this.name);
   })();
  }, 1000);
 }
}; 
nameObj.waitShowName();// 'Bob'[形成匿名函数，this指向全局变量] 

This article introduces 9 application scenarios in this to you. I hope it will be helpful to you. This site will be updated with new content every day. Please continue to pay attention to this site. Thank you.