Detailed explanation of this pointing issue in JavaScript strict mode

In addition to the normal running mode, ECMAscript 5 adds a second running mode: "strict mode". The following article mainly introduces to you some relevant information about this in JavaScript strict mode. Friends who need it can refer to it. Let’s take a look together.

Preface

#I believe that many people have been confused by this in JavaScript when they were learning or using Javascript. , then this article will summarize the several directions of this in strict mode.

1. This in the global scope

#In strict mode, in the global scope, this points to the window object

 "use strict";
 
 console.log("严格模式");
 console.log("在全局作用域中的this");
 console.log("this.document === document",this.document === document);
 console.log("this === window",this === window);
 this.a = 9804;
 console.log('this.a === window.a===',window.a);

2. This

in the function in the global scope In strict mode, this in this function is equal to undefined

 "use strict";
 
 console.log("严格模式");
 console.log('在全局作用域中函数中的this');
 function f1(){
 console.log(this);
 }
 
 function f2(){
 function f3(){
 console.log(this);
 }
 f3();
 }
 f1();
 f2();

## 3. This# in the function (method) of the object
##In strict mode, this in the object's function points to the object instance of the calling function

 "use strict";
 
 console.log("严格模式");
 console.log("在对象的函数中的this");
 var o = new Object();
 o.a = 'o.a';
 o.f5 = function(){
 return this.a;
 }
 console.log(o.f5());

4. This in the constructor
In strict mode, this in the constructor points to the object instance created by the constructor.

 "use strict";
 
 console.log("严格模式");
 console.log("构造函数中的this");
 function constru(){
 this.a = 'constru.a';
 this.f2 = function(){
 console.log(this.b);
 return this.a;
 }
 }
 var o2 = new constru();
 o2.b = 'o2.b';
 console.log(o2.f2());

5. This
in the event processing function is strictly In mode, in the event handling function, this points to the target object that triggered the event.

 "use strict";
 
 function blue_it(e){
 if(this === e.target){
 this.style.backgroundColor = "#00f";
 }
 }
 var elements = document.getElementsByTagName('*');
 for(var i=0 ; i<elements.length ; i++){
 elements[i].onclick = blue_it;
 }
 
 //这段代码的作用是使被单击的元素背景色变为蓝色

6. This in the inline event handler function
In strict mode, in the inline event handler function , there are the following two situations:

 <button onclick="alert((function(){&#39;use strict&#39;; return this})());">
 内联事件处理1
 </button>
 <!-- 警告窗口中的字符为undefined -->
 
 <button onclick="&#39;use strict&#39;; alert(this.tagName.toLowerCase());">
 内联事件处理2
 </button>
 <!-- 警告窗口中的字符为button -->

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