The problem of this direction in javasript and the method of changing this direction

Wahaha a 2020-11-09 22:18:56
problem direction javasript method changing

I'm learning javascript We tend to be this The direction of the problem made dizzy turn , Let's learn today this Direction problem of , And change this The way to point .

One .this Direction problem of

I'm learning this We need to understand two things before we point to the problem :

1:this Always point to an object ;
2:this The direction of the function depends entirely on the location of the function call ;

We can understand the first point , Because in javascript Everything in is the object . The second point is actually easy to understand , When the position of function call is different ,this It's not the same thing , So you could say this Can be changed dynamically , Let's start with a simple example this The direction of is changing

function fun(){
var change={
var name ='world'
var result=change.f()//hello

Through the above example, we can see clearly that this Changes in the direction of , Because there's a function in the object change Inside , therefore this It's the object outside the function that points to , So output hello.

After reading the above examples, we will have a good understanding of this Dynamic point to switch has a certain understanding of .


So next , We are right. this Make a summary of the most frequently used situations , The most common basic is the following 3 Kind of :

Methods in objects , Event binding , Constructors , Timer

There's no need to talk about the first two , Let's take a look at the timer this Point to the problem ,

var obj = {
this ;
setInterval(,1000); // this Point to window object 
setInterval('',1000); // this Point to obj object 

setInterval()  yes window Object , Take two parameters , The first parameter is allowed to be a function or an executable section JS Code , The second parameter is the interval between the execution of the previous function or code ;

In the code above ,setInterval(,1000)  The first parameter of obj Object's fun , because JS Functions in can be passed by reference as values , In fact, the address of this function is passed as an argument to  setInterval  Method , In other words  setInterval  The first parameter of takes a function , So at this time 1000 Milliseconds later , The function is already running in window Under the object , That is, the caller of the function has become window object , So one of them this It points to the overall situation window object ;

And in the  setInterval('',1000)  First parameter in , Actually, it's the incoming executable segment JS Code ;1000 Millisecond later when JS When the engine comes to execute this code , It is through  obj  Object to find  fun  Function and call to execute , So the environment of the function is still object  obj  Inside , So inside the function this It points to  obj  object ;

In addition to these, we need to understand three things that can change this Function pointed to , Including the arrow function ,call(),apply()

Arrow function : There's an official explanation , One of the reasons for the introduction of the arrow function , It's not bound this; In the arrow function , Arrowhead function this Set as a closed lexical environment , let me put it another way , In the arrow function this Depending on the environment in which the function was created .

var globalObject = this;
var foo = (() => this);
console.log(foo() === globalObject); // true

// And then the code above
// Call as a method of an object
var obj = {foo: foo};
console.log( === globalObject); // true
// Try to use call To set this
console.log( === globalObject); // true
// Try to use bind To set this
foo = foo.bind(obj);
console.log(foo() === globalObject); // true

in any case ,foo  Of  this  Is set to the environment in which it was created ( In the example above , It's the global object ). The same applies to arrow functions created within other functions : These arrow functions are this Set as a closed lexical environment .

// Create a containing bar Methodical obj object ,
// bar Returns a function ,
// This function returns this,
// The returned function is created with the arrow function ,
// So it's this Is permanently bound to its outer function this.
// bar The value of can be set in the call , This in turn sets the value of the return function .
var obj = {
bar: function() {
var x = (() => this);
return x;
// As obj Object to call bar, It's this Bound to the obj.
// Assign the reference of the returned function to fn.
var fn =;
// Call directly fn It is not set this,
// Usually ( That is, without using the arrow function ) Default to global object 
// In strict mode, it is undefined
console.log(fn() === obj); // true
// But notice , If you just quote obj Methods ,
// Instead of calling it 
var fn2 =;
// So after calling the arrow function ,this Point to window, Because it comes from bar Inherited this.
console.log(fn2()() == window); // true
call and apply Method : Take an object as call perhaps apply The first parameter of ,this Will be bound to this parameter object 
var obj = {parent:' male '};
var parent = '28';
function child(obj){
child(); // 28; // male 
child.apply(obj); // male 


本文为[Wahaha a]所创,转载请带上原文链接,感谢

  1. 【计算机网络 12(1),尚学堂马士兵Java视频教程
  2. 【程序猿历程,史上最全的Java面试题集锦在这里
  3. 【程序猿历程(1),Javaweb视频教程百度云
  4. Notes on MySQL 45 lectures (1-7)
  5. [computer network 12 (1), Shang Xuetang Ma soldier java video tutorial
  6. The most complete collection of Java interview questions in history is here
  7. [process of program ape (1), JavaWeb video tutorial, baidu cloud
  8. Notes on MySQL 45 lectures (1-7)
  9. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  10. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  11. 精进 Spring Boot 03:Spring Boot 的配置文件和配置管理,以及用三种方式读取配置文件
  12. Refined spring boot 03: spring boot configuration files and configuration management, and reading configuration files in three ways
  13. 【递归,Java传智播客笔记
  14. [recursion, Java intelligence podcast notes
  15. [adhere to painting for 386 days] the beginning of spring of 24 solar terms
  16. K8S系列第八篇(Service、EndPoints以及高可用kubeadm部署)
  17. K8s Series Part 8 (service, endpoints and high availability kubeadm deployment)
  18. 【重识 HTML (3),350道Java面试真题分享
  19. 【重识 HTML (2),Java并发编程必会的多线程你竟然还不会
  20. 【重识 HTML (1),二本Java小菜鸟4面字节跳动被秒成渣渣
  21. [re recognize HTML (3) and share 350 real Java interview questions
  22. [re recognize HTML (2). Multithreading is a must for Java Concurrent Programming. How dare you not
  23. [re recognize HTML (1), two Java rookies' 4-sided bytes beat and become slag in seconds
  24. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  25. RPC 1: how to develop RPC framework from scratch
  26. 造轮子系列之RPC 1:如何从零开始开发RPC框架
  27. RPC 1: how to develop RPC framework from scratch
  28. 一次性捋清楚吧,对乱糟糟的,Spring事务扩展机制
  29. 一文彻底弄懂如何选择抽象类还是接口,连续四年百度Java岗必问面试题
  30. Redis常用命令
  31. 一双拖鞋引发的血案,狂神说Java系列笔记
  32. 一、mysql基础安装
  33. 一位程序员的独白:尽管我一生坎坷,Java框架面试基础
  34. Clear it all at once. For the messy, spring transaction extension mechanism
  35. A thorough understanding of how to choose abstract classes or interfaces, baidu Java post must ask interview questions for four consecutive years
  36. Redis common commands
  37. A pair of slippers triggered the murder, crazy God said java series notes
  38. 1、 MySQL basic installation
  39. Monologue of a programmer: despite my ups and downs in my life, Java framework is the foundation of interview
  40. 【大厂面试】三面三问Spring循环依赖,请一定要把这篇看完(建议收藏)
  41. 一线互联网企业中,springboot入门项目
  42. 一篇文带你入门SSM框架Spring开发,帮你快速拿Offer
  43. 【面试资料】Java全集、微服务、大数据、数据结构与算法、机器学习知识最全总结,283页pdf
  44. 【leetcode刷题】24.数组中重复的数字——Java版
  45. 【leetcode刷题】23.对称二叉树——Java版
  46. 【leetcode刷题】22.二叉树的中序遍历——Java版
  47. 【leetcode刷题】21.三数之和——Java版
  48. 【leetcode刷题】20.最长回文子串——Java版
  49. 【leetcode刷题】19.回文链表——Java版
  50. 【leetcode刷题】18.反转链表——Java版
  51. 【leetcode刷题】17.相交链表——Java&python版
  52. 【leetcode刷题】16.环形链表——Java版
  53. 【leetcode刷题】15.汉明距离——Java版
  54. 【leetcode刷题】14.找到所有数组中消失的数字——Java版
  55. 【leetcode刷题】13.比特位计数——Java版
  56. oracle控制用户权限命令
  57. 三年Java开发,继阿里,鲁班二期Java架构师
  58. Oracle必须要启动的服务
  59. 万字长文!深入剖析HashMap,Java基础笔试题大全带答案
  60. 一问Kafka就心慌?我却凭着这份,图灵学院vip课程百度云