What does this in js mean?
This is always used in JS. Today I will talk to you about what this is that can easily confuse people and how to use this.
This points to 1, ordinary function points to the caller of the function: a simple way is to see if there is a dot in front of the function. If there is a dot, then point to the value in front of the dot; 2. The arrow function points to the domain where the function is located: Pay attention to understanding the scope. Only the {} of the function constitutes the scope. The {} of the object and if(){} do not constitute the scope;
const obj = { name: 'objName', say() { console.log(this.name); }, read: () => { console.log(this.name); } } obj.say(); // objName obj.read(); // undefined
Ordinary function, the caller is obj, so the result is objname; also understand that say() is an ordinary function, with a dot in front, so this points to obj;
arrow Function, this points to the scope where the function is located. The current scope is the global environment, so this.name is undefined.
Take the following example to understand more clearly where the arrow function this points, where the arrow function is located The scope is the ordinary function say, and the caller of say() is obj
const obj = { say: function () { setTimeout(() => { console.log(this) }); } }
obj.say(); // obj, at this time this refers to the definition of him obj
function test() { console.log(this); }test( ); //The default this for the global environment in window
is {}, and the default this for ordinary functions is globalconsole.log(this); // {}
function test() {
console.log(this);
}
test(); //global
Come on two questions to check whether you have mastered it
example1
const length = 10; function fn() { console.log(this.length); } const obj = { length: 5, method: function(fn) { fn(); arguments[0](); } }; obj.method(fn, 1);
Output 10, 2
I just saw this question at the beginning I was also confused about the question, and now I finally understand it. The method function passes in two parameters, one parameter is fn(), fn() is an ordinary function, and this points to the caller of the function, which at this time points to the whole world (you can also See that there is no dot in front of this function), so the running result is 10. arguments are all parameters of the function, which is an object of class
array, arguments[0] (), which can be regarded as arguments.0() , what calls this function is arguments, at this time this refers to arguments, this.length refers to angument.length, which is the total number of parameters passed in 2
Note: The running result of the above example in the node environment is undefined 2, const length = 10 is changed to global.length = 10; because the
defined globally in the node environment will not be bound to global, and the browser It will also be automatically bound to the global environment windowWhat will be the result if it is changed to the following?
const length = 10; function fn() { console.log(this.length); } const obj = { length: 5, method: function(fn) { fn(); const fun = arguments[0]; fun(); } }; obj.method(fn, 1); 10, 10 example 2 window.val = 1; var obj = { val: 2, dbl: function() { this.val *= 2 val *= 2; console.log(val); console.log(this.val); } } obj.dbl(); // 2 4 var func = obj.dbl; func(); // 8 8
This is a bit roundabout, but step by step It is easy to understand when analyzing:
obj.dbl(); When executing this line of code, this refers to obj, so this.val === obj.val*=2, and the final result is 4 ,val*=2 === window.val *= 2, the final result is 2
func(), when executing this line of code, func() does not have any prefix, this refers to window.func( ); So at this time the value of this is window, this.val === window.val *= 2, at this time window.val is 4, val*=2 === window.val *2, the final result is 8, finally console.log(this.val), and console.log(val) both refer to window.val.
The above is my little understanding of this. If you have any questions about this article of mine, please feel free to communicate with me.
Related recommendations:
How to understand JavaScript object-oriented basics and this pointing issuesDetailed explanation of the difference between this and event in JSIntroduction to this, apply, call, and bind in JSThe above is the detailed content of How to use this in js. For more information, please follow other related articles on the PHP Chinese website!

The shift from C/C to JavaScript requires adapting to dynamic typing, garbage collection and asynchronous programming. 1) C/C is a statically typed language that requires manual memory management, while JavaScript is dynamically typed and garbage collection is automatically processed. 2) C/C needs to be compiled into machine code, while JavaScript is an interpreted language. 3) JavaScript introduces concepts such as closures, prototype chains and Promise, which enhances flexibility and asynchronous programming capabilities.

Different JavaScript engines have different effects when parsing and executing JavaScript code, because the implementation principles and optimization strategies of each engine differ. 1. Lexical analysis: convert source code into lexical unit. 2. Grammar analysis: Generate an abstract syntax tree. 3. Optimization and compilation: Generate machine code through the JIT compiler. 4. Execute: Run the machine code. V8 engine optimizes through instant compilation and hidden class, SpiderMonkey uses a type inference system, resulting in different performance performance on the same code.

JavaScript's applications in the real world include server-side programming, mobile application development and Internet of Things control: 1. Server-side programming is realized through Node.js, suitable for high concurrent request processing. 2. Mobile application development is carried out through ReactNative and supports cross-platform deployment. 3. Used for IoT device control through Johnny-Five library, suitable for hardware interaction.

I built a functional multi-tenant SaaS application (an EdTech app) with your everyday tech tool and you can do the same. First, what’s a multi-tenant SaaS application? Multi-tenant SaaS applications let you serve multiple customers from a sing

This article demonstrates frontend integration with a backend secured by Permit, building a functional EdTech SaaS application using Next.js. The frontend fetches user permissions to control UI visibility and ensures API requests adhere to role-base

JavaScript is the core language of modern web development and is widely used for its diversity and flexibility. 1) Front-end development: build dynamic web pages and single-page applications through DOM operations and modern frameworks (such as React, Vue.js, Angular). 2) Server-side development: Node.js uses a non-blocking I/O model to handle high concurrency and real-time applications. 3) Mobile and desktop application development: cross-platform development is realized through ReactNative and Electron to improve development efficiency.

The latest trends in JavaScript include the rise of TypeScript, the popularity of modern frameworks and libraries, and the application of WebAssembly. Future prospects cover more powerful type systems, the development of server-side JavaScript, the expansion of artificial intelligence and machine learning, and the potential of IoT and edge computing.

JavaScript is the cornerstone of modern web development, and its main functions include event-driven programming, dynamic content generation and asynchronous programming. 1) Event-driven programming allows web pages to change dynamically according to user operations. 2) Dynamic content generation allows page content to be adjusted according to conditions. 3) Asynchronous programming ensures that the user interface is not blocked. JavaScript is widely used in web interaction, single-page application and server-side development, greatly improving the flexibility of user experience and cross-platform development.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

SublimeText3 Linux new version
SublimeText3 Linux latest version

EditPlus Chinese cracked version
Small size, syntax highlighting, does not support code prompt function

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Notepad++7.3.1
Easy-to-use and free code editor