search
HomeWeb Front-endJS Tutorial(Common interview questions) js basics from setTimeout to asynchronous principle of js

These are some very basic but easily overlooked issues. Everyone knows how to use this method but has not studied in depth what the process is. Without further ado, look at the code below

 <input type="text" value="a" name="input" onkeydown="alert(this.value)" />
<input type="text" value="a" name="input" onkeydown="var me=this;setTimeout(function(){alert(me.value)},0)" >

When one keydown is performed, the original value in the input pops up, but when the second keydown is performed, the updated value pops up. This is because setTimeout, although its delay is set to 0, is triggered almost immediately. But it is still added to the back of the execution queue, but for this asynchronous process, the rendering has been completed. When the callback function is executed, the output is already the updated value. Another problem here is that this indicates different objects inside and outside different functions. When there is a function in the function, pay more attention to this. It is easy to make mistakes. Just be careful and don't say much.

Next, js js is single-threaded. It is conceivable that if there is no multi-threading, the entire program will be stuck. Fortunately, the browser is multi-threaded, and the browser makes js asynchronous. Some attributes of js: js is a single-threaded language. The browser only assigns one main thread to js to execute tasks (functions), but only one task can be executed at a time. These tasks form a task queue waiting to be executed, but certain tasks on the front end Some tasks are very time-consuming, such as network requests, timers and event monitoring. If they are queued up to wait for execution like other tasks, the execution efficiency will be very low and even cause the page to freeze. Therefore, the browser has opened up additional threads for these time-consuming tasks, mainly including http request threads, browser timing triggers, and browser event trigger threads. These tasks are asynchronous.

The browser opens a separate thread for asynchronous tasks such as network requests. So the question is, how does the main thread know after these asynchronous tasks are completed? The answer is the callback function. The entire program is event-driven. Each event will be bound to the corresponding callback function. For example, there is a piece of code that sets a timer.

setTimeout(function(){
    console.log(time is out);
},500);

When this code is executed, browse The server performs timing operations asynchronously. When 500ms arrives, a timing event will be triggered. At this time, the callback function will be placed in the task queue. The entire program is driven by such events.
So, js has always been single-threaded, and the browser is the key to achieving asynchronous

The following is transferred from the Internet:

js has been doing a job, which is to retrieve data from the task queue Extract the task and put it into the main thread for execution. Let’s have a deeper understanding below.
event loop
The picture comes from Philip Roberts' speech "Help, I'm stuck in an event-loop" is very profound!
Let’s map the concept we just learned to the picture. The threads that the browser opens for asynchronous tasks mentioned above can be collectively understood as WebAPIs. The task queue mentioned above is the callback queue. What we call The main thread is the part composed of dotted lines. The heap and stack together form the js main thread. The execution of functions is achieved by pushing and popping the stack. For example, there is a foo() function in the picture. The main thread pushes it into the stack. When executing the function body, it finds that the above functions still need to be executed, so it pushes these functions into the stack. When the function is executed, the function is popped off the stack. When the stack is cleared, it means that a task has been executed. At this time, the next task will be searched from the callback queue and pushed into the stack (this search process is called event loop, because it always loops to find whether there is a task in the task queue. There are also tasks).

Related articles:

Detailed explanation of js asynchronous file loader, detailed explanation of js asynchronous loading

How does native JS implement asynchronous requests to implement Ajax

Related videos:

JS#jQuery width and height understanding and application

The above is the detailed content of (Common interview questions) js basics from setTimeout to asynchronous principle of js. For more information, please follow other related articles on the PHP Chinese website!

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Replace String Characters in JavaScriptReplace String Characters in JavaScriptMar 11, 2025 am 12:07 AM

Detailed explanation of JavaScript string replacement method and FAQ This article will explore two ways to replace string characters in JavaScript: internal JavaScript code and internal HTML for web pages. Replace string inside JavaScript code The most direct way is to use the replace() method: str = str.replace("find","replace"); This method replaces only the first match. To replace all matches, use a regular expression and add the global flag g: str = str.replace(/fi

Custom Google Search API Setup TutorialCustom Google Search API Setup TutorialMar 04, 2025 am 01:06 AM

This tutorial shows you how to integrate a custom Google Search API into your blog or website, offering a more refined search experience than standard WordPress theme search functions. It's surprisingly easy! You'll be able to restrict searches to y

Build Your Own AJAX Web ApplicationsBuild Your Own AJAX Web ApplicationsMar 09, 2025 am 12:11 AM

So here you are, ready to learn all about this thing called AJAX. But, what exactly is it? The term AJAX refers to a loose grouping of technologies that are used to create dynamic, interactive web content. The term AJAX, originally coined by Jesse J

Example Colors JSON FileExample Colors JSON FileMar 03, 2025 am 12:35 AM

This article series was rewritten in mid 2017 with up-to-date information and fresh examples. In this JSON example, we will look at how we can store simple values in a file using JSON format. Using the key-value pair notation, we can store any kind

8 Stunning jQuery Page Layout Plugins8 Stunning jQuery Page Layout PluginsMar 06, 2025 am 12:48 AM

Leverage jQuery for Effortless Web Page Layouts: 8 Essential Plugins jQuery simplifies web page layout significantly. This article highlights eight powerful jQuery plugins that streamline the process, particularly useful for manual website creation

What is 'this' in JavaScript?What is 'this' in JavaScript?Mar 04, 2025 am 01:15 AM

Core points This in JavaScript usually refers to an object that "owns" the method, but it depends on how the function is called. When there is no current object, this refers to the global object. In a web browser, it is represented by window. When calling a function, this maintains the global object; but when calling an object constructor or any of its methods, this refers to an instance of the object. You can change the context of this using methods such as call(), apply(), and bind(). These methods call the function using the given this value and parameters. JavaScript is an excellent programming language. A few years ago, this sentence was

Improve Your jQuery Knowledge with the Source ViewerImprove Your jQuery Knowledge with the Source ViewerMar 05, 2025 am 12:54 AM

jQuery is a great JavaScript framework. However, as with any library, sometimes it’s necessary to get under the hood to discover what’s going on. Perhaps it’s because you’re tracing a bug or are just curious about how jQuery achieves a particular UI

10 Mobile Cheat Sheets for Mobile Development10 Mobile Cheat Sheets for Mobile DevelopmentMar 05, 2025 am 12:43 AM

This post compiles helpful cheat sheets, reference guides, quick recipes, and code snippets for Android, Blackberry, and iPhone app development. No developer should be without them! Touch Gesture Reference Guide (PDF) A valuable resource for desig

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
2 weeks agoBy尊渡假赌尊渡假赌尊渡假赌
Repo: How To Revive Teammates
1 months agoBy尊渡假赌尊渡假赌尊渡假赌
Hello Kitty Island Adventure: How To Get Giant Seeds
4 weeks agoBy尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Safe Exam Browser

Safe Exam Browser

Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

SublimeText3 Linux new version

SublimeText3 Linux new version

SublimeText3 Linux latest version

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)