This article will introduce you to the event loop in node.js, look at the event loop model, the various stages of the event loop, and share two interview questions about the event loop. Let’s take a look. Look!
Event Loop Model
The JavaScript engine itself does not implement the event loop mechanism. The event loop in the browser is implemented by the browser, and the event loop in NodeJS is mainly implemented by the underlying Libuv library.
The Libuv library is event-driven, which encapsulates and unifies API implementations on different platforms. NodeJS's cross-platform and event loop mechanisms are implemented through the Libuv library. [Recommended learning: "nodejs Tutorial"]
The above is divided into four steps:
1. NodeJS's V8 engine parses JavaScript code, and then call the Node API
2. NodeAPI allocates tasks to the Libuv library for processing.
3. Libuv manages the execution of these tasks through the event loop mechanism, and returns the results to V8 engine.
4. V8 engine callback processing Libuv returns
Micro tasks and macro tasks
Divide different task queues according to task types, micro task queue and macro task queue.
Macro tasks: setTimeout, setInterval, setImmediate
Micro tasks: promise.then(), promise.catch(), process.nextTick()
Events Each stage of the cycle
- timers stage: execute all setTimeout and setInterval callbacks
- pendding callback stage: callback for certain system operations. Such as TCP connection errors
- poll phase: polling to wait for new links and requests and other events, and execute I/O callbacks. For example: file reading operation
- check phase: execute setImmedidate callback
- close callback phase: close callback execution, such as socket.on( close...)
Each stage will execute the task queue of the current stage, and then execute the microtask queue of the current stage. Only after all microtasks have been executed will the entry Next stage.
Differences in event loops in browsers:
There are no multiple stages in the browser; priority levels: synchronization code, micro tasks, macro tasks
In the browser, there may be multiple queues for macro tasks, but there is only one micro task. Each time, the micro task will be executed first and then the macro task
Statement summary
Mainly learned the NodeJS event loop model, parsing the JS code from the V8 engine, and then calling NodeAPI. NodeAPI assigns tasks to the underlying Libuv library for execution. The main event loop mechanisms are all controlled by the Libuv library. Completed.
The entire event loop is divided into 6 stages: timer stage, pending callback stage, poll stage, check stage, close callback stage. The order of execution is: after the task queue in each stage is executed, the microprocessor is executed. Task, the next stage will be executed only after the microtask execution is completed. The browser's event loop is not divided into stages, and the order of priority is: synchronization code, microtasks, and macrotasks.
Interview questions
Question 1: What is your understanding of the event loop of NodeJS?
Answer: My understanding of the event loop is that Node is single-threaded. The main thread puts all tasks in the circular queue, and then the underlying libuv library takes out the tasks from the circular event queue and assigns them to Different threads handle it, and the main thread also performs callback processing at the same time, and the entire process forms an event loop.
Differences from the browser event loop:
1. The browser does not divide into multiple stages
2. Browser task priority: synchronization code, micro tasks, macros Task
Question 2: What is the order of task execution in the event queue?
Answer: The NodeJS event loop is divided into multiple stages. The execution sequence is: after one stage is completed, the microtasks are executed, and then the next stage is executed.
For more programming-related knowledge, please visit: Introduction to Programming! !
The above is the detailed content of Understand the event loop in node.js in one article. For more information, please follow other related articles on the PHP Chinese website!

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.


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

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.

Dreamweaver CS6
Visual web development tools

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

SublimeText3 Linux new version
SublimeText3 Linux latest version

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