What is the principle of browser rendering in js

The principle of browser rendering in

js is to parse HTML, CSS and JS codes, build a rendering tree, perform layout and drawing, and finally display the rendering results on the screen. Detailed description: 1. When the browser receives the HTML document, it will parse it into a DOM tree; 2. The browser will parse the CSS style sheet in the HTML document and convert it into a CSS object model; 3. The browser will The DOM tree and CSSOM are merged to generate a rendering tree; 4. The browser calculates the position and size of each element on the screen based on the layout information of the rendering tree, etc.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

JavaScript (JS) is a widely used programming language, especially in web development. In browsers, JS is used to achieve dynamic web page effects, interactivity, and user experience improvements. However, to understand how JS works in the browser, we first need to understand the browser's rendering process.

The browser rendering process can be divided into the following steps:

1. Parse HTML: When the browser receives the HTML document, it will parse it into DOM (Document Object Model) Tree. The DOM tree is a tree structure composed of HTML tags and the relationships between them, which represents the structure and content of the document.

2. Parse CSS: The browser will parse the CSS style sheet in the HTML document and convert it into a CSS Object Model (CSSOM). CSSOM represents the style information of the document, including color, font, layout, etc.

3. Build a rendering tree: The browser merges the DOM tree and CSSOM to generate a rendering tree. The rendering tree only contains elements that need to be displayed on the screen. Hidden elements will not be included in the rendering tree. Each node in the render tree contains style and layout information for the element.

4. Layout (rearrangement): The browser calculates the position and size of each element on the screen based on the layout information of the rendering tree. This process is called layout or rearrangement. Layout is a computationally intensive process because it requires taking into account the size, position, and relationship of elements.

5. Drawing (redrawing): After layout is completed, the browser converts the elements in the rendering tree into pixels on the screen. This process is called painting or repainting. The drawing process involves applying the element's style to the pixels and drawing them to the screen.

6. Synthesis: After the drawing is completed, the browser synthesizes the drawn images into one or more bitmaps and then displays them on the screen. The compositing process includes operations such as merging bitmaps, handling transparency, and blending.

In this process, the execution of JS will affect the rendering. When the browser encounters JS code, it stops the rendering process, executes the JS code, and then resumes rendering. This is because JS can modify the DOM tree and CSSOM, thus affecting the construction and layout of the rendering tree.

In order to improve performance, the browser will optimize the JS code. For example, browsers will parse JS code into bytecode or machine code to improve execution speed. In addition, browsers will also use technologies such as asynchronous loading and lazy loading to separate the downloading and execution of JS code from the rendering process to avoid blocking the loading and rendering of the page.

Summary

The rendering principle of JS in the browser is to parse HTML, CSS and JS code, build a rendering tree, layout and draw, and finally render the result displayed on the screen. The execution of JS will affect the rendering, but the browser will optimize the JS code to improve performance. Understanding how JS works in the browser will help us write efficient JS code and improve the loading and rendering speed of web pages.

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