Improve web page performance: reduce the stress of reflowing and redrawing

Optimize web page performance: reduce the burden caused by reflow and redrawing, specific code examples are required

In the current era of rapid development of the Internet, website performance is crucial to user experience and It is very important for website ranking. Users expect to be able to see content immediately when they open a website, rather than waiting for the loading process. Therefore, optimizing web page performance has become one of the goals that every web developer should pursue.

Optimizing web page performance can start from many aspects, among which reducing reflow and repaint operations is crucial to improving web page performance. Reflow and redraw are basic operations when browsers render web pages, but their frequent occurrence will cause web page rendering to slow down, thus affecting user experience. This article explores how to reduce reflow and redraw operations and provides specific code examples.

The concept of reflow and redraw
Reflow (reflow) refers to when the browser renders the web page, it calculates the layout of the web page based on the size and position of the DOM elements, and redraws it to the screen. process. Repaint refers to the process of redrawing elements according to the style of DOM elements. Reflow and redraw operations are time-consuming operations, so we should try to avoid their frequent occurrence.

How to reduce reflow and redraw operations

1. Use the transform and opacity properties of CSS3
   When changing the position and size of elements, you can use the transform property of CSS3 instead of Directly modify the left, top, width and height properties of the element. Because the transform attribute will only cause redrawing, not reflow. Similarly, you can use the opacity attribute to modify the transparency of an element, which will only cause a redraw.

// Example: Use transform to replace the left and top attributes
// Not recommended:
element.style.left = '100px';
element.style.top = '100px';

// Recommended:
element.style.transform = 'translate(100px, 100px)';

2. Batch operation of DOM elements
   when needed When making modifications to multiple DOM elements, avoid using multiple individual operations and instead combine them into a single batch operation. Because each operation on the DOM will trigger reflow and redraw operations, batch operations can reduce the number of reflow and redraw operations and improve performance.

// Example: Batch operation of DOM elements
// Not recommended:
element1.style.width = '100px';
element2.style.width = '200px' ;
// Each operation triggers a reflow and redraw

// Recommended:
element1.style.width = '100px';
element2.style.width = '200px ';
// One operation only triggers one reflow and redraw

3. Avoid frequent access to layout information
   Accessing the layout information of DOM elements (such as offsetLeft, offsetHeight, etc.) through JavaScript will trigger The browser performs a reflow operation. Therefore, try to avoid frequent access to layout information. You can cache the layout information to avoid repeated calculations.

// Example: Avoid frequent access to layout information
// Not recommended:
const height = element.offsetHeight;
// Accessing offsetHeight will trigger a reflow operation

// Recommendation:
const height = element.offsetHeight;
// Cache layout information to avoid repeated calculations

4. Use DocumentFragment
   Dynamically using JavaScript When creating a large number of DOM elements, you can use DocumentFragment to reduce reflow and redraw operations. DocumentFragment is a lightweight document fragment that can be operated offline and finally inserted into the document to reduce the browser's rendering burden.

// Example: Using DocumentFragment
// Not recommended:
for (let i = 0; i const element = document.createElement ('div');
document.body.appendChild(element);
}
// Each time an element is inserted, reflow and redraw will be triggered

// Recommended:
const fragment = document.createDocumentFragment();
for (let i = 0; i const element = document.createElement('div');
fragment.appendChild( element);
}
document.body.appendChild(fragment);
// Insert all elements at once, triggering only one reflow and redraw

Summary
By reducing reflow and redraw operations, we can greatly improve web page performance and enhance user experience. This article describes several ways to reduce reflow and redraw operations, and provides specific code examples. I hope these methods will help you optimize web page performance. Remember, think about whether every operation will cause reflow and redraw. By optimizing the code and reducing unnecessary operations, your web page will be smoother and faster.

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