An effective way to improve performance: Maximize use of reflow and redraw functions

How to efficiently utilize reflow and redraw for performance optimization

1. Overview
In front-end development, performance optimization is a very important link. Reflow and repaint are two key factors that affect page performance. This article will introduce how to effectively utilize reflow and redraw for performance optimization, and give some specific code examples.

2. The definition and difference between reflow and repaint
Reflow and repaint are both part of the browser's work when rendering the page, but what are their specific meanings and differences? ?

Reflow, that is, recalculating the position and size of elements in the document flow and drawing them onto the page again. Reflow will cause a certain performance loss because it requires reverse calculation of the layout and geometric properties of the page. Common operations that trigger reflow include modifying the position, size, text content, and style of elements.

Redrawing refers to redrawing elements on the page, but does not involve calculation of position and size. Redrawing is relatively inexpensive because it only requires changing the element's draw color and pixels.

3. How to use reflow and redraw for performance optimization

1. Avoid frequent style changes
   When performing DOM operations, try to focus style modifications on Together, to reduce the number of reflows and redraws. Avoid frequently calling the setting method of style attributes separately, such as element.style.width = '100px'. Instead, modify the style by adding/deleting classes.

   Wrong example:

   for (let i = 0; i < elements.length; i++) {
       elements[i].style.width = '100px';
       elements[i].style.height = '100px';
       elements[i].style.color = 'red';
       // ...
   }

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   Correct example:

   for (let i = 0; i < elements.length; i++) {
       elements[i].classList.add('modified-style');
   }

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2. Using cache and batch processing
   In cases where a large number of styles need to be modified, you can Use caching and batch processing for performance optimization. First, save the style that needs to be modified in a temporary variable, then modify the element's style all at once, and finally reset the temporary variable.

   Sample code:

   const tempStyles = []; 
   for (let i = 0; i < elements.length; i++) {
       const tempStyle = {
           width: '100px',
           height: '100px',
           color: 'red',
           // ...
       };
       tempStyles.push(tempStyle);
   }
   
   // 批量修改样式
   for (let i = 0; i < elements.length; i++) {
       const element = elements[i];
       const tempStyle = tempStyles[i];
       Object.assign(element.style, tempStyle);
   }
   
   // 重置临时变量
   tempStyles.length = 0;

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3. Using virtual DOM
   Virtual DOM is a technology that uses JavaScript objects to represent elements and states on the page. By modifying and comparing the virtual DOM and then updating the real DOM in batches, the number of reflows and redraws can be reduced. Common virtual DOM libraries include React, Vue, etc.

   Sample code:

   const list = document.getElementById('list');
   const fragment = document.createDocumentFragment();
   
   for (let i = 0; i < 1000; i++) {
       const li = document.createElement('li');
       li.textContent = 'Item ' + i;
       fragment.appendChild(li);
   }
   
   list.appendChild(fragment);

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4. Summary
Reflow and redrawing have an important impact on page performance. Reasonable use of reflow and redrawing for performance optimization is the front-end An essential part of development. By avoiding frequent style changes, using caching and batch processing, and using virtual DOM and other optimization methods, the number of reflows and redraws can be effectively reduced and the rendering performance of the page can be improved. However, it should be noted that in actual development, the most appropriate optimization method should be selected according to specific scenarios to achieve performance optimization.

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