Coping with performance bottlenecks: redrawing and reflow solutions for front-end engineers

Redraw and reflow decryption: How front-end engineers deal with performance bottlenecks

Introduction:
With the rapid development of the Internet, the role of front-end engineers is becoming more and more important . They need to handle the design and development of user interfaces while also focusing on optimizing website performance. In front-end performance optimization, redrawing and reflow are common performance bottlenecks. This article will introduce the principles of redrawing and reflow in detail, and provide some practical code examples to help front-end engineers deal with performance bottlenecks.

1. What is redrawing and reflow

1. Repaint: When the appearance of an element changes but does not affect its layout, repaint will be triggered. Redrawing usually occurs when CSS properties change, such as color, border, etc. Redrawing only redraws the visible part of the page and does not affect the layout and position of other elements.
2. Reflow: When the layout of an element changes, reflow will be triggered. Reflow recalculates the position and size of elements and rebuilds the render tree. Reflow will affect the layout of the entire page and may cause other elements to be reflowed and redrawn.

2. Reasons for redrawing and reflow

1. DOM operations: When we manipulate DOM elements, such as inserting, deleting or modifying elements, redrawing and reflow will be triggered . Therefore, frequent DOM operations can lead to performance degradation.
2. CSS style change: When we modify the CSS style of an element, such as changing color, size, etc., it will cause redrawing and reflow. Therefore, CSS styles need to be used carefully to avoid unnecessary performance losses.
3. Trigger certain properties and methods: When we call some properties and methods that need to be rendered, such as offsetLeft, clientHeight, etc., redrawing and reflow will be triggered.

3. How to optimize redrawing and reflow

1. Reduce DOM operations: avoid frequent DOM operations and merge multiple operations into one operation as much as possible. Reduce the number of redraws and reflows.
   Sample code:

   let container = document.getElementById('container');
   let fragment = document.createDocumentFragment();
   for(let i = 0; i < 1000; i++) {
     let div = document.createElement('div');
     fragment.appendChild(div);
   }
   container.appendChild(fragment);

2. Batch modification styles: Try to use CSS class names to modify the styles of elements in batches, and avoid directly modifying the styles of individual elements to reduce redrawing and reflow. frequency.
   Sample code:

   let elements = document.getElementsByClassName('box');
   for(let i = 0; i < elements.length; i++) {
     elements[i].classList.add('highlight');
   }

3. Use DocumentFragment to cache DOM operations: Put DOM operations into DocumentFragment, and then insert them into the document at one time, which can reduce the cost of redrawing and reflowing frequency.
   Sample code:

   let container = document.getElementById('container');
   let fragment = document.createDocumentFragment();
   for(let i = 0; i < 1000; i++) {
     let div = document.createElement('div');
     fragment.appendChild(div);
   }
   container.appendChild(fragment);

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