Master the performance bottlenecks of reflow and redraw: methods to optimize page performance

Improve page performance: To understand the performance bottlenecks of reflow and redraw, specific code examples are required

Overview:
When developing web applications, page performance is a Very important consideration. A high-performance web page not only provides a better experience for users, but also improves search engine rankings. To improve page performance, it is critical to understand the performance bottlenecks of reflow and redrawing.

Reflow and redraw refer to the process by which the browser calculates and renders the page based on CSS styles. Reflow refers to the process by which the browser completes all calculations and re-lays out the page, while redraw refers to the process by which the browser redraws the page according to the new style. Frequent occurrences of reflows and redraws will lead to reduced page performance, so we need to avoid this situation as much as possible.

Performance bottlenecks of reflow and redraw:

1. Change the size and position of elements: When we change the size and position of an element, the browser needs to recalculate and adjust other elements layout, triggering reflow and redraw. In this case, we should try to avoid changing the size and position of the element frequently.
2. Modify the content of an element: When we modify the text content of an element or insert or delete the node of an element, the browser also needs to recalculate and draw the page, triggering reflow and redrawing. Therefore, when modifying element content, we should try to reduce the number of operations on the DOM. We can consider batch updates or using document fragments to operate.
3. Adjust the style of elements: Changing the style of elements, such as changing the background color, font size, etc., will also cause reflow and redrawing. In order to avoid this situation, we can change the style by switching the class name instead of directly modifying the style of the element.

Code examples:
The following are some common code examples that easily lead to reflow and redrawing:

1. Frequently modify element styles:

const element = document.getElementById('myElement');
for (let i = 0; i < 1000; i++) {
  element.style.width = '100px';
  element.style.height = '100px';
  // ...其他样式修改
}

Improvement method:

const element = document.getElementById('myElement');
element.classList.add('myClassName');

2. Frequently modify element content:

const element = document.getElementById('myElement');
for (let i = 0; i < 1000; i++) {
  element.innerHTML += '<div>' + i + '</div>';
}

Improvement method:

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

3. Get element position information:

const element = document.getElementById('myElement');
const width = element.offsetWidth;
const height = element.offsetHeight;
// ...

Improvement method: Minimize the number of times to obtain element position information.

Conclusion:
In actual development, we need to optimize page performance, avoid triggering frequent reflows and redraws, and improve user experience and web page performance. By understanding the performance bottlenecks of reflow and redraw, we can optimize specific code fragments to reduce unnecessary calculation work and improve code running efficiency. Especially in scenarios involving a large number of DOM operations, proper use of the optimization methods in the code examples can significantly improve page performance.

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