Detailed explanation of the implementation principle of lazy loading of images in js

Advantages of lazy loading of images:

Enhance user experience;

Optimize code;

Reduce http requests;

Reduce server-side pressure;

On-demand loading of the server;

Picture lazy loading principle:

First set the src of the image to the same image or not set it, and set a special attribute for the img tag, for example: data-src is used to store the real preview address of the image; if the image does not enter the visible area , display the same picture or not display the picture directly, no http request will occur at this time. When the picture enters the visible area, the value on data-src is assigned to src, and then the http request is sent.

Key: Determine whether the image enters the visible area (key function)

Visible area width of the page: document.body.clientWidth;

Web page Visible area height: document.body.clientHeight;

Visible area width of web page: document.body.offsetWidth (including the width of the edge); Including the width of the edge);

The full text width of the web page body: document.body.scrollWidth;

The full text height of the web page body: document.body.scrollHeight;

The web page is scrolled The height of the page: document.body.scrollTop;

The left side of the web page being scrolled: document.body.scrollLeft; Text part left: window.screenLeft;

The height of the screen resolution: window.screen.height;

The width of the screen resolution: window.screen.width;

Screen available work area height: window.screen.availHeight;

The distance of the current element relative to the top of its offsetParent element: HTMLElement.offsetTop;

The viewport height of the browser window (In pixels): window.innerHeight; (If there is a horizontal scroll bar, also includes the scroll bar height)

(Recommended tutorial:

js tutorial

)

Code implementation:

html part

<div>
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
  <img src="/static/imghwm/default1.png"  data-src="./img/1.jpg"  class="lazy"   data- alt="">
 </div>

js part

window.onload = () => {
   // 获取某节点与浏览器顶部的距离
   function getTop(e) {
    if (!e) return
    return e.offsetTop
   }
   let imgs = document.getElementsByTagName('img')
   function lazyLoading(imgs) {
    // 可是区域高度
    let innerHeight = window.innerHeight;
    // 滚动区域高度
    let scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
    for (let i = 0; i < imgs.length; i++) {
     // 图片距离顶部的距离大于可视区域和滚动区域之和时加载
     if (scrollTop + innerHeight > getTop(imgs[i])) {
      imgs[i].src = imgs[i].getAttribute(&#39;data-src&#39;)
     }
    }
   }
   lazyLoading(imgs)
   window.onscroll = () => {
    lazyLoading(imgs)
   }
  }

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