前言

cdn.jsdelivr.net 有时候会出现国内无法访问的情况，以至于造成网站 js, css, image，字体等文件无法正常显示。
因此 BestTools 大神开发出自动检查 cdn.jsdelivr.net 是否可用的脚本， 如果不可用时，会自动把所有资源地址切换到其他可用的域名。比如：gcore.jsdelivr.net，fastly.jsdelivr.net等其他 CDN。

项目地址

Github 地址：https://github.com/PipecraftNet/jsdelivr-auto-fallback

使用方法

1. 直接复制 index.js 或 index.min.js 里的内容，加到网站里。强烈建议添加到 head 标签最上面。
2. 所有 script 标签加上 defer 属性。如果原来有 async 属性，可以跳过。这个可以避免 pending 状态带来的等待时间，大大提升性能。
3. 如果是 hexo 生成的网站，可以安装 hexo-filter-jsdelivr-auto-fallback 插件，自动添加。

• 示例：

  # 可以把 js 文件放到其他目录下进行引用
  <script defer src="index.js"></script>
  <script defer src="index.min.js"></script>

• index.js 代码：

  ((document) => {
  'use strict';
  let fastNode;
  let failed;
  let isRunning;
  const DEST_LIST = [
  'cdn.jsdelivr.net',
  'fastly.jsdelivr.net',
  'gcore.jsdelivr.net',
  'cdn.zenless.top',
  'testingcf.jsdelivr.net',
  'test1.jsdelivr.net'
  ];
  const PREFIX = '//';
  const SOURCE = DEST_LIST[0];
  const starTime = Date.now();
  const TIMEOUT = 2000;
  const STORE_KEY = 'jsdelivr-auto-fallback';
  const TEST_PATH = '/gh/PipecraftNet/jsdelivr-auto-fallback@main/empty.css?';
  const shouldReplace = (text) => text && text.includes(PREFIX + SOURCE);
  const replace = (text) => text.replace(PREFIX + SOURCE, PREFIX + fastNode);
  const setTimeout = window.setTimeout;
  const $ = document.querySelectorAll.bind(document);
  
  const replaceElementSrc = () => {
  let element;
  let value;
  for (element of $('link[rel="stylesheet"]')) {
    value = element.href;
    if (shouldReplace(value) && !value.includes(TEST_PATH)) {
      element.href = replace(value);
    }
  }
  
  for (element of $('script')) {
    value = element.src;
    if (shouldReplace(value)) {
      const newNode = document.createElement('script');
      newNode.src = replace(value);
      element.defer = true;
      element.src = '';
      element.before(newNode);
      element.remove();
    }
  }
  
  for (element of $('img')) {
    value = element.src;
    if (shouldReplace(value)) {
      // Used to cancel loading. Without this line it will remain pending status.
      element.src = '';
      element.src = replace(value);
    }
  }
  
  // All elements that have a style attribute
  for (element of $('*[style]')) {
    value = element.getAttribute('style');
    if (shouldReplace(value)) {
      element.setAttribute('style', replace(value));
    }
  }
  
  for (element of $('style')) {
    value = element.innerHTML;
    if (shouldReplace(value)) {
      element.innerHTML = replace(value);
    }
  }
  };
  
  const tryReplace = () => {
  if (!isRunning && failed && fastNode) {
    console.warn(SOURCE + ' is not available. Use ' + fastNode);
    isRunning = true;
    setTimeout(replaceElementSrc, 0);
    // Some need to wait for a while
    setTimeout(replaceElementSrc, 20);
    // Replace dynamically added elements
    setInterval(replaceElementSrc, 500);
  }
  };
  
  const checkAvailable = (url, callback) => {
  let timeoutId;
  const newNode = document.createElement('link');
  const handleResult = (isSuccess) => {
    if (!timeoutId) {
      return;
    }
  
    clearTimeout(timeoutId);
    timeoutId = 0;
    // Used to cancel loading. Without this line it will remain pending status.
    if (!isSuccess) newNode.href = 'data:text/plain;base64,';
    newNode.remove();
    callback(isSuccess);
  };
  
  timeoutId = setTimeout(handleResult, TIMEOUT);
  
  newNode.addEventListener('error', () => handleResult(false));
  newNode.addEventListener('load', () => handleResult(true));
  newNode.rel = 'stylesheet';
  newNode.text = 'text/css';
  newNode.href = url + TEST_PATH + starTime;
  document.head.insertAdjacentElement('afterbegin', newNode);
  };
  
  const cached = (() => {
  try {
    return Object.assign(
      {},
      JSON.parse(localStorage.getItem(STORE_KEY) || '{}')
    );
  } catch {
    return {};
  }
  })();
  
  const main = () => {
  cached.time = starTime;
  cached.failed = false;
  cached.fastNode = null;
  
  for (const url of DEST_LIST) {
    checkAvailable('https://' + url, (isAvailable) => {
      // console.log(url, Date.now() - starTime, Boolean(isAvailable));
      if (!isAvailable && url === SOURCE) {
        failed = true;
        cached.failed = true;
      }
  
      if (isAvailable && !fastNode) {
        fastNode = url;
      }
  
      if (isAvailable && !cached.fastNode) {
        cached.fastNode = url;
      }
  
      tryReplace();
    });
  }
  
  setTimeout(() => {
    // If all domains are timeout
    if (failed && !fastNode) {
      fastNode = DEST_LIST[1];
      tryReplace();
    }
  
    localStorage.setItem(STORE_KEY, JSON.stringify(cached));
  }, TIMEOUT + 100);
  };
  
  if (
  cached.time &&
  starTime - cached.time < 60 * 60 * 1000 &&
  cached.failed &&
  cached.fastNode
  ) {
  failed = true;
  fastNode = cached.fastNode;
  tryReplace();
  setTimeout(main, 1000);
  } else {
  main();
  }
  })(document);

• index.min.js 代码：

  (n=>{"use strict";let r,s,e;const l=["cdn.jsdelivr.net","fastly.jsdelivr.net","gcore.jsdelivr.net","cdn.zenless.top","testingcf.jsdelivr.net","test1.jsdelivr.net"],t="//",a=l[0],i=Date.now(),o=2e3,c="jsdelivr-auto-fallback",f="/gh/PipecraftNet/jsdelivr-auto-fallback@main/empty.css?",d=e=>e&&e.includes(t+a),m=e=>e.replace(t+a,t+r),u=window.setTimeout,v=n.querySelectorAll.bind(n),g=()=>{let e,t;for(e of v('link[rel="stylesheet"]'))t=e.href,d(t)&&!t.includes(f)&&(e.href=m(t));for(e of v("script"))if(t=e.src,d(t)){const r=n.createElement("script");r.src=m(t),e.defer=!0,e.src="",e.before(r),e.remove()}for(e of v("img"))t=e.src,d(t)&&(e.src="",e.src=m(t));for(e of v("*[style]"))t=e.getAttribute("style"),d(t)&&e.setAttribute("style",m(t));for(e of v("style"))t=e.innerHTML,d(t)&&(e.innerHTML=m(t))},y=()=>{!e&&s&&r&&(console.warn(a+" is not available. Use "+r),e=!0,u(g,0),u(g,20),setInterval(g,500))},b=(()=>{try{return Object.assign({},JSON.parse(localStorage.getItem(c)||"{}"))}catch{return{}}})();var h=()=>{b.time=i,b.failed=!1,b.fastNode=null;for(const t of l)((e,t)=>{let r;const s=n.createElement("link"),l=e=>{r&&(clearTimeout(r),r=0,e||(s.href="data:text/plain;base64,"),s.remove(),t(e))};r=u(l,o),s.addEventListener("error",()=>l(!1)),s.addEventListener("load",()=>l(!0)),s.rel="stylesheet",s.text="text/css",s.href=e+f+i,n.head.insertAdjacentElement("afterbegin",s)})("https://"+t,e=>{e||t!==a||(s=!0,b.failed=!0),e&&!r&&(r=t),e&&!b.fastNode&&(b.fastNode=t),y()});u(()=>{s&&!r&&(r=l[1],y()),localStorage.setItem(c,JSON.stringify(b))},o+100)};b.time&&i-b.time<36e5&&b.failed&&b.fastNode?(s=!0,r=b.fastNode,y(),u(h,1e3)):h()})(document);

用户脚本

作为用户，你也可以使用油猴脚本将网站中的 cdn.jsdelivr.net 替换为可以访问的域名。

1. 浏览器安装 Tampermonkey。
2. 安装脚本: https://greasyfork.org/zh-CN/scripts/445701-jsdelivr-auto-fallback

jsdelivr 可用节点比较

参考文章

• https://www.neosey.com/archives/55.html
• https://iui.su/167/