前言

最近做一个玩的东西需要用到天气API,便从今天头条首页抓了一个想自己用。https://www.toutiao.com/stream/widget/local_weather/data/,可当我高高兴兴的引入JQuery并打算发起请求

<script type="text/javascript" src="https://cdn.staticfile.org/jquery/3.4.1/jquery.min.js"></script>
  <script type="text/javascript">
    $.get(
      'https://www.toutiao.com/stream/widget/local_weather/data/',
      function(data){
        alert(data);
      },'json'
    )
  </script>

但是无论如何都获取不到数据,打开F12看到如下报错:

Access to XMLHttpRequest at 'https://www.toutiao.com/stream/widget/local_weather/data/' from origin 'null' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

开始

官方定义,CORS (Cross-Origin Resource Sharing,跨域资源共享)是一个系统,它由一系列传输的HTTP头组成,这些HTTP头决定浏览器是否阻止前端 JavaScript 代码获取跨域请求的响应。同源安全策略 默认阻止“跨域”获取资源。但是 CORS 给了web服务器这样的权限,即服务器可以选择,允许跨域请求访问到它们的资源。通俗一点来说呢,就是浏览器有权决定是否阻止网页上的JavaScript从不同域名下调取数据的行为,但是你也可以通过服务器返回的HTTP头部来决定浏览器不去阻止此请求。

所以上面我调用头条API的行为就被浏览器阻止了,因为头条的服务器并没有设置一个Access-Control-Allow-Origin来允许我调用(没设置头部的话,同域名是正常使用的)。解决的办法很简单,就是让头条把API开放给大家用,显然是不现实的。以我目前的能力可以有两个方案来解决此问题:

  • 使用动态语言来获取数据后再返回

    该部分不在本篇内容重点,以后一定写!

  • 转发此请求,为我所用

    如果使用此方案的话,比较常见的是使用nginx来处理。不过对于帅7(贫穷)的我来说,那也太枯燥(费钱)了。

    正好我在CloudFlare的Worker模板中看到了一个正对我需求的内容,所以就以此来理解CORS跨域吧!这篇在<https://developers.cloudflare.com/workers/templates/pages/cors_header_proxy/>,其中用到的脚本在这里<https://github.com/ashleygwilliams/template-registry/blob/master/templates/javascript/cors_header_proxy.js>这个脚本主要功能是为了解决跨域问题,并且写了转发请求和原始请求的对比,我直接写注释以阐述我的理解与想法。(废话较多,需要的同学可以直接划过看下面的总结)

    async function handleRequest(request) {
      const url = new URL(request.url)
      const apiurl = url.searchParams.get('apiurl')
      // Rewrite request to point to API url. This also makes the request mutable
      // so we can add the correct Origin header to make the API server think
      // that this request isn't cross-site.
      request = new Request(apiurl, request)
      request.headers.set('Origin', new URL(apiurl).origin)
        //为请求添加一个Origin属性,保证原始请求可以正常发送。
      let response = await fetch(request)
      // Recreate the response so we can modify the headers
      response = new Response(response.body, response)
      // Set CORS headers
      response.headers.set('Access-Control-Allow-Origin', url.origin)
      // Append to/Add Vary header so browser will cache response correctly
      response.headers.append('Vary', 'Origin')
        //设置响应头中的Access-Control-Allow-Origin为此次访问本次浏览器请求URL的origin,并说明此属性将跟随Vary变化
      return response
    }
    // 所以这部分我们可以总结到:
    //Access-Control-Allow-Origin 正是控制请求的关键参数,如果我们需要对不同域名请求做出变化的话,还需要使用Vary参数告知浏览器。
    function handleOptions(request) {
      // Make sure the necesssary headers are present
      // for this to be a valid pre-flight request
      if (
        request.headers.get('Origin') !== null &&
        request.headers.get('Access-Control-Request-Method') !== null &&
        request.headers.get('Access-Control-Request-Headers') !== null
      ) {
        // Handle CORS pre-flight request.
        // If you want to check the requested method + headers
        // you can do that here.
        return new Response(null, {
          headers: corsHeaders,
        })
      } else {
        // Handle standard OPTIONS request.
        // If you want to allow other HTTP Methods, you can do that here.
        return new Response(null, {
          headers: {
            Allow: 'GET, HEAD, POST, OPTIONS',
          },
        })
      }
    }
    addEventListener('fetch', event => {
      const request = event.request
      const url = new URL(request.url)
      if (url.pathname.startsWith(proxyEndpoint)) {
        if (request.method === 'OPTIONS') {
          // Handle CORS preflight requests
          event.respondWith(handleOptions(request))
        } else if (
          request.method === 'GET' ||
          request.method === 'HEAD' ||
          request.method === 'POST'
        ) {
          // Handle requests to the API server
          event.respondWith(handleRequest(request))
        } else {
          event.respondWith(async () => {
            return new Response(null, {
              status: 405,
              statusText: 'Method Not Allowed',
            })
          })
        }
      } else {
        // Serve demo page
        event.respondWith(rawHtmlResponse(demoPage))
      }
    })
    // We support the GET, POST, HEAD, and OPTIONS methods from any origin,
    // and accept the Content-Type header on requests. These headers must be
    // present on all responses to all CORS requests. In practice, this means
    // all responses to OPTIONS requests.
    const corsHeaders = {
      'Access-Control-Allow-Origin': '*',
        //允许所有域名发起请求
      'Access-Control-Allow-Methods': 'GET, HEAD, POST, OPTIONS',
        //允许的请求方式
      'Access-Control-Allow-Headers': 'Content-Type',
        //允许的头部,比如post发送Json数据,或者需要`authorization`头部时候会用到
    }
    // The URL for the remote third party API you want to fetch from
    // but does not implement CORS
    const apiurl = 'https://workers-tooling.cf/demos/demoapi'
    // The endpoint you want the CORS reverse proxy to be on
    const proxyEndpoint = '/corsproxy/'
    // The rest of this snippet for the demo page
    async function rawHtmlResponse(html) {
      return new Response(html, {
        headers: {
          'content-type': 'text/html;charset=UTF-8',
        },
      })
    }
    const demoPage = `
    <!DOCTYPE html>
    <html>
    <body>
      <h1>API GET without CORS Proxy</h1>
      <a target='_blank' href='https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch#Checking_that_the_fetch_was_successful'>Shows TypeError: Failed to fetch since CORS is misconfigured</a>
      <p id='noproxy-status'/>
      <code id='noproxy'>Waiting</code>
      <h1>API GET with CORS Proxy</h1>
      <p id='proxy-status'/>
      <code id='proxy'>Waiting</code>
      <h1>API POST with CORS Proxy + Preflight</h1>
      <p id='proxypreflight-status'/>
      <code id='proxypreflight'>Waiting</code>
      <script>
      let reqs = {};
      reqs.noproxy = async () => {
        let response = await fetch('${apiurl}')
        return await response.json()
      }
      reqs.proxy = async () => {
        let response = await fetch(window.location.origin + '${proxyEndpoint}?apiurl=${apiurl}')
        return await response.json()
      }
      reqs.proxypreflight = async () => {
        const reqBody = {
          msg: "Hello world!"
        }
        let response = await fetch(window.location.origin + '${proxyEndpoint}?apiurl=${apiurl}', {
          method: "POST",
          headers: {
            "Content-Type": "application/json"
          },
          body: JSON.stringify(reqBody),
        })
        return await response.json()
      }
      (async () => {
        for (const [reqName, req] of Object.entries(reqs)) {
          try {
            let data = await req()
            document.getElementById(reqName).innerHTML = JSON.stringify(data)
          } catch (e) {
            document.getElementById(reqName).innerHTML = e
          }
        }
      })()
      </script>
    </body>
    </html>`
    

    总结一下以上用到的参数:

    请求端:

    Origin:请求中用来标示源的字段

    Access-Control-Request-Method:Preflight request(预请求)中标示本次请求方式的字段

    Access-Control-Request-Headers:Preflight request(预请求)中标示本次请求头部的字段

    响应端:

    Access-Control-Allow-Origin:响应中标示允许源的字段

    Vary:响应中标示此次请求响应是以何种方式判别(好像很拗口),就是用来告诉浏览器如何控制缓存的啦。如果本次请求返回’Vary: Origin’,说明响应是根据源来响应的,下次同源的请求就可以使用上次的缓存了。

    Access-Control-Allow-Methods:响应中标示允许的请求方式

    Access-Control-Allow-Headers:响应中标示允许的头部

    所以当我们遇到跨域问题,就可以去检查响应端的这些参数,是否对请求的类型允许。

文档

[]: https://developer.mozilla.org/zh-CN/docs/Glossary/CORS “CORS 术语解释”
[]: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Access_control_CORS “CORS 详解”
[]: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/Vary “Vary参数 详解”
[]: https://developer.mozilla.org/zh-CN/docs/Glossary/Preflight_request “预加载 术语解释”
[]: https://developer.mozilla.org/zh-CN/docs/Web/HTTP/Headers/Access-Control-Request-Method “Access-Control-Request-Method参数 详解”