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CORS header proxy

Add the necessary CORS headers to a third party API response.

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This creates a repository in your GitHub account and deploys the application to Cloudflare Workers.

export default {
  async fetch(request) {
    const corsHeaders = {
      "Access-Control-Allow-Origin": "*",
      "Access-Control-Allow-Methods": "GET,HEAD,POST,OPTIONS",
      "Access-Control-Max-Age": "86400",
    };


    // The URL for the remote third party API you want to fetch from
    // but does not implement CORS
    const API_URL = "https://examples.cloudflareworkers.com/demos/demoapi";


    // The endpoint you want the CORS reverse proxy to be on
    const PROXY_ENDPOINT = "/corsproxy/";


    // The rest of this snippet for the demo page
    function rawHtmlResponse(html) {
      return new Response(html, {
        headers: {
          "content-type": "text/html;charset=UTF-8",
        },
      });
    }


    const DEMO_PAGE = `
      <!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 = () => {
          return fetch("${API_URL}").then(r => r.json())
        }
        reqs.proxy = async () => {
          let href = "${PROXY_ENDPOINT}?apiurl=${API_URL}"
          return fetch(window.location.origin + href).then(r => r.json())
        }
        reqs.proxypreflight = async () => {
          let href = "${PROXY_ENDPOINT}?apiurl=${API_URL}"
          let response = await fetch(window.location.origin + href, {
            method: "POST",
            headers: {
              "Content-Type": "application/json"
            },
            body: JSON.stringify({
              msg: "Hello world!"
            })
          })
          return 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>
    `;


    async function handleRequest(request) {
      const url = new URL(request.url);
      let apiUrl = url.searchParams.get("apiurl");


      if (apiUrl == null) {
        apiUrl = API_URL;
      }


      // Rewrite request to point to API URL. This also makes the request mutable
      // so you can add the correct Origin header to make the API server think
      // that this request is not cross-site.
      request = new Request(apiUrl, request);
      request.headers.set("Origin", new URL(apiUrl).origin);
      let response = await fetch(request);
      // Recreate the response so you 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");


      return response;
    }


    async function handleOptions(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 preflight requests.
        return new Response(null, {
          headers: {
            ...corsHeaders,
            "Access-Control-Allow-Headers": request.headers.get(
              "Access-Control-Request-Headers",
            ),
          },
        });
      } else {
        // Handle standard OPTIONS request.
        return new Response(null, {
          headers: {
            Allow: "GET, HEAD, POST, OPTIONS",
          },
        });
      }
    }


    const url = new URL(request.url);
    if (url.pathname.startsWith(PROXY_ENDPOINT)) {
      if (request.method === "OPTIONS") {
        // Handle CORS preflight requests
        return handleOptions(request);
      } else if (
        request.method === "GET" ||
        request.method === "HEAD" ||
        request.method === "POST"
      ) {
        // Handle requests to the API server
        return handleRequest(request);
      } else {
        return new Response(null, {
          status: 405,
          statusText: "Method Not Allowed",
        });
      }
    } else {
      return rawHtmlResponse(DEMO_PAGE);
    }
  },
};