CORS header proxy

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

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);    }  },
};

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);    }  },
} satisfies ExportedHandler;