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CORS Header Proxy


Add necessary CORS headers to a third party API response

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) 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') return response } 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': '*', 'Access-Control-Allow-Methods': 'GET, HEAD, POST, OPTIONS', 'Access-Control-Allow-Headers': 'Content-Type', } // 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>`