Cloudflare Workers CORS Proxy (supports websockets)

cf-worker.js

// We support the GET, POST, HEAD, and OPTIONS methods from any origin,
// and allow any header on requests. These headers must be present
// on all responses to all CORS preflight 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-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://SERVICE_NAME.datahub.figment.io"
const WS_API_URL = "wss://WEBSOCKET_SERVICE_NAME.datahub.figment.io"
const API_AUTH_KEY = "YOUR_API_KEY_HERE"

// List all of the domains here that you want to be able to access this proxy
const ALLOWED_DOMAINS = [
  'my.example.com'
]

/**
 * Receives a HTTP request, proxies the request, and returns the response. If the request is a websocket requests,
 * it hands the request off to a separate handler for creating a websocket proxy.
 * @param {Request} request
 * @returns {Promise<Response>}
 */
async function handleRequest(request) {
  const { url, headers } = request
  const { host, pathname } = new URL(url)
  const dhURL = API_URL + pathname
  const request_origin = headers.get("Origin")

  const origin = request_origin ? new URL(request_origin) : { host: headers.get('Host') }

  if (ALLOWED_DOMAINS.includes(origin.host)){
    let response

    // Websocket requests are identified with an "Upgrade:websocket" HTTP header
    const upgradeHeader = request.headers.get("Upgrade")
    if (upgradeHeader && upgradeHeader === "websocket") {
      const dataHubWebsocketURL = WS_API_URL + pathname
      response = await handleWebsocketRequest(dataHubWebsocketURL)
    } else {
      dataHubRequest = new Request(dhURL, request)
      dataHubRequest.headers.set("Authorization", API_AUTH_KEY);
      dataHubRequest.headers.set("Origin", new URL(dhURL).origin)
      response = await fetch(dataHubRequest)
      
      // 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", headers.get("Origin"))

    // Append to/Add Vary header so browser will cache response correctly
    response.headers.append("Vary", "Origin")

    return response
  }
  else {
    return new Response("Not Found for " + host, { status: 404 })
  }
}

/**
 * Receives a HTTP request and replies with a websocket proxy
 * @param {Request} request
 * @returns {Promise<Response>}
 */
 async function handleWebsocketRequest(dataHubWebsocketURL) {
  // Establish the websocket connection to DataHub
  const dataHubResponse = await fetch(dataHubWebsocketURL, { headers: { "Upgrade": "websocket", "Authorization": API_AUTH_KEY } })
  if (dataHubResponse.status !== 101) {
    return new Response(null, {
      status: dataHubResponse.status,
      statusText: dataHubResponse.statusText
    })
  }
  const dataHubSocket = dataHubResponse.webSocket
  dataHubSocket.accept()

  // Create a client/server to act as the proxy layer
  const proxyWebSocketPair = new WebSocketPair()
  const [client, server] = Object.values(proxyWebSocketPair)

  // tell the Workers runtime that it should listen for WebSocket data and keep the connection open with client
  server.accept()

  // Any messages from the client are forwarded to the DataHub socket
  server.addEventListener("message", event => {
    dataHubSocket.send(event.data)
  })

  // Any messages coming from DataHub are forwarded back to the client
  dataHubSocket.addEventListener("message", event => {
    server.send(event.data)
  })

  const response = new Response(null, {
    status: 101,
    webSocket: client
  })
  
  return response
}

/**
 * Responds with an uncaught error.
 * @param {Error} error
 * @returns {Response}
 */
function handleError(error) {
  console.error('Uncaught error:', error)

  const { stack } = error
  return new Response(stack || error, {
    status: 500,
    headers: {
      'Content-Type': 'text/plain;charset=UTF-8'
    }
  })
}

function handleOptions(request) {
  // Make sure the necessary headers are present
  // for this to be a valid pre-flight request
  let headers = request.headers;
  if (
    headers.get("Origin") !== null &&
    headers.get("Access-Control-Request-Method") !== null &&
    headers.get("Access-Control-Request-Headers") !== null
  ){
    // Handle CORS pre-flight request.
    // If you want to check or reject the requested method + headers
    // you can do that here.
    let respHeaders = {
      ...corsHeaders,
    // Allow all future content Request headers to go back to browser
    // such as Authorization (Bearer) or X-Client-Name-Version
      "Access-Control-Allow-Headers": request.headers.get("Access-Control-Request-Headers"),
    }

    return new Response(null, {
      headers: respHeaders,
    })
  }
  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

  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(
      new Response(null, {
        status: 405,
        statusText: "Method Not Allowed",
      }),
    )
  }
})

The blog you referenced simply calls return await fetch(request) without ever setting up the WebSocket pair or calling client.accept()

That's correct. Simply return await fetch(request) is all you need to proxy any kind of HTTP request/response, including WebSockets.

Great hint! I searched everywhere for a client to origin server websocket proxy through cloudflare workers. Simply returning an await fetch(request) does the job :)