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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): Promise<Response> {
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).textContent = JSON.stringify(data)
} catch (e) {
document.getElementById(reqName).textContent = 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;
from pyodide.ffi import to_js as _to_js
from js import Response, URL, fetch, Object, Request
def to_js(x):
return _to_js(x, dict_converter=Object.fromEntries)
async def on_fetch(request):
cors_headers = {
"Access-Control-Allow-Origin": "*",
"Access-Control-Allow-Methods": "GET,HEAD,POST,OPTIONS",
"Access-Control-Max-Age": "86400",
}
api_url = "https://examples.cloudflareworkers.com/demos/demoapi"
proxy_endpoint = "/corsproxy/"
def raw_html_response(html):
return Response.new(html, headers=to_js({"content-type": "text/html;charset=UTF-8"}))
demo_page = f'''
<!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 def handle_request(request):
url = URL.new(request.url)
api_url2 = url.searchParams["apiurl"]
if not api_url2:
api_url2 = api_url
request = Request.new(api_url2, request)
request.headers["Origin"] = (URL.new(api_url2)).origin
print(request.headers)
response = await fetch(request)
response = Response.new(response.body, response)
response.headers["Access-Control-Allow-Origin"] = url.origin
response.headers["Vary"] = "Origin"
return response
async def handle_options(request):
if "Origin" in request.headers and "Access-Control-Request-Method" in request.headers and "Access-Control-Request-Headers" in request.headers:
return Response.new(None, headers=to_js({
**cors_headers,
"Access-Control-Allow-Headers": request.headers["Access-Control-Request-Headers"]
}))
return Response.new(None, headers=to_js({"Allow": "GET, HEAD, POST, OPTIONS"}))
url = URL.new(request.url)
if url.pathname.startswith(proxy_endpoint):
if request.method == "OPTIONS":
return handle_options(request)
if request.method in ("GET", "HEAD", "POST"):
return handle_request(request)
return Response.new(None, status=405, statusText="Method Not Allowed")
return raw_html_response(demo_page)