How to use: CORS Headers for Express
CORS Headers for Express is a free online Web Tools tool that generate correct Express CORS middleware with an explicit origin allow-list and preflight handling instantly in your browser. Paste your data, adjust the options and get the result in real time — no installation, no sign-up, and nothing is ever uploaded to a server.
Use CORS Headers for Express now →
Description
Generate correct Express CORS middleware with an explicit origin allow-list and preflight handling.
- Web Tools
- Because everything is computed client-side, CORS Headers for Express is safe to use with sensitive data such as API keys, tokens or private logs.
How to use
Using CORS Headers for Express takes three steps:
- Paste your data into the input box (or load the example).
- Configure the options to match your target format or scenario.
- Copy, download or inspect the output — it updates live as you type.
Run
Examples
List the allowed origins in the options; a wildcard combined with credentials is rejected because browsers forbid it.
Output (generated live by the tool just now)
// Express — explicit allow-list, no wildcard with credentials
const ALLOWED = new Set(["https://app.example.com"]);
app.use((req, res, next) => {
const origin = req.headers.origin;
if (ALLOWED.has(origin)) {
res.setHeader('Access-Control-Allow-Origin', origin);
res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PUT,PATCH,DELETE,OPTIONS');
res.setHeader('Access-Control-Allow-Headers', 'Content-Type,Authorization,X-Requested-With');
res.setHeader('Access-Control-Expose-Headers', 'Content-Length,X-Request-Id');
res.setHeader('Access-Control-Max-Age', '86400');
}
if (req.method === 'OPTIONS') return res.sendStatus(204);
next();
});
Notes
• Preflight is cached for 86400s, so browsers skip the OPTIONS round trip during that window.
• CORS protects browsers, not your server — it is not an authentication mechanism. Enforce authorisation server-side too.
This output was produced by actually running the tool, not written by hand.
Use cases
Common situations where CORS Headers for Express helps:
- Generate correct Express CORS middleware with an explicit origin allow-list and preflight handling.
Parameters
This tool has no options — it works on the input alone.
FAQ
What is CORS Headers for Express?
CORS Headers for Express generate correct Express CORS middleware with an explicit origin allow-list and preflight handling. It runs entirely in your browser, so your data stays private.
Is CORS Headers for Express free to use?
Yes. CORS Headers for Express is free, unlimited and works without registration or installation.
Does my data get uploaded?
No. All processing happens locally in your browser; nothing is sent to any server.
Good to know
- Everything runs client-side, so extremely large inputs are limited by your browser tab’s memory rather than by a server upload limit.