res.headersSent Property

res.headersSent is a boolean that tells you whether the response headers have already been sent to the client.

Syntax

res.headersSent

Example

const express = require("express");
const app = express();
const PORT = 4800;

app.get("/", (req, res) => {
    console.log("before send:", res.headersSent); // false
    res.send("OK");
    console.log("after send:", res.headersSent);   // true
});

app.listen(PORT, () => console.log(`Listening on ${PORT}`));

Use cases

• Preventing "headers already sent" errors by checking before calling res.set() again.
• Writing custom error-handling middleware that avoids double responses.
• Deciding whether a response can still be modified.