res.append() Function

res.append() adds a value to a response header. If the header doesn't exist yet, Express creates it.

Syntax

res.append(field [, value])

Example

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

app.get("/", (req, res) => {
    res.append("X-Powered-By", "PixelMart-API");
    res.append("Set-Cookie", "seen=true");
    res.append("Set-Cookie", "region=bd");
    res.send("Headers appended");
});

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

Calling res.append() twice on the same header (like Set-Cookie) accumulates values instead of overwriting them.

Use cases

Adding multiple values to the same response header.
Building Link headers for pagination or related resources.
Attaching custom metadata headers dynamically.