`res.append()`

Appends a value to an HTTP response header, preserving existing values.

Since Express 4.11 Spec ↗

Syntax

res.append(field, value)

Parameters

Name	Type	Required	Description
`field`	`string`	Yes	The header name.
`value`	`string \| string[]`	Yes	The value(s) to add to the header.

Returns

Response — The response object, for chaining.

Examples

app.get('/', (req, res) => {
  res.append('Set-Cookie', 'a=1');
  res.append('Set-Cookie', 'b=2');
  res.send('ok');
});

Output

Set-Cookie: a=1
Set-Cookie: b=2

app.get('/x', (req, res) => {
  res.append('Vary', 'Accept');
  res.append('Vary', 'Accept-Encoding');
  res.end();
});

Output

Vary: Accept, Accept-Encoding

Notes

Unlike `res.set`, which replaces, `res.append` adds another value - important for multi-value headers like `Set-Cookie`, `Vary`, and `Link`. Calling `res.set` afterward for the same field overwrites appended values.