expressjs.com/_includes/api/en/3x/req-header.md

req.get(field)

Get the case-insensitive request header field. The "Referrer" and "Referer" fields are interchangeable.

req.get('Content-Type')
// => "text/plain"

req.get('content-type')
// => "text/plain"

req.get('Something')
// => undefined

p Aliased as req.header(field).