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= cowboy_req:header(3)

== Name

cowboy_req:header - HTTP header

== Description

[source,erlang]
----
header(Name, Req)          -> header(Name, Req, undefined)
header(Name, Req, Default) -> binary() | Default

Name    :: binary()          %% lowercase; case insensitive
Req     :: cowboy_req:req()
Default :: any()
----

Return the value for the given HTTP header.

The header name must be given as a lowercase binary string.
While header names are case insensitive, Cowboy requires them
to be given as lowercase to function properly.

Headers can also be obtained using pattern matching:

[source,erlang]
----
#{headers := #{Name := Value}} = Req.
----

Note that this snippet will crash if the header is missing.

== Arguments

Name::

Desired HTTP header name as a lowercase binary string.

Req::

The Req object.

Default::

Default value returned when the header is missing.

== Return value

The header value is returned as a binary string. When the
header is missing, the default argument is returned.

== Changelog

* *2.0*: Only the header value is returned, it is no longer wrapped in a tuple.
* *1.0*: Function introduced.

== Examples

.Get the accept header
[source,erlang]
----
Accept = cowboy_req:header(<<"accept">>, Req).
----

.Get the content-length header with a default value
[source,erlang]
----
Length = cowboy_req:header(<<"content-length">>, Req, <<"0">>).
----

== See also

link:man:cowboy_req(3)[cowboy_req(3)],
link:man:cowboy_req:headers(3)[cowboy_req:headers(3)],
link:man:cowboy_req:parse_header(3)[cowboy_req:parse_header(3)]