[For Cowboy 2.0] A Cowboy middleware to set some CORS headers for every request, and to handle OPTIONS requests without needing to implement them in every handler.

ca_cowboy_middleware.erl

-module(ca_cowboy_middleware).

-behaviour(cowboy_middleware).

-export([execute/2]).

execute(Req, Env) ->
    {ok, ReqWithCorsHeaders} = set_cors_headers(Req),
    Method = cowboy_req:method(ReqWithCorsHeaders),

    case Method of
	<<"OPTIONS">> ->
	    ReqFinal = cowboy_req:reply(200, ReqWithCorsHeaders),
	    {stop, ReqFinal};
	_ ->
	    %% continue as normal
	    {ok, ReqWithCorsHeaders, Env}
    end.

%% ===================================================================
%% Helpers
%% ===================================================================

set_headers(Headers, Req) ->
    ReqWithHeaders = lists:foldl(fun({Header, Value}, ReqIn) ->
					 ReqWithHeader = cowboy_req:set_resp_header(Header, Value, ReqIn),
					 ReqWithHeader
				 end, Req, Headers),
    {ok, ReqWithHeaders}.
    

set_cors_headers(Req) ->
    Headers = [{<<"access-control-allow-origin">>, <<"*">>},
	       {<<"access-control-allow-methods">>, <<"POST, GET, OPTIONS">>},
	       {<<"access-control-allow-headers">>, <<"Origin, X-Requested-With, Content-Type, Accept">>},
	       {<<"access-control-max-age">>, <<"1000">>}],
    {ok, Req2} = set_headers(Headers, Req),
    {ok, Req2}.

gistfile1.txt

{middlewares, [cowboy_router, ca_cowboy_middleware, cowboy_handler]}