CORS with Phoenix

router.ex

defmodule App.Router do
  use Phoenix.Router
  
  resources "users", Controller.Users
  options  "/users", Controller.Users, :options
end

users.ex

defmodule Controller.Users do
  use Phoenix.Controller
  
  def index(conn) do
    {:ok, user} = # ... get user JSON
    json allow_cors(conn), user
  end
  
  def options(conn) do
    text allow_cors(conn), ""
  end
  
  defp allow_cors(conn), do: put_resp_header(conn, "Access-Control-Allow-Origin", "*")
end

In Phoenix 0.13, you need to set header with lowercase:
defp allow_cors(conn), do: put_resp_header(conn, "access-control-allow-origin", "*")

@remiq Downcasing is no longer true. I've been able to do put_resp_header(conn, "Access-Control-Allow-Origin", "*") without issue in Phoenix 1.1.4 :)