This is a small portion of the core.cj file for call-timeline that aimed to add CORS support using two middlewares.

add-cors.cj

(defn wrap-options
  [app]
  (fn [request]
    (if (= (request :request-method) :options)
      (into request {:status 204})
      (app request))))

(defn add-header
  [r name value]
  (assoc-in r [:headers name] value))

(defn wrap-cors
  [app]
    (fn
      ([request]
        (-> (app request)
          (add-header "Access-Control-Allow-Origin" "*")
          (add-header "Access-Control-Allow-Methods" "GET, OPTIONS")
          (add-header "Access-Control-Allow-Headers" "Content-Type, Authorization, X-Account-Id")))))

(def handler
  (-> app
      wrap-options
      wrap-cors))