refactor: simplify (#2120)

Author: jamesgeorge007

lib/Server.js

@@ -280,14 +280,12 @@ class Server {
      * ]
      */
     this.options.proxy.forEach((proxyConfigOrCallback) => {
-      let proxyConfig;
       let proxyMiddleware;
 
-      if (typeof proxyConfigOrCallback === 'function') {
-        proxyConfig = proxyConfigOrCallback();
-      } else {
-        proxyConfig = proxyConfigOrCallback;
-      }
+      let proxyConfig =
+        typeof proxyConfigOrCallback === 'function'
+          ? proxyConfigOrCallback()
+          : proxyConfigOrCallback;
 
       proxyMiddleware = getProxyMiddleware(proxyConfig);