htmllint.js: use flatMap instead of concat & map (#171)

Author: XhmikosR

lib/htmllint.js

@@ -42,14 +42,13 @@ function parseErrorMessages(errors, config) {
   if (config.server) {
     // Extract "messages" property from each object and set the url of each message.
     // This results in an array of arrays instead of array of objects, which is then
-    // flattened by concatenation
-    // eslint-disable-next-line unicorn/prefer-spread
-    messages = [].concat(...messages, parsed.map(file => {
+    // flattened
+    messages = parsed.flatMap(file => {
       return file.messages.map(message => {
         message.url = file.url;
         return message;
       });
-    }));
+    });
   }
 
   return messages;