computed example

hello/app.js

@@ -20,5 +20,10 @@ var app = new Vue({
         reverseMessage: function () {
             this.message = this.message.split('').reverse().join('')
         }
+    },
+    computed: {
+        lenMessage: function () {
+            return this.message.length
+        }
     }
 })

hello/index.html

@@ -27,7 +27,7 @@
 
             <button v-on:click="reverseMessage">Reverse Message</button>
 
-            <input v-model="message">
+            <input v-model="message"> <span>{{ lenMessage }}</span>
         </div>
         
         <script src="app.js"></script>