[add] result_value

src/App.vue

@@ -26,11 +26,13 @@ const parameter = ref({
   padding: '20px',
   margin: '15px',
 })
+
+const result_value = ref("");
 </script>
 
 <template>
   <div class="calculator">
-    <div class="display">0</div>
+    <div class="display">{{ result_value || 0 }}</div>
 
     <div>
       <button v-for="e in elements" :key="e" :style="parameter">