vue/simple-watcher/index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Watch the input value</title>
</head>
<body>
  <div id="app">
    <input type="text" v-model="inputValue">
    <p>Five seconds ago value was 
      <u>{{ oldInputValue }}</u>
      </p>
  </div>
  <script src="https://cdn.jsdelivr.net/npm/vue@2.6.10/dist/vue.js"></script>
  <script>
    new Vue({
      el: '#app',
      data: {
        inputValue: 'initial value',
        oldInputValue: ''
      },
      watch: {
        inputValue(val, oldVal) {
          // val = this.inputValue
          let newValue = this.inputValue;
          setTimeout(() => {
            this.oldInputValue = newValue
          }, 5000)
        }
      }
    })
  </script>
</body>
</html>