Vuejs get old value when on change event

This solution assumes that you need the old value only when handling the change coming from user input - which is probably the case, and not a generic model change. Below I added a sketch of solution watching all changes in if it's necessary, though perhaps it's better to control the other changes at the places they occur, and not in this component.

Drop v-model and replace it with manual handling.

It is important to note that "although a bit magical, v-model is essentially syntax sugar for updating data on user input events (...)". So without much harm you can remove it and handle the events directly, as you already almost do.

First, replace v-model with :value. The input will still follow the item.age updates, but will not update the age automatically.

<input type="text" name="qty"       :value="item.age"       @change="changeAge(item,$event)">

Then, in order to actually update the value, add item.age = event.target.value to changeAge, like this:

changeAge: function(item, event){  alert(item.age);  // Old value  alert(event.target.value);  // New value  item.age = event.target.value;  // Actual assignment}

Note: you may want to use @input instead, that's what v-model actually uses.

And that's it, it should do the trick. If you need to prevent the change, you can just omit the assignment, however, you need to reset the input value. item.age = item.age of Vue.set(item, 'age', item.age) may do the trick, but I'm not quite sure it will.

Note: event.target.value is a string, if you need a number, use parseInt().

Create a separate component instance for every item and create a watch there

In case you need to watch all the changes. However, if you need that, perhaps you should do that in some other place, not this component.

I don't have all the details ready, so I'll just the general draft: in your v-for, instead of plain <input>, you place another component, say <my-item>. You put v-model=item on it. Inside the component, you create the <input> and forward the value prop to it, as well as forward it's input/change events outside. In your component single item is a single prop, so you can attach a watcher directly to it. Relevant docs: Component Basics, Customizing Component v-model, Props.

You dont get old value of element on-change as explained here. Also when watching an object, you can not get older value as explained here until the reference of object changes.

To work around these you can create a computed property which will be clone of your items data, and you can put a watcher over this computed property to get to know the old value as well, See working code below:

var app = new Vue({  el:"#table1",  data:{   items:[{name:'long name One',age:21},{name:'long name Two',age:22}]  },  watch:{    clonedItems: function(newVal, oldVal){      alert(JSON.stringify(newVal));      alert(JSON.stringify(oldVal));    }  },  computed:{    clonedItems: function(){       return JSON.parse(JSON.stringify(this.items))    }  }});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><script src="https://unpkg.com/vue/dist/vue.js"></script><table class="table table-cart" id="table1">   <thead>     <tr>       <th>Name</th>       <th>Age</th>     </tr>  </thead>   <tbody>     <tr v-for="(item,index) in items">       <td>{{item.name}} :</td>              <td>         <input type="text" name="qty"                v-model="item.age">       </td>     </tr>   </tbody> </table>