VueJS How can I use computed property with v-for

You can't create a computed property for each iteration. Ideally, each of those items would be their own component so each one can have its own fullName computed property.

What you can do, if you don't want to create a user component, is use a method instead. You can move fullName right from the computed property to methods, then you can use it like:

{{ fullName(user) }}

Also, side note, if you find yourself needing to pass an arguments to a computed you likely want a method instead.

What you're missing here is that your items is an array, which holds all the items, but the computed is a single fullName, which just can't express all the fullNames in items. Try this:

var vm = new Vue({
    el: '#el',
    data: { items: items },
    computed: {
        // corrections start
        fullNames: function() {
            return this.items.map(function(item) {
                return item.firstname + ' ' + item.lastname;
            });
        }
        // corrections end
    }
});

Then in the view:

<div id="el">
    <p v-for="(item, index) in items">
        <span>{{fullNames[index]}}</span>
    </p>
</div>

The way Bill introduces surely works, but we can do it with computed props and I think it's a better design than method in iterations, especially when the app gets larger. Also, computed has a performance gain compared to method on some circumstances: http://vuejs.org/guide/computed.html#Computed-Caching-vs-Methods