How do you communicate from child to parent using events?

If you want child wants to communicate back up to the parent, then emit an event from child using $emit object to parent,

Vue.component('todo-item', {
props: ['todo'],
template: `
<h3>{{ todo.title }}</h3>
<button v-on:click="$emit('increment-count', 1)">
Add
</button>
`
})

Now you can use this todo-item in parent component to access the count value.

<ul v-for="todo in todos">
<li>
<todo-item
v-bind:key="todo.id"
v-bind:todo="todo"
v-on:increment-count="total += 1"
/></todo-item>
</li>
</ul>
<span> Total todos count is {{total}}</span>