[Vuejs]-Add to cart not working Laravel and vue js?

The template contains:

<span class="d-block" id="itemsCountVue">@{{itemsCount}}</span>

itemsCount is not defined inside the vue data object, and it’s not defined as a computed property either.

You could try:

el: '#app1',
data: {
    itemsCount: 0,
},

Then everywhere you’re setting nav.itemsCount also set this.itemsCount