Loading Default Component - Vue.js CDN

For the vue router you can use optional parameter.

So instead of path: '/product/:id' you can write path: '/product/:id? (added ? at the end)

<script src="https://unpkg.com/vue/dist/vue.js"></script>
<script src="https://unpkg.com/vue-router/dist/vue-router.js"></script>

<div id="app">
    <p>
        <router-link to="/product">/product</router-link>
        <router-link to="/product/foo">/product/foo</router-link>
    </p>

    <router-view></router-view>
</div>

<script>
    const Product = {
        /*
            router `id` will be passed as prop
            so we can use `id` in our templates instead of $route.params.id
            this way we can also specify the default value is nothing was passed
         */
        template: `<div class="user"> <h2>User {{ id }}</h2></div> `,
        props: {
            id: {
                type:    String,
                // specify default value if id is not passed to the router
                default: 'N/A'
            }
        }
    }

    const router = new VueRouter({
        mode: 'history',
        routes: [
            {
                // :id? means that it's an optional parameter
                path: '/product/:id?',
                component: Product,
                // Passing Props to Route Components
                props:     true
            },
        ],
    })

    const app = new Vue({
        router
    })
    .$mount('#app')

</script>