Introduction

In a JavaScript web application, a router is the part that syncs the currently displayed view with the browser address bar content.

In other words, it’s the part that makes the URL change when you click something in the page, and helps to show the correct view when you hit a specific URL.

Traditionally the Web is built around URLs. When you hit a certain URL, a specific page is displayed.

With the introduction of applications that run inside the browser and change what the user sees, many applications broke this interaction, and you had to manually update the URL with the browser’s History API.

You need a router when you need to sync URLs to views in your app. It’s a very common need, and all the major modern frameworks now allow you to manage routing.

The Vue Router library is the way to go for Vue.js applications. Vue does not enforce the use of this library. You can use whatever generic routing library you want, or also create your own History API integration, but the benefit of using Vue Router is that it’s official.

This means it’s maintained by the same people who maintain Vue, so you get a more consistent integration in the framework, and the guarantee that it’s always going to be compatible in the future, no matter what.

Installation

Vue Router is available via npm with the package named vue-router.

If you use Vue via a script tag, you can include Vue Router using

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

unpkg.com is a very handy tool that makes every npm package available in the browser with a simple link

If you use the Vue CLI, install it using

npm install vue-router

Once you install vue-router and make it available either using a script tag or via Vue CLI, you can now import it in your app.

You import it after vue, and you call Vue.use(VueRouter) to install it inside the app:

import Vue from 'vue'
import VueRouter from 'vue-router'

Vue.use(VueRouter)

After you call Vue.use() passing the router object, in any component of the app you have access to these objects:

  • this.$router is the router object
  • this.$route is the current route object

The router object

The router object, accessed using this.$router from any component when the Vue Router is installed in the root Vue component, offers many nice features.

We can make the app navigate to a new route using

  • this.$router.push()
  • this.$router.replace()
  • this.$router.go()

which resemble the pushState, replaceState and go methods of the History API.

push() is used to go to a new route, adding a new item to the browser history. replace() is the same, except it does not push a new state to the history.

Usage samples:

this.$router.push('about') //named route, see later
this.$router.push({ path: 'about' })
this.$router.push({ path: 'post', query: { post_slug: 'hello-world' } }) //using query parameters (post?post_slug=hello-world)
this.$router.replace({ path: 'about' })

go() goes back and forth, accepting a number that can be positive or negative to go back in the history:

this.$router.go(-1) //go back 1 step
this.$router.go(1) //go forward 1 step

Defining the routes

I’m using a Vue Single File Component in this example.

In the template I use a nav tag that has 3 router-link components, which have a label (Home/Login/About) and a URL assigned through the to attribute.

The router-view component is where the Vue Router will put the content that matches the current URL.

<template>
  <div id="app">
    <nav>
      <router-link to="/">Home</router-link>
      <router-link to="/login">Login</router-link>
      <router-link to="/about">About</router-link>
    </nav>
    <router-view></router-view>
  </div>
</template>

A router-link component renders an a tag by default (you can change that). Every time the route changes, either by clicking a link or by changing the URL, a router-link-active class is added to the element that refers to the active route, allowing you to style it.

In the JavaScript part we first include and install the router, then we define 3 route components.

We pass them to the initialization of the router object, and we pass this object to the Vue root instance.

Here’s the code:

<script>
import Vue from 'vue'
import VueRouter from 'vue-router'

Vue.use(Router)

const Home  = {
  template: '<div>Home</div>'
}

const Login = {
  template: '<div>Login</div>'
}

const About = {
  template: '<div>About</div>'
}

const router = new VueRouter({
  routes: [
    { path: '/', component: Home },
    { path: '/login', component: Login },
    { path: '/about', component: About }
  ]
})

new Vue({
  router
}).$mount('#app')
</script>

Usually, in a Vue app you instantiate and mount the root app using:

new Vue({
  render: h => h(App)
}).$mount('#app')

When using the Vue Router, you don’t pass a render property but instead, you use router.

The syntax used in the above example:

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

is a shorthand for

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

See in the example, we pass a routes array to the VueRouter constructor. Each route in this array has a path and component params.

If you pass a name param too, you have a named route.

Using named routes to pass parameters to the router push and replace methods

Remember how we used the Router object to push a new state before?

this.$router.push({ path: 'about' })

With a named route we can pass parameters to the new route:

this.$router.push({ name: 'post', params: { post_slug: 'hello-world' } })

the same goes for replace():

this.$router.replace({ name: 'post', params: { post_slug: 'hello-world' } })

The application will render the route component that matches the URL passed to the link.

The new route component that handles the URL is instantiated and its guards called, and the old route component will be destroyed.

Route guards

Since we mentioned guards, let’s introduce them.

You can think of them of life cycle hooks or middleware, those are functions called at specific times during the execution of the application. You can jump in and alter the execution of a route, redirecting or simply canceling the request.

You can have global guards by adding a callback to the beforeEach() and afterEach() property of the router.

  • beforeEach() is called before the navigation is confirmed
  • beforeResolve() is called when beforeEach is executed and all the components beforeRouterEnter and beforeRouteUpdate guards are called, but before the navigation is confirmed. The final check, if you want
  • afterEach() is called after the navigation is confirmed

What does “the navigation is confirmed” mean? We’ll see it in a second. In the meantime think of it as “the app can go to that route”.

The usage is:

this.$router.beforeEach((to, from, next) => {
  // ...
})
this.$router.afterEach((to, from) => {
  // ...
})

to and from represent the route objects that we go to and from. beforeEach has an additional parameter next which if we call with false as the parameter, will block the navigation, and cause it to be unconfirmed. Like in Node middleware, if you’re familiar, next() should always be called otherwise execution will get stuck.

Single route components also have guards:

  • beforeRouteEnter(from, to, next) is called before the current route is confirmed
  • beforeRouteUpdate(from, to, next) is called when the route changes but the component that manages it is still the same (with dynamic routing, see next)
  • beforeRouteLeave(from, to, next) is called when we move away from here

We mentioned navigation. To determine if the navigation to a route is confirmed, Vue Router performs some checks:

  • it calls beforeRouteLeave guard in the current component(s)
  • it calls the router beforeEach() guard
  • it calls the beforeRouteUpdate() in any component that needs to be reused, if any exist
  • it calls the beforeEnter() guard on the route object (I didn’t mention it but you can look here)
  • it calls the beforeRouterEnter() in the component that we should enter into
  • it calls the router beforeResolve() guard
  • if all was fine, the navigation is confirmed!
  • it calls the router afterEach() guard

You can use the route-specific guards (beforeRouteEnter and beforeRouteUpdate in case of dynamic routing) as life cycle hooks, so you can start data fetching requests for example.

Dynamic routing

The example above shows a different view based on the URL, handling the /, /login and /about routes.

A very common need is to handle dynamic routes, like having all posts under /post/, each with the slug name:

  • /post/first
  • /post/another-post
  • /post/hello-world

You can achieve this using a dynamic segment.

Those were static segments:

const router = new VueRouter({
  routes: [
    { path: '/', component: Home },
    { path: '/login', component: Login },
    { path: '/about', component: About }
  ]
})

we add in a dynamic segment to handle blog posts:

const router = new VueRouter({
  routes: [
    { path: '/', component: Home },
    { path: '/post/:post_slug', component: Post },
    { path: '/login', component: Login },
    { path: '/about', component: About }
  ]
})

Notice the :post_slug syntax. This means that you can use any string, and that will be mapped to the post_slug placeholder.

You’re not limited to this kind of syntax. Vue relies on this library to parse dynamic routes, and you can go wild with Regular Expressions.

Now inside the Post route component we can reference the route using $route, and the post slug using $route.params.post_slug:

const Post = {
  template: '<div>Post: {{ $route.params.post_slug }}</div>'
}

We can use this parameter to load the contents from the backend.

You can have as many dynamic segments as you want, in the same URL:

/post/:author/:post_slug

Remember when before we talked about what happens when a user navigates to a new route?

In the case of dynamic routes, what happens is a little different.

Vue to be more efficient instead of destroying the current route component and re-instantiating it, it reuses the current instance.

When this happens, Vue calls the beforeRouteUpdate life cycle event. There you can perform any operation you need:

const Post = {
  template: '<div>Post: {{ $route.params.post_slug }}</div>'
  beforeRouteUpdate(to, from, next) {
    console.log(`Updating slug from ${from} to ${to}`)
    next() //make sure you always call next()
  }
}

Using props

In the examples, I used $route.params.* to access the route data. A component should not be so tightly coupled with the router, and instead, we can use props:

const Post = {
  props: ['post_slug'],
  template: '<div>Post: {{ post_slug }}</div>'
}

const router = new VueRouter({
  routes: [
    { path: '/post/:post_slug', component: Post, props: true }
  ]
})

Notice the props: true passed to the route object to enable this functionality.

Nested routes

Before I mentioned that you can have as many dynamic segments as you want, in the same URL, like:

/post/:author/:post_slug

So, say we have an Author component taking care of the first dynamic segment:

<template>
  <div id="app">
    <router-view></router-view>
  </div>
</template>

<script>
import Vue from 'vue'
import VueRouter from 'vue-router'

Vue.use(Router)

const Author  = {
  template: '<div>Author: {{ $route.params.author}}</div>'
}

const router = new VueRouter({
  routes: [
    { path: '/post/:author', component: Author }
  ]
})

new Vue({
  router
}).$mount('#app')
</script>

We can insert a second router-view component instance inside the Author template:

const Author  = {
  template: '<div>Author: {{ $route.params.author}}<router-view></router-view></div>'
}

we add the Post component:

const Post = {
  template: '<div>Post: {{ $route.params.post_slug }}</div>'
}

and then we’ll inject the inner dynamic route in the VueRouter configuration:

const router = new VueRouter({
  routes: [{
    path: '/post/:author',
    component: Author,
    children: [
      path: ':post_slug',
      component: Post
    ]
  }]
})