feat: expose START_LOCATION

Close #2718

Author: posva

docs/api/README.md

@@ -319,6 +319,26 @@ Since it's just a component, it works with `<transition>` and `<keep-alive>`. Wh
 
   The current route represented as a [Route Object](#the-route-object).
 
+### router.START_LOCATION
+
+- type: `Route`
+
+  Initial route location represented as a [Route Object](#the-route-object) where the router starts at. Can be used in navigation guards to differentiate the initial navigation.
+
+  ```js
+  import Router from 'vue-router'
+
+  const router = new Router({
+    // ...
+  })
+
+  router.beforeEach((to, from) => {
+    if (from === START_LOCATION) {
+      // initial navigation
+    }
+  })
+  ```
+
 ## Router Instance Methods
 
 ### router.beforeEach

src/index.js

@@ -280,6 +280,7 @@ VueRouter.install = install
 VueRouter.version = '__VERSION__'
 VueRouter.isNavigationFailure = isNavigationFailure
 VueRouter.NavigationFailureType = NavigationFailureType
+VueRouter.START_LOCATION = START
 
 if (inBrowser && window.Vue) {
   window.Vue.use(VueRouter)

types/router.d.ts

@@ -75,6 +75,8 @@ export declare class VueRouter {
   static NavigationFailureType: {
     [k in keyof typeof NavigationFailureType]: NavigationFailureType
   }
+
+  static START_LOCATION: Route
 }
 
 export enum NavigationFailureType {