Add string literal type for router events

packages/next/client/page-loader.ts

@@ -1,8 +1,7 @@
 import { ComponentType } from 'react'
 import type { ClientSsgManifest } from '../build'
 import type { ClientBuildManifest } from '../build/webpack/plugins/build-manifest-plugin'
-import mitt from '../next-server/lib/mitt'
-import type { MittEmitter } from '../next-server/lib/mitt'
+import mitt, { Emitter } from '../next-server/lib/mitt'
 import {
   addBasePath,
   markLoadingError,
@@ -116,7 +115,7 @@ export default class PageLoader {
   private buildId: string
   private assetPrefix: string
   private pageCache: Record<string, PageCacheEntry>
-  private pageRegisterEvents: MittEmitter
+  private pageRegisterEvents: Emitter<any, any>
   private loadingRoutes: Record<string, boolean>
   private promisedBuildManifest?: Promise<ClientBuildManifest>
   private promisedSsgManifest?: Promise<ClientSsgManifest>
@@ -129,7 +128,7 @@ export default class PageLoader {
     this.assetPrefix = assetPrefix
 
     this.pageCache = {}
-    this.pageRegisterEvents = mitt()
+    this.pageRegisterEvents = mitt<any, any>()
     this.loadingRoutes = {
       // By default these 2 pages are being loaded in the initial html
       '/_app': true,

packages/next/client/router.ts

@@ -1,6 +1,9 @@
 /* global window */
 import React from 'react'
-import Router, { NextRouter } from '../next-server/lib/router/router'
+import Router, {
+  NextRouter,
+  RouterEventMap,
+} from '../next-server/lib/router/router'
 import { RouterContext } from '../next-server/lib/router-context'
 
 type ClassArguments<T> = T extends new (...args: infer U) => any ? U : any
@@ -38,7 +41,7 @@ const urlPropertyFields = [
   'isFallback',
   'basePath',
 ]
-const routerEvents = [
+const routerEvents: Array<keyof RouterEventMap> = [
   'routeChangeStart',
   'beforeHistoryChange',
   'routeChangeComplete',
@@ -85,7 +88,7 @@ coreMethodFields.forEach((field) => {
 
 routerEvents.forEach((event) => {
   singletonRouter.ready(() => {
-    Router.events.on(event, (...args) => {
+    Router.events.on(event, (...args: any[]) => {
       const eventField = `on${event.charAt(0).toUpperCase()}${event.substring(
         1
       )}`

packages/next/next-server/lib/mitt.ts

@@ -1,44 +1,64 @@
 /*
 MIT License
-
 Copyright (c) Jason Miller (https://jasonformat.com/)
-
 Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
-
 The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
-
 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */
 
 // This file is based on https://github.com/developit/mitt/blob/v1.1.3/src/index.js
 // It's been edited for the needs of this script
 // See the LICENSE at the top of the file
 
-type Handler = (...evts: any[]) => void
+export type EventType =
+  | 'routeChangeStart'
+  | 'beforeHistoryChange'
+  | 'routeChangeComplete'
+  | 'routeChangeError'
+  | 'hashChangeStart'
+  | 'hashChangeComplete'
+
+export type Listeners = {
+  [K in keyof EventMap]?: Array<(p: EventMap[K]) => void>
+}
+
+export type ArgumentTypes<F extends Function> = F extends (
+  ...args: infer A
+) => any
+  ? A
+  : never
+
+export type EventMap = Record<string, any>
+export type EventHandlersMap = Record<string, any>
+export type EventKey<T extends EventMap> = string & keyof T
 
-export type MittEmitter = {
-  on(type: string, handler: Handler): void
-  off(type: string, handler: Handler): void
-  emit(type: string, ...evts: any[]): void
+export interface Emitter<T extends EventMap, H extends EventHandlersMap> {
+  on<K extends EventKey<T>>(eventName: K, fn: H[K]): void
+  off<K extends EventKey<T>>(eventName: K, fn: H[K]): void
+  emit<K extends EventKey<T>>(
+    eventName: K,
+    ...params: ArgumentTypes<H[K]>
+  ): void
 }
 
-export default function mitt(): MittEmitter {
-  const all: { [s: string]: Handler[] } = Object.create(null)
+export default function mitt<
+  T extends EventMap,
+  H extends EventHandlersMap
+>(): Emitter<T, H> {
+  const all: Listeners = Object.create(null)
 
   return {
-    on(type: string, handler: Handler) {
+    on(type, handler) {
       ;(all[type] || (all[type] = [])).push(handler)
     },
-
-    off(type: string, handler: Handler) {
+    off(type, handler) {
       if (all[type]) {
-        all[type].splice(all[type].indexOf(handler) >>> 0, 1)
+        all[type]?.splice((all[type] || []).indexOf(handler) >>> 0, 1)
       }
     },
-
-    emit(type: string, ...evts: any[]) {
+    emit(type, ...evts) {
       // eslint-disable-next-line array-callback-return
-      ;(all[type] || []).slice().map((handler: Handler) => {
+      ;(all[type] || []).slice().map((handler: any) => {
         handler(...evts)
       })
     },

packages/next/next-server/lib/router/router.ts

@@ -3,13 +3,13 @@
 import { ParsedUrlQuery } from 'querystring'
 import { ComponentType } from 'react'
 import { UrlObject } from 'url'
+import mitt, { Emitter } from '../mitt'
 import {
   normalizePathTrailingSlash,
   removePathTrailingSlash,
 } from '../../../client/normalize-trailing-slash'
 import { GoodPageCache, StyleSheetTuple } from '../../../client/page-loader'
 import { denormalizePagePath } from '../../server/denormalize-page-path'
-import mitt, { MittEmitter } from '../mitt'
 import {
   AppContextType,
   formatWithValidation,
@@ -39,6 +39,16 @@ interface NextHistoryState {
 
 type HistoryState = null | { __N: false } | ({ __N: true } & NextHistoryState)
 
+interface TransitionOptions {
+  shallow?: boolean
+}
+
+interface NextHistoryState {
+  url: string
+  as: string
+  options: TransitionOptions
+}
+
 const basePath = (process.env.__NEXT_ROUTER_BASEPATH as string) || ''
 
 function buildCancellationError() {
@@ -237,6 +247,21 @@ export type NextRouter = BaseRouter &
     | 'isFallback'
   >
 
+export type RouterEventError = {
+  [key: string]: any
+  cancelled: boolean
+}
+
+export type RouterHandlersMap = {
+  routeChangeStart: (url: string) => void
+  beforeHistoryChange: (url: string) => void
+  routeChangeComplete: (url: string) => void
+  routeChangeError: (err: RouterEventError, url: string) => void
+  hashChangeStart: (url: string) => void
+  hashChangeComplete: (url: string) => void
+}
+export type RouterEventMap = Record<keyof RouterHandlersMap, any>
+
 export type PrefetchOptions = {
   priority?: boolean
 }
@@ -324,14 +349,14 @@ export default class Router implements BaseRouter {
   clc: ComponentLoadCancel
   pageLoader: any
   _bps: BeforePopStateCallback | undefined
-  events: MittEmitter
+  events: Emitter<RouterEventMap, RouterHandlersMap>
   _wrapApp: (App: AppComponent) => any
   isSsr: boolean
   isFallback: boolean
   _inFlightRoute?: string
   _shallow?: boolean
 
-  static events: MittEmitter = mitt()
+  static events = mitt<RouterEventMap, RouterHandlersMap>()
 
   constructor(
     pathname: string,

packages/next/next-server/server/render.tsx

@@ -29,7 +29,7 @@ import { defaultHead } from '../lib/head'
 import { HeadManagerContext } from '../lib/head-manager-context'
 import Loadable from '../lib/loadable'
 import { LoadableContext } from '../lib/loadable-context'
-import mitt, { MittEmitter } from '../lib/mitt'
+import mitt, { Emitter } from '../lib/mitt'
 import postProcess from '../lib/post-process'
 import { RouterContext } from '../lib/router-context'
 import { NextRouter } from '../lib/router/router'
@@ -68,7 +68,7 @@ class ServerRouter implements NextRouter {
   events: any
   isFallback: boolean
   // TODO: Remove in the next major version, as this would mean the user is adding event listeners in server-side `render` method
-  static events: MittEmitter = mitt()
+  static events: Emitter<any, any> = mitt()
 
   constructor(
     pathname: string,

test/integration/typescript/pages/routerEvents.tsx

@@ -0,0 +1,33 @@
+import React, { useEffect, useState } from 'react'
+import Router from 'next/router'
+
+const RouterEvents: React.FC = () => {
+  const [routerEvent, setRouterEvent] = useState('')
+  const [routerEventError, setRouterEventError] = useState('')
+
+  useEffect(() => {
+    Router.events.on('routeChangeStart', () =>
+      setRouterEvent('routeChangeStart')
+    )
+
+    Router.events.on('routeChangeComplete', () =>
+      setRouterEvent('routeChangeComplete')
+    )
+
+    Router.events.on('routeChangeError', (error) => {
+      setRouterEvent('routeChangeError')
+
+      setRouterEventError(error.message)
+    })
+  }, [])
+
+  return (
+    <div>
+      {routerEvent && <p>{routerEvent}</p>}
+
+      {routerEventError && <p>{routerEventError}</p>}
+    </div>
+  )
+}
+
+export default RouterEvents