diff --git a/packages/next/client/experimental-script.tsx b/packages/next/client/experimental-script.tsx
index 77e914f331bc7..d5b645a58a571 100644
--- a/packages/next/client/experimental-script.tsx
+++ b/packages/next/client/experimental-script.tsx
@@ -16,7 +16,7 @@ interface Props extends ScriptHTMLAttributes<HTMLScriptElement> {
   preload?: boolean
 }
 
-const loadScript = (props: Props) => {
+const loadScript = (props: Props): void => {
   const {
     src = '',
     onLoad = () => {},
@@ -83,7 +83,7 @@ const loadScript = (props: Props) => {
   document.body.appendChild(el)
 }
 
-export default function Script(props: Props) {
+export default function Script(props: Props): JSX.Element | null {
   const {
     src = '',
     onLoad = () => {},
diff --git a/packages/next/client/head-manager.ts b/packages/next/client/head-manager.ts
index 4ea0ef9597149..9e2da8f6007d1 100644
--- a/packages/next/client/head-manager.ts
+++ b/packages/next/client/head-manager.ts
@@ -40,7 +40,7 @@ function reactElementToDOM({ type, props }: JSX.Element): HTMLElement {
   return el
 }
 
-function updateElements(type: string, components: JSX.Element[]) {
+function updateElements(type: string, components: JSX.Element[]): void {
   const headEl = document.getElementsByTagName('head')[0]
   const headCountEl: HTMLMetaElement = headEl.querySelector(
     'meta[name=next-head-count]'
@@ -84,7 +84,10 @@ function updateElements(type: string, components: JSX.Element[]) {
   headCountEl.content = (headCount - oldTags.length + newTags.length).toString()
 }
 
-export default function initHeadManager() {
+export default function initHeadManager(): {
+  mountedInstances: Set<unknown>
+  updateHead: (head: JSX.Element[]) => void
+} {
   let updatePromise: Promise<void> | null = null
 
   return {
diff --git a/packages/next/client/link.tsx b/packages/next/client/link.tsx
index 0ea989d872b7b..7a89a7e4843c8 100644
--- a/packages/next/client/link.tsx
+++ b/packages/next/client/link.tsx
@@ -62,7 +62,7 @@ function prefetch(
   prefetched[href + '%' + as + (curLocale ? '%' + curLocale : '')] = true
 }
 
-function isModifiedEvent(event: React.MouseEvent) {
+function isModifiedEvent(event: React.MouseEvent): boolean {
   const { target } = event.currentTarget as HTMLAnchorElement
   return (
     (target && target !== '_self') ||
diff --git a/packages/next/client/page-loader.ts b/packages/next/client/page-loader.ts
index 01d1350a0644e..b95c0e7e331c2 100644
--- a/packages/next/client/page-loader.ts
+++ b/packages/next/client/page-loader.ts
@@ -13,7 +13,7 @@ import createRouteLoader, {
   RouteLoader,
 } from './route-loader'
 
-function normalizeRoute(route: string) {
+function normalizeRoute(route: string): string {
   if (route[0] !== '/') {
     throw new Error(`Route name should start with a "/", got "${route}"`)
   }
@@ -83,13 +83,14 @@ export default class PageLoader {
   /**
    * @param {string} href the route href (file-system path)
    * @param {string} asPath the URL as shown in browser (virtual path); used for dynamic routes
+   * @returns {string}
    */
   getDataHref(
     href: string,
     asPath: string,
     ssg: boolean,
     locale?: string | false
-  ) {
+  ): string {
     const { pathname: hrefPathname, query, search } = parseRelativeUrl(href)
     const { pathname: asPathname } = parseRelativeUrl(asPath)
     const route = normalizeRoute(hrefPathname)
diff --git a/packages/next/client/performance-relayer.ts b/packages/next/client/performance-relayer.ts
index 83ff892f58f3b..ecd25791f0e71 100644
--- a/packages/next/client/performance-relayer.ts
+++ b/packages/next/client/performance-relayer.ts
@@ -12,7 +12,7 @@ const initialHref = location.href
 let isRegistered = false
 let userReportHandler: ReportHandler | undefined
 
-function onReport(metric: Metric) {
+function onReport(metric: Metric): void {
   if (userReportHandler) {
     userReportHandler(metric)
   }
@@ -57,7 +57,7 @@ function onReport(metric: Metric) {
   }
 }
 
-export default (onPerfEntry?: ReportHandler) => {
+export default (onPerfEntry?: ReportHandler): void => {
   // Update function if it changes:
   userReportHandler = onPerfEntry
 
diff --git a/packages/next/client/request-idle-callback.ts b/packages/next/client/request-idle-callback.ts
index cea89be0ddabc..46127657673a3 100644
--- a/packages/next/client/request-idle-callback.ts
+++ b/packages/next/client/request-idle-callback.ts
@@ -18,7 +18,9 @@ declare global {
 
 const requestIdleCallback =
   (typeof self !== 'undefined' && self.requestIdleCallback) ||
-  function (cb: (deadline: RequestIdleCallbackDeadline) => void) {
+  function (
+    cb: (deadline: RequestIdleCallbackDeadline) => void
+  ): NodeJS.Timeout {
     let start = Date.now()
     return setTimeout(function () {
       cb({
diff --git a/packages/next/client/route-loader.ts b/packages/next/client/route-loader.ts
index ce3073a5a348c..ba6023cf7047f 100644
--- a/packages/next/client/route-loader.ts
+++ b/packages/next/client/route-loader.ts
@@ -55,7 +55,7 @@ function withFuture<T>(
     return Promise.resolve(entry)
   }
   let resolver: (entrypoint: T) => void
-  const prom = new Promise<T>((resolve) => {
+  const prom: Promise<T> = new Promise<T>((resolve) => {
     resolver = resolve
   })
   map.set(key, (entry = { resolve: resolver!, future: prom }))
@@ -120,7 +120,7 @@ export function markAssetError(err: Error): Error {
   return Object.defineProperty(err, ASSET_LOAD_ERROR, {})
 }
 
-export function isAssetError(err?: Error) {
+export function isAssetError(err?: Error): boolean | undefined {
   return err && ASSET_LOAD_ERROR in err
 }
 
@@ -166,7 +166,9 @@ export function getClientBuildManifest(): Promise<ClientBuildManifest> {
     return Promise.resolve(self.__BUILD_MANIFEST)
   }
 
-  const onBuildManifest = new Promise<ClientBuildManifest>((resolve) => {
+  const onBuildManifest: Promise<ClientBuildManifest> = new Promise<
+    ClientBuildManifest
+  >((resolve) => {
     // Mandatory because this is not concurrent safe:
     const cb = self.__BUILD_MANIFEST_CB
     self.__BUILD_MANIFEST_CB = () => {
@@ -229,7 +231,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
   > = new Map()
 
   function maybeExecuteScript(src: string): Promise<unknown> {
-    let prom = loadedScripts.get(src)
+    let prom: Promise<unknown> | undefined = loadedScripts.get(src)
     if (prom) {
       return prom
     }
@@ -244,7 +246,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
   }
 
   function fetchStyleSheet(href: string): Promise<RouteStyleSheet> {
-    let prom = styleSheets.get(href)
+    let prom: Promise<RouteStyleSheet> | undefined = styleSheets.get(href)
     if (prom) {
       return prom
     }
@@ -269,7 +271,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
     whenEntrypoint(route: string) {
       return withFuture(route, entrypoints)
     },
-    onEntrypoint(route, execute) {
+    onEntrypoint(route: string, execute: () => unknown) {
       Promise.resolve(execute)
         .then((fn) => fn())
         .then(
@@ -285,7 +287,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
           if (old && 'resolve' in old) old.resolve(input)
         })
     },
-    loadRoute(route) {
+    loadRoute(route: string) {
       return withFuture<RouteLoaderEntry>(route, routes, async () => {
         try {
           const { scripts, css } = await getFilesForRoute(assetPrefix, route)
@@ -296,7 +298,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
             Promise.all(css.map(fetchStyleSheet)),
           ] as const)
 
-          const entrypoint = await Promise.race([
+          const entrypoint: RouteEntrypoint = await Promise.race([
             this.whenEntrypoint(route),
             idleTimeout<RouteLoaderEntry>(
               MS_MAX_IDLE_DELAY,
@@ -315,7 +317,7 @@ function createRouteLoader(assetPrefix: string): RouteLoader {
         }
       })
     },
-    prefetch(route) {
+    prefetch(route: string): Promise<void> {
       // https://github.com/GoogleChromeLabs/quicklink/blob/453a661fa1fa940e2d2e044452398e38c67a98fb/src/index.mjs#L115-L118
       // License: Apache 2.0
       let cn
diff --git a/packages/next/client/router.ts b/packages/next/client/router.ts
index 093a310f3b60d..ff3c66dc92013 100644
--- a/packages/next/client/router.ts
+++ b/packages/next/client/router.ts
@@ -66,7 +66,7 @@ Object.defineProperty(singletonRouter, 'events', {
   },
 })
 
-urlPropertyFields.forEach((field) => {
+urlPropertyFields.forEach((field: string) => {
   // Here we need to use Object.defineProperty because, we need to return
   // the property assigned to the actual router
   // The value might get changed as we change routes and this is the
@@ -79,7 +79,7 @@ urlPropertyFields.forEach((field) => {
   })
 })
 
-coreMethodFields.forEach((field) => {
+coreMethodFields.forEach((field: string) => {
   // We don't really know the types here, so we add them later instead
   ;(singletonRouter as any)[field] = (...args: any[]) => {
     const router = getRouter() as any
@@ -87,7 +87,7 @@ coreMethodFields.forEach((field) => {
   }
 })
 
-routerEvents.forEach((event) => {
+routerEvents.forEach((event: string) => {
   singletonRouter.ready(() => {
     Router.events.on(event, (...args) => {
       const eventField = `on${event.charAt(0).toUpperCase()}${event.substring(
diff --git a/packages/next/client/use-intersection.tsx b/packages/next/client/use-intersection.tsx
index 93aa6c385f43e..ad198e4d7f5ff 100644
--- a/packages/next/client/use-intersection.tsx
+++ b/packages/next/client/use-intersection.tsx
@@ -4,6 +4,11 @@ import requestIdleCallback from './request-idle-callback'
 type UseIntersectionObserverInit = Pick<IntersectionObserverInit, 'rootMargin'>
 type UseIntersection = { disabled?: boolean } & UseIntersectionObserverInit
 type ObserveCallback = (isVisible: boolean) => void
+type Observer = {
+  id: string
+  observer: IntersectionObserver
+  elements: Map<Element, ObserveCallback>
+}
 
 const hasIntersectionObserver = typeof IntersectionObserver !== 'undefined'
 
@@ -11,7 +16,7 @@ export function useIntersection<T extends Element>({
   rootMargin,
   disabled,
 }: UseIntersection): [(element: T | null) => void, boolean] {
-  const isDisabled = disabled || !hasIntersectionObserver
+  const isDisabled: boolean = disabled || !hasIntersectionObserver
 
   const unobserve = useRef<Function>()
   const [visible, setVisible] = useState(false)
@@ -49,12 +54,12 @@ function observe(
   element: Element,
   callback: ObserveCallback,
   options: UseIntersectionObserverInit
-) {
+): () => void {
   const { id, observer, elements } = createObserver(options)
   elements.set(element, callback)
 
   observer.observe(element)
-  return function unobserve() {
+  return function unobserve(): void {
     elements.delete(element)
     observer.unobserve(element)
 
@@ -66,15 +71,8 @@ function observe(
   }
 }
 
-const observers = new Map<
-  string,
-  {
-    id: string
-    observer: IntersectionObserver
-    elements: Map<Element, ObserveCallback>
-  }
->()
-function createObserver(options: UseIntersectionObserverInit) {
+const observers = new Map<string, Observer>()
+function createObserver(options: UseIntersectionObserverInit): Observer {
   const id = options.rootMargin || ''
   let instance = observers.get(id)
   if (instance) {
diff --git a/packages/next/client/with-router.tsx b/packages/next/client/with-router.tsx
index 59f50c8dc26f8..217a65bd15c1d 100644
--- a/packages/next/client/with-router.tsx
+++ b/packages/next/client/with-router.tsx
@@ -17,7 +17,7 @@ export default function withRouter<
 >(
   ComposedComponent: NextComponentType<C, any, P>
 ): React.ComponentType<ExcludeRouterProps<P>> {
-  function WithRouterWrapper(props: any) {
+  function WithRouterWrapper(props: any): JSX.Element {
     return <ComposedComponent router={useRouter()} {...props} />
   }