fix(nuxt): make `onNuxtReady` safe to run on server-side (#18706)

docs/3.api/3.utils/on-nuxt-ready.md

@@ -17,3 +17,7 @@ export default defineNuxtPlugin(() => {
 ```
 
 It is 'safe' to run even after your app has initialized. In this case, then the code will be registered to run in the next idle callback.
+
+::alert
+`onNuxtReady` only runs on the client-side.
+::

packages/nuxt/src/app/compat/idle-callback.ts

@@ -1,7 +1,7 @@
 // Polyfills for Safari support
 // https://caniuse.com/requestidlecallback
 export const requestIdleCallback: Window['requestIdleCallback'] = process.server
-  ? undefined as any
+  ? (() => {}) as any
   : (globalThis.requestIdleCallback || ((cb) => {
       const start = Date.now()
       const idleDeadline = {
@@ -12,5 +12,5 @@ export const requestIdleCallback: Window['requestIdleCallback'] = process.server
     }))
 
 export const cancelIdleCallback: Window['cancelIdleCallback'] = process.server
-  ? null as any
+  ? (() => {}) as any
   : (globalThis.cancelIdleCallback || ((id) => { clearTimeout(id) }))

packages/nuxt/src/app/composables/ready.ts

@@ -2,6 +2,8 @@ import { useNuxtApp } from '../nuxt'
 import { requestIdleCallback } from '../compat/idle-callback'
 
 export const onNuxtReady = (callback: () => any) => {
+  if (process.server) { return }
+
   const nuxtApp = useNuxtApp()
   if (nuxtApp.isHydrating) {
     nuxtApp.hooks.hookOnce('app:suspense:resolve', () => { requestIdleCallback(callback) })