What are route loaders in TanStack Router and how do they integrate with TanStack Query?

Route loaders run before the component renders. Using queryClient.ensureQueryData() inside a loader pre-populates the Query cache so the component renders synchronously with no loading spinner.

import { createRoute }  from "@tanstack/vue-router"
import { queryClient }  from "../queryClient"
import { userKeys }     from "../queries/userKeys"

export const userDetailRoute = createRoute({
  getParentRoute: ()=>rootRoute,
  path:           "/users/$userId",
  component:      UserDetailPage,
  loader: async ({ params })=>{
    await queryClient.ensureQueryData({
      queryKey: userKeys.detail(Number(params.userId)),
      queryFn:  ()=>fetchUser(Number(params.userId)),
    })
    // Component will find data already in cache — renders instantly
  },
})

// In UserDetailPage.vue
import { useParams } from "@tanstack/vue-router"
import { useQuery }  from "@tanstack/vue-query"
const { userId } = useParams({ from:"/users/$userId" })
const { data:user } = useQuery({
  queryKey: userKeys.detail(Number(userId)),
  queryFn:  ()=>fetchUser(Number(userId)),
})

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