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How do you implement optimistic updates with TanStack Query?

Optimistic updates change the UI immediately before the server responds. On error the UI rolls back to the snapshot saved in onMutate.

const { mutate:toggleTodo }=useMutation({
  mutationFn: (todo:Todo)=>
    fetch(`/api/todos/${todo.id}`,{
      method:"PATCH",
      body:JSON.stringify({done:!todo.done}),
    }).then(r=>r.json()),

  onMutate: async (todo)=>{
    // Cancel in-flight fetches so they don't overwrite our update
    await qc.cancelQueries({queryKey:["todos"]})
    const prev=qc.getQueryData<Todo[]>(["todos"])
    qc.setQueryData<Todo[]>(["todos"],
      old=>old?.map(t=>t.id===todo.id?{...t,done:!t.done}:t)??[])
    return { prev }
  },

  onError: (_,__,ctx)=>{
    if(ctx?.prev) qc.setQueryData(["todos"],ctx.prev)
  },

  onSettled: ()=>{ qc.invalidateQueries({queryKey:["todos"]}) },
})

Why call cancelQueries inside onMutate?
What must onMutate return for rollback to work?

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