Hooks 源码：useMemo

function mountMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null
): T {
  const hook = mountWorkInProgressHook()
  const nextDeps = deps === undefined ? null : deps
  const nextValue = nextCreate()
  hook.memoizedState = [nextValue, nextDeps]
  return nextValue
}

function updateMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null
): T {
  const hook = updateWorkInProgressHook()
  const nextDeps = deps === undefined ? null : deps
  const prevState = hook.memoizedState
  if (prevState !== null) {
    // Assume these are defined. If they're not, areHookInputsEqual will warn.
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1]
      if (areHookInputsEqual(nextDeps, prevDeps)) {
        return prevState[0]
      }
    }
  }
  const nextValue = nextCreate()
  hook.memoizedState = [nextValue, nextDeps]
  return nextValue
}