Javascript 这在React代码库中起什么作用？

我在React代码库中看到以下内容：

export type TypeOfWork = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10;

它在干什么

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来源：

这是flow或typescript语法

它定义的自定义类型只能是0到10之间的数字

export type TypeOfWork = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10;
let x: TypeOfWork = 0;

x = 12; // <-- compiler would throw an error here

正如你在照片中看到的那样，哪一个会缩小得更好

export const enum TypeOfWork {
  IndeterminateComponent = 0, // Before we know whether it is functional or class
  FunctionalComponent = 1,
  ClassComponent = 2,
  HostRoot = 3, // Root of a host tree. Could be nested inside another node.
  HostPortal = 4, // A subtree. Could be an entry point to a different renderer.
  HostComponent = 5,
  HostText = 6,
  CoroutineComponent = 7,
  CoroutineHandlerPhase = 8,
  YieldComponent = 9,
  Fragment = 10
};