Typescript枚举类型

Question

问题1：如下写法为何不报错呢？我理解变量 a 的值应该是 1 ~ 7?

enum Weekday {
  Monday = 1,
  Tuesday,
  Wednesday,
  Thursday,
  Friday,
  Saturday,
  Sunday,
}
//不报错
const a: Weekday = 100;

问题2：如下写法为何会报错呢？

enum Direction {
  Up = 'UP',
  Down = 'DOWN',
  Left = 'LEFT',
  Right = 'RIGHT',
}
//error：不能将类型“"DOWN"”分配给类型“Direction”
const b: Direction = 'DOWN';

问题3：那如何实现上面的这种需求呢？

Answer 1

这个问题上周研究了半天，大佬们回答的 断言 和 Direction.DOWN 取代 'DOWN' 也可以临时规避此问题。

此类问题在typescript github上有很多issue反馈，似乎是ts设计问题。

#32690中，提问者给出一种解决方法：

namespace MyEnum {
  export const Zero = 0;
  export type Zero = typeof Zero;

  export const One = 1;
  export type One = typeof One;
}
type MyEnum = typeof MyEnum[keyof typeof MyEnum];
const foo: MyEnum.Zero = 0 // okay as expected
const bar: MyEnum.Zero = 1 // error as expected

其它相关issues：#31834，#30629，#26362，#22464，#22311

Answer 2

- const a: Weekday = 100;
+ const a = 100 as Weekday;

- const b: Direction = 'DOWN';
+ const b = 'DOWN' as Direction;

我再补充一下，如果你想校验有效性可以这样：

if (a in Weekday) {}

if (b === Direction.Down) {}

Answer 3

你为啥不写的规范一点呢

const b: Direction =Direction.DOWN ;