TypeScript 属性在 typeof 枚举类型上不存在，在检查它确实存在之后答案

【问题标题】：TypeScript Property does not exist on type typeof enum, after checking that it definitely existsTypeScript 属性在 typeof 枚举类型上不存在，在检查它确实存在之后
【发布时间】：2021-05-06 15:10:45
【问题描述】：

在下面的例子中，一个角色可以执行几个动作，但是，我们只有一些动作的动词。

我们在返回之前检查动词是否存在。

TypeScript 不会接受动词存在，即使我们已经在 Enum 中检查了它。

type Action =
| 'WALK'
| 'RUN'
| 'SLEEP'
| 'SWIM';

enum Verbs {
  'WALK' = 'walking',
  'RUN' = 'running',
  'SWIM' = 'swimming',
}

const getVerb = (character: string, action: Action): string => {
  if (action in Verbs) {
    // Property 'SLEEP' does not exist on type 'typeof Verbs'.ts(7053) ─┐
    const  verb = Verbs[action]; // <───────────────────────────────────┘
    return `${character} is ${verb}`;
  }
  return `${character} is doing something weird`;
}

示例代码框： https://codesandbox.io/s/nostalgic-grass-7vrpt?file=/src/index.ts

我已经通过创建PartialRecord 以Action 为键来解决它，但最好了解它为什么不适用于Enum

const verbs: Partial<Record<Action, string>> = {
  'WALK': 'walking',
  'RUN': 'running',
  'SWIM': 'swimming',
};
const getVerb = (character: string, action: Action): string => {
  if (action in verbs) {
    return `${character} is ${verbs[action]}`; // Works ????
  }
  return `${character} is doing something weird`;
};

【问题讨论】：

标签： typescript enums

【解决方案1】：

您必须使用类型谓词。它看起来像这样：

type Action =
| 'WALK'
| 'RUN'
| 'SLEEP'
| 'SWIM';

enum Verbs {
  'WALK' = 'walking',
  'RUN' = 'running',
  'SWIM' = 'swimming',
}

const getVerb = (character: string, action: Action): string => {
  if (isVerb(action)) {
    const  verb = Verbs[action];
    return `${character} is ${verb}`;
  }
  return `${character} is doing something weird`;
}

// predicate here
const isVerb = (action: Action): action is keyof typeof Verbs => {
  return action in Verbs; 
}

const character = 'Gom';
const action: Action = 'RUN';
const message = getVerb(character, action);
console.log(message);

TypeScript playground

【讨论】：