Question

我有一个带字符串的类型保护，我想知道它是否是联合类型的一部分，但是如果我们在联合类型中添加新的字符串，我不想通过也添加新的字符串来管理我的类型保护： / p>

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如果我在联合类型“ GreatAnimal”中添加Fish，则不需要更新我的类型防护来管理Fish。

我使用的是最后的打字稿版本

Answer 1

const GreatAnimalList = ['Dog', 'Cat', 'Fish'] as const; // TS3.4 syntax
type GreatAnimal = typeof GreatAnimalList[number]; // 'Dog'|'Cat'|'Fish';

function isGreatAnimal(pet: string): pet is GreatAnimal {
  // return GreatAnimalList.includes(pet)
  return GreatAnimalList.indexOf(pet as GreatAnimal) !== -1
}

function Foo(animal: string) {
  if (isGreatAnimal(animal)) {
    // do something
    console.log('isGreatAnimal true')
  }
  else {
    // do something else
    console.log('isGreatAnimal false')
  }
}

Foo('Dog');      // isGreatAnimal true
Foo('Turtle');   // isGreatAnimal false

明天，如果您将新动物添加到GreatAnimalList中，它将被自动添加到联合类型GreatAnimal中。无需更改isGreatAnimal()函数。

Answer 2

您不能以编程方式使用类型，因为在已转译的JavaScript代码中将不存在它们。

例如代码的第一部分

type GreatAnimal = 'Dog' | 'Cat'

function isGreatAnimal(pet: string): pet is GreatAnimal {
    return pet === 'Dog' || pet === 'Cat';
}

转换为

function isGreatAnimal(pet) {
    return pet === 'Dog' || pet === 'Cat';
}

因此，您需要对GreatAnimals进行一些编程引用才能隐式检查它们。有很多方法可以做到这一点-一个例子就是枚举（将其转换为简单的对象）：

enum GreatAnimal {
  DOG = "Dog",
  CAT = "Cat"
}

function isGreatAnimal(pet: string): pet is GreatAnimal {
  return pet in Object.values(GreatAnimal);
}

function Foo(animal: string) {
  if (isGreatAnimal(animal)) {
    // do something
  } else {
    // do something else
  }
}

（请注意，此示例使用Object.values，它是ES2017的一部分。InternetExplorer从v7开始不支持它，并且节点也支持它）。

如何检查我的字符串是联合类型的一部分

2 个答案: