为什么打字稿类型保护不适用于对象的内部属性

Question

使用打字稿，当一个函数 return 是一个具有可能为 null 的属性的对象时。为什么在这些内部属性上使用类型保护不允许打字稿推断保护后的内部属性不能为空？

这是一个最简单的例子。 Try it

interface DatabaseResponse {
  settings: string | null
}
interface MainResponse {
  settings: string
}

const retrieveFromDatabase = (): DatabaseResponse => {
  return {
    settings: 'always a string but lets pretend it could be null sometimes'
  }
}

const main = (): MainResponse | Error => {
  const data = retrieveFromDatabase()

  if (data.settings === null) {
    throw new Error()
  }

  return data

}

主函数return的错误是

Type 'DatabaseResponse' is not assignable to type 'MainResponse | Error'.
  Type 'DatabaseResponse' is not assignable to type 'MainResponse'.
    Types of property 'settings' are incompatible.
      Type 'string | null' is not assignable to type 'string'.
        Type 'null' is not assignable to type 'string'.

Answer 1

那不是真正的类型保护。您可以这样做：

interface DatabaseResponse {
    settings: string | null
}
interface MainResponse {
    settings: string
}

const retrieveFromDatabase = (): DatabaseResponse => {
    return {
        settings: 'always a string but lets pretend it could be null sometimes'
    } as DatabaseResponse
}

const main = (): MainResponse | Error => {
    const data = retrieveFromDatabase()

    if (!isMainResponse(data)) {
        throw new Error()
    }

    return data
}

const isMainResponse = (data: DatabaseResponse | MainResponse): data is MainResponse {
  return !!data.settings;
}

或者类似地，创建一个具有相同类型保护的新的非 Nullable 类型

type NonNullableDB = { [K in keyof DatabaseResponse]: NonNullable<DatabaseResponse[K]> }

const isMainResponse = (data: DatabaseResponse | MainResponse): data is NonNullableDB => {
    return !!data.settings;
}

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