typescript - 为什么 TypeScript 在尝试从联合类型中读取 never 时不报错？

Question

我认为 TypeScript尝试阅读属性时会提示 length什么可能是array或 never ，然后，我需要使用类型谓词缩小类型范围。不是这种情况。为什么 ？我怎样才能更改类型以便 TypeScript 强制我检查我不是在处理空对象？

// strictNullChecks = true
type Empty = Record<string, never>;

type NonEmpty = {
  documents: Array<number>;
};

function isEmpty(x: Empty | NonEmpty): x is Empty {
  return !("documents" in x);
}

const count = (x: Empty | NonEmpty) => {
  // TypeScript does not complain about reading `length` property of `never`
  // if (isEmpty(x)) {
  //   return 0;
  // }
  return x.documents.length;
};

Answer 1

下面是一个逐步类型的销毁示例:

type Empty = Record<string, never>;

type NonEmpty = {
  documents: Array<number>;
};

function isEmpty(x: Empty | NonEmpty): x is Empty {
  return !("documents" in x);
}

const count = (x: Empty | NonEmpty) => {

  type D0 = typeof x.documents
  type D1 =  (Empty | NonEmpty)[`documents`]
  type D2 = Empty[`documents`] | NonEmpty[`documents`]
  type D3 =  never | NonEmpty[`documents`]
  type D4 =  NonEmpty[`documents`]
  type D5 =  Array<number>

  return x.documents.length; // x.documents is number[], that's why no error here
};

以下是可能符合您需要的预期行为示例:

type EmptyExpected = Record<string, undefined>;

const countExpected = (x: EmptyExpected | NonEmpty) => {

  type D0 = typeof x.documents
  type D1 =  (EmptyExpected | NonEmpty)[`documents`]
  type D2 = EmptyExpected[`documents`] | NonEmpty[`documents`]
  type D3 =  undefined | NonEmpty[`documents`]
  type D4 =  undefined | number[]

  return x.documents.length; // x.documents is number[] | undefined, that's why it is an error here
};

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