如何通过布尔值输入守卫？

Question

我有以下几点：

exampleFunction = (param: string) => {
  try {
    const result = processString(param) as { resultExample: string };
    
    return { isSuccess: true, ...result };
  catch (error) {
    return { isSuccess: false };
  }
}

理想情况下，返回的类型为：{ isSuccess: true, resultExample: string} | { isSuccess: false }，但它的类型为 { isSuccess: boolean, resultExample: string } | { isSuccess: boolean }，这令人沮丧，因为这意味着我无法利用类型保护。

const data = exampleFunction("exampleParam");

if (data.isSuccess) {
  functionExpectingResultExample(data.resultExample); // Type Error because "resultExample: string | {}" is not assignable to "resultExample: string"
}

以上内容不应出错，因为 true 和 false 应用作类型保护，而是将它们键入为布尔值。

我唯一的解决方案是手动输入真假吗？

Answer 1

您需要将 isSuccess 返回类型转换为 const，以便 TypeScript 知道该值不是布尔值，而是特定的、不变的布尔值：

const exampleFunction = (param: string) => {
  try {
    const result = processString(param) as { resultExample: string };
    
    return { isSuccess: true as const, ...result };
  } catch (error) {
    return { isSuccess: false as const };
  }
}

查看proof-of-concept on TypeScript Playground。

---

如果您愿意，也可以将整个返回的对象转换为 const：

const exampleFunction = (param: string) => {
  try {
    const result = processString(param) as { resultExample: string };
    
    return { isSuccess: true , ...result } as const;
  } catch (error) {
    return { isSuccess: false } as const;
  }
}

See the second proof-of-concept here.