如何检查对象是否是 TypeScript 中的只读数组？

Question

如何使用只读数组 (ReadonlyArray) 进行数组检查（如 Array.isArray()）？

举个例子：

type ReadonlyArrayTest = ReadonlyArray<string> | string | undefined;

let readonlyArrayTest: ReadonlyArrayTest;

if (readonlyArrayTest && !Array.isArray(readonlyArrayTest)) {
  // Here I expect `readonlyArrayTest` to be a string
  // but the TypeScript compiler thinks it's following:
  // let readonlyArrayTest: string | readonly string[]
}

对于通常的数组，TypeScript 编译器正确识别它必须是 if 条件内的字符串。

Answer 1

Here's 打字稿中的相关问题。

@jcalz 建议的解决方法是在 isArray 的声明中添加重载：

declare global {
    interface ArrayConstructor {
        isArray(arg: ReadonlyArray<any> | any): arg is ReadonlyArray<any>
    }
}

Answer 2

应该是这样的

interface ArrayConstructor {
  isArray(arg: unknown): arg is unknown[] | readonly unknown[];
}

并在打字稿中进行测试

const a = ['a', 'b', 'c'];
if (Array.isArray(a)) {
  console.log(a); // a is string[]
} else {
  console.log(a); // a is never
}

const b: readonly string[] = ['1', '2', '3']

if (Array.isArray(b)) {
  console.log(b); // b is readonly string[]
} else {
  console.log(b); // b is never
}

function c(val: string | string[]) {
  if (Array.isArray(val)) {
    console.log(val); // val is string[]
  }
  else {
    console.log(val); // val is string
  }
}

function d(val: string | readonly string[]) {
  if (Array.isArray(val)) {
    console.log(val); // val is readonly string[]
  }
  else {
    console.log(val); // val is string
  }
}

function e(val: string | string[] | readonly string[]) {
  if (Array.isArray(val)) {
    console.log(val); // val is string[] | readonly string[]
  }
  else {
    console.log(val); // val is string
  }
}

如何检查对象是否是 TypeScript 中的只读数组？

How to check if an object is a readonly array in TypeScript?

arrays

typescript

typechecking