Typescript `ensureArray` 辅助泛型类型

Question

我想创建一个辅助函数来确保值被包装在数组中。

如果值为 null 或 undefined return 空数组
如果值为数组，则保持原样
否则将值包装在数组中

我不明白的是 typescript 抱怨什么，如何修复它以及有没有办法获得更具描述性的错误消息：

function ensureArray<T>(
  value: T,
): T extends undefined | null ? [] : T extends Array<any> ? T : T[] {
  if (value === undefined || value === null) {
    // Type '[]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.(2322)
    return []; // should I just use as any?
  }

  // Type '(T & any[]) | [T]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.
  //  Type 'T & any[]' is not assignable to type 'T extends null | undefined ? [] : T extends any[] ? T : T[]'.(2322)
  return Array.isArray(value) ? value : [value];
}


const x1: number[] = ensureArray(1)
const x2: number[] = ensureArray([1, 2])
const x3: string[] | [] = ensureArray('str' as string | null)

Answer 1

您不需要条件类型！

function ensureArray<T>(
  value: T | Array<T> | undefined | null,
): T[] {
  if (value === undefined || value === null) {
    return []; // should I just use as any?
  }

  return Array.isArray(value) ? value : [value];
}


const x1: number[] = ensureArray(1)
const x2: number[] = ensureArray([1, 2])
const x3: string[] = ensureArray('str' as string | null)

Playground

Typescript `ensureArray` 辅助泛型类型

Typescript `ensureArray` helper generic type

typescript