如何在 TypeScript 中继承 PromiseLike 接口？答案

【问题标题】：How to inherit PromiseLike interface in TypeScript?如何在 TypeScript 中继承 PromiseLike 接口？
【发布时间】：2020-01-29 18:17:15
【问题描述】：

我想扩展 PromiseLike 接口。

（我使用 TypeScript v3.6.3）

我取了原来的PromiseLike接口：

interface PromiseLike<T> {
    then<TResult1 = T, TResult2 = never>(
        onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null,
        onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null,
    ): PromiseLike<TResult1 | TResult2>
}

那我只继承 PromiseLike 并将嵌套的 PromiseLike 类型替换为 Thenable：

interface Thenable<T = any> extends PromiseLike<T> {
    then<TResult1 = T, TResult2 = never>(
        onfulfilled?: ((value: T) => TResult1 | Thenable<TResult1>) | undefined | null,
        onrejected?: ((reason: any) => TResult2 | Thenable<TResult2>) | undefined | null,
    ): Thenable<TResult1 | TResult2>
}

得到了这个错误：

Error:(28, 18) TS2430: Interface 'Thenable<T>' incorrectly extends interface 'PromiseLike<T>'.
  Types of property 'then' are incompatible.
    Type '<TResult1 = T, TResult2 = never>(onfulfilled?: (value: T) => TResult1 | Thenable<TResult1>, onrejected?: (reason: any) => TResult2 | Thenable<TResult2>) => Thenable<TResult1 | TResult2>' is not assignable to type '<TResult1 = T, TResult2 = never>(onfulfilled?: (value: T) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>'.
      Types of parameters 'onrejected' and 'onrejected' are incompatible.
        Type 'TResult2 | PromiseLike<TResult2>' is not assignable to type 'TResult2 | Thenable<TResult2>'.
          Type 'PromiseLike<TResult2>' is not assignable to type 'TResult2 | Thenable<TResult2>'.
            Type 'PromiseLike<TResult2>' is not assignable to type 'Thenable<TResult2>'.
              Types of property 'then' are incompatible.
                Type '<TResult1 = TResult2, TResult2 = never>(onfulfilled?: (value: TResult2) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>' is not assignable to type '<TResult1 = TResult2, TResult2 = never>(onfulfilled?: (value: TResult2) => TResult1 | Thenable<TResult1>, onrejected?: (reason: any) => TResult2 | Thenable<TResult2>) => Thenable<TResult1 | TResult2>'.
                  Types of parameters 'onrejected' and 'onrejected' are incompatible.
                    Type 'TResult2 | Thenable<TResult2>' is not assignable to type 'TResult2 | PromiseLike<TResult2>'.
                      Type 'Thenable<TResult2>' is not assignable to type 'TResult2 | PromiseLike<TResult2>'.
                        Type 'Thenable<TResult2>' is not assignable to type 'PromiseLike<TResult2>'.
                          Types of property 'then' are incompatible.
                            Type '<TResult1 = TResult2, TResult2 = never>(onfulfilled?: (value: TResult2) => TResult1 | Thenable<TResult1>, onrejected?: (reason: any) => TResult2 | Thenable<TResult2>) => Thenable<TResult1 | TResult2>' is not assignable to type '<TResult1 = TResult2, TResult2 = never>(onfulfilled?: (value: TResult2) => TResult1 | PromiseLike<TResult1>, onrejected?: (reason: any) => TResult2 | PromiseLike<TResult2>) => PromiseLike<TResult1 | TResult2>'.
                              Types of parameters 'onrejected' and 'onrejected' are incompatible.
                                Type 'TResult2 | PromiseLike<TResult2>' is not assignable to type 'TResult2 | Thenable<TResult2>'.
                                  Type 'PromiseLike<TResult2>' is not assignable to type 'TResult2 | Thenable<TResult2>'.

如何解决这个错误？

此外，我想像这样扩展这个接口：

export interface Thenable<T = any> extends PromiseLike<T> {
    then<TResult1 = T, TResult2 = never>(
        onfulfilled?: ((value: T) => TResult1 | ThenableOrIteratorOrValue<TResult1>) | undefined | null,
        onrejected?: ((reason: any) => TResult2 | ThenableOrIteratorOrValue<TResult2>) | undefined | null,
    ): ThenableOrIteratorOrValue<TResult1 | TResult2>
}

并且我希望它与 PromiseLike 保持兼容，以便异步等待功能与之兼容。

【问题讨论】：

您不能扩展接口并覆盖属性。请看stackoverflow.com/questions/41285211/…
为什么要扩展 PromiseLike 接口？你想达到什么目标？谢谢
我使用生成器（yield 而不是 await）实现了异步函数的同步版本。我使用 Iterator 类型作为同步 Promise。现在我想结合所有类似接口的承诺。
union 类型会有帮助吗？我不知道您的ThenableOrIteratorOrValue<T> 的代码。也许以下内容可以帮助您：type PromiseLikeOrThenable<T> = PromiseLike<T> | Thenable<T> | IteratorResult<T>;

标签： typescript promise

【解决方案1】：

保持与PromiseLike 兼容意味着您的Thenable 可以作为参数传递给需要PromiseLike 的代码，并且该代码仍然可以工作。为此，then 应该接受返回PromiseLike 的onfulfilled 回调，而不是您的Thenable - 因为这是使用PromiseLike 的代码将传递给您的then 实现的回调类型。

因此，将PromiseLike 添加到可以由回调返回的类型联合中，这样错误就会消失：

    onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null,

由于我不明白的原因，onrejected 是兼容的，显然是因为它的类型不依赖于泛型参数T。

【讨论】：