typescript 使用带有 `typeof T` 的泛型类型参数
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Using a generic type argument with `typeof T`
提问by Aaron Beall
I have a factory-like function that is meant to return an sub-class instance of BaseApi. It currently looks something like this (trimmed out irrelevant parts):
我有一个类似工厂的函数，用于返回BaseApi. 它目前看起来像这样（修剪掉不相关的部分）：
function getApi<T extends BaseApi>(apiClass: typeof BaseApi): T {
return new apiClass();
}
And I use it like this:
我像这样使用它：
const someApi = getApi<SomeApi>(SomeApi);
This works, but I would like <SomeApi>to be inferred by virtue of the fact that I'm passing the SomeApiconstructor in. If I omit <SomeApi>then someApiis inferred to be of type BaseApi, not SomeApi. Worse, there's really no compiler correlation between <T extends BaseApi>and typeof BaseApibeing the same thing, so you can incorrectly do something like getApi<SecondApi>(FirstApi)without a compiler error.
这是有效的，但我想<SomeApi>根据我正在传递SomeApi构造函数的事实来推断。如果我省略，<SomeApi>则someApi推断为类型BaseApi，而不是SomeApi。更糟的是，真的没有编译器之间的相关性<T extends BaseApi>，并typeof BaseApi为同样的事情，这样你就可以正确地做这样的事情getApi<SecondApi>(FirstApi)没有一个编译器错误。
So I tried defining the apiClassas typeof T:
所以我尝试将其定义apiClass为typeof T：
function getApi<T extends BaseApi>(apiClass: typeof T): T {
return new apiClass();
}
And I found that TS did not understand this usage of T. Is there any way to do this?
而且我发现 TS 不理解T. 有没有办法做到这一点？
回答by Ryan Cavanaugh
What you actually want is new() => T, since you intend to use the argument as a constructor and produce a T. Even if you could write typeof T, that wouldn't be what you want, since Tmight not have a zero-argument constructor.
您真正想要的是new() => T，因为您打算将参数用作构造函数并生成T. 即使您可以编写typeof T，那也不是您想要的，因为T可能没有零参数构造函数。
Remember that the typeofoperator takes a valueand produces a value. Tis already a type; it is not a value.
请记住，typeof运算符接受一个值并产生一个值。T已经是一个类型；它不是一个值。
Naturally, this is addressed in the TypeScript FAQ https://github.com/Microsoft/TypeScript/wiki/FAQ#why-cant-i-write-typeof-t-new-t-or-instanceof-t-in-my-generic-function
自然，这在 TypeScript 常见问题解答https://github.com/Microsoft/TypeScript/wiki/FAQ#why-cant-i-write-typeof-t-new-t-or-instanceof-t-in-my 中得到解决-通用功能
