typescript - 为什么 Typescript 接口(interface)允许自引用？

Question

type SomeGeneric<T> = {
    x: T;
}

type TTest = SomeGeneric<TTest> & { a: string; }; // Type alias 'TTest' circularly references itself.

interface ITest extends SomeGeneric<ITest> { // OK
    a: string;
}

我不明白为什么允许接口(interface)在它们自己的声明中引用它们自己而类型却不允许。

Answer 1

类型别名不能递归(大多数情况下)，而接口(interface)可以。如果您搜索 GitHub，可以找到几个关于原因的答案。

例如 RyanCavanaugh 解释 here :

Type aliases and interfaces are subtly different and there are rules about self-recursion for aliases that don't apply to interfaces. Because an alias is supposed to always be "immediately expandable" (it's as if it were an in-place expansion of its referand), there are things you can do interfaces you can't do with type aliases.

或here作者:丹尼尔罗森瓦瑟

The restriction that a type alias can't be referenced by itself at the top level has been the behavior since we implemented type aliases; however, you might recall that a while back, we started allowing type aliases to be referenced from within an object type. ... However, it has no problem provided that the type can be expanded, or "unrolled" one level at a time. In other words, as long as you have some sort of box-y thing containing the type:

如果您需要递归类型，一般的解决方法是使用接口(interface)。

请注意，这些规则并没有完全一致地执行。例如，在您的情况下 SomeGeneric<TTest>可以扩展为 { x: TTest }编译器甚至不尝试它，它只是失败了。然而，以下递归定义将起作用:

type TTest = { x: TTest } & { a: string; };