我可以在 DU 键上定义通用类型约束吗？

Question

有没有办法定义泛型类型约束，使得

//this will compile
type Contrained = StrongConstraint<"a" | "b" | "c", "a" | "b">

//this wont compile as "a" | "b" | "d" is not a strict subset of "a" | "b" | "c"
type Contrained = StrongConstraint<"a" | "b" | "c", "a" | "b" | "d">

这类似于Exclude只是更强，因为我不喜欢第二个参数中不属于第一个参数的键。

Answer 1

您可以强制第二个参数扩展第一个：

type ExcludeConstrained<T, U extends T> = Exclude<T, U>

type T1 = ExcludeConstrained<"a" | "b" | "c", "a" | "b">; // OK, T1 = 'c'

type T2 = ExcludeConstrained<"a" | "b" | "c", "a" | "b" | "d">; // ERROR
// -----------------------------------------> ~~~~~~~~~~~~~~~
// Error: Type '"d"' is not assignable to type '"a" | "b" | "c"'

Answer 2

我不认为你的方法是可能的（至少我不知道）。 但是，根据您的用例，您可以检查Partial 。

type StrongConstraint<T> = {
    left: T,
    right: Partial<T>,
}

// success
const foo: StrongConstraint<'a' | 'b' | 'c'> = {
    left: 'a',
    right: 'b'
}

// fail
const bar: StrongConstraint<'a' | 'b' | 'c'> = {
    left: 'a',
    right: 'd'
}