Typescript 文字联合

Question

I have two literal types, and I want to exclude B from union A.

type A = 'a' | 'b'
type B = 'a'

When I do it like this:

type MyExclude<A2, B2> = A2 extends B2 ? never : A2
type C2 = MyExclude<A, B>
// C2 = 'b'

It works correctly, ie i get 'b'.

But when I do it directly like this:

type C = A extends B ? never : A
// C = 'a' | 'b'

It doesn't. Why should it matter whether I do it via a intermediary generic type or directly?

Thanks.

Answer 1

When you do it as a generic, Typescript is going to apply the check distributively to each member of the union. So type C2 = MyExclude<A, B> is going to be evaluated not as 'a | b' extends 'a'? never: 'a | 'b' 'a | b' extends 'a'? never: 'a | 'b' 'a | b' extends 'a'? never: 'a | 'b' , but rather ('a' extends 'a'? never: 'a') | ('b' extends 'a'? never: 'b') ('a' extends 'a'? never: 'a') | ('b' extends 'a'? never: 'b') . 'a' does extend B, so that results in never . 'b' does not extend B, so that results in 'b'. And the final result is never | 'b' never | 'b' , which is just 'b' .

The distributive part does not happen if it's not a generic. type C = A extends B? never: A type C = A extends B? never: A is just going to be evaluated once, as 'a | b' extends 'a'? never: 'a | 'b' 'a | b' extends 'a'? never: 'a | 'b' 'a | b' extends 'a'? never: 'a | 'b' . 'a' | 'b' 'a' | 'b' does not extend 'a' , so the result is the whole type, 'a' | 'b' 'a' | 'b'

See distributive conditional types

Distributive conditional types

Conditional types in which the checked type is a naked type parameter are called distributive conditional types. Distributive conditional types are automatically distributed over union types during instantiation. For example, an instantiation of T extends U? X: Y T extends U? X: Y with the type argument A | B | C A | B | C A | B | C for T is resolved as (A extends U? X: Y) | (B extends U? X: Y) | (C extends U? X: Y) (A extends U? X: Y) | (B extends U? X: Y) | (C extends U? X: Y) (A extends U? X: Y) | (B extends U? X: Y) | (C extends U? X: Y) .