If
Challenge
Implement a utils If which accepts condition C, a truthy return type T,
and a falsy return type F. C is expected to be either true or false
while T and F can be any type.
For example:
type A = If<true, "a", "b">; // expected to be 'a'
type B = If<false, "a", "b">; // expected to be 'b'
Solution
If you are not sure when to use conditional types in TypeScript, it is when you need to use an “if” statement on types. Exactly what we tasked to do here.
If the condition type evaluates to true, we need to take a “true” branch,
otherwise “false” branch:
type If<C, T, F> = C extends true ? T : F;
Going that way we will get a compilation error, because we are trying to assign
C to boolean type and not having a constraint that shows that. So let us fix
it by adding extends boolean to the type parameter C:
type If<C extends boolean, T, F> = C extends true ? T : F;
Comments