How can I restrict an interface type based on a generic parameter?

Given:

interface Foo<T> {
  data: T[] | SomeWrapper<T>
}

I want to change this so that data can only be an array if T = string. So if this syntax worked, I’d do

interface Foo<T> {
  data: T === typeof(string) 
          ? T[] | SomeWrapper<T> 
          : SomeWrapper<T>
}

I feel like there’s some solution with never here, but not sure how to construct it…

>Solution :

you can use conditional types for this.

interface Foo<T> {
  data: T extends string ? SomeWrapper<T>[] : SomeWrapper<T>
}

The syntax is similar as for ternary expressions and lets you define types according to some logic around what a given type extends. So a basic and clear example would be

type IsString<T> = T extends string ? true : false

Given the above type, IsString<"hi"> has type true and IsString<5> has type false.