TypeScript doesn't check return type when use generic

Issue

say we have the following code :

class Foo<T> {
  lift<R>(): Foo<R> {
    const bar = new Foo<T>()
    return bar
  }
}

why there is no error? Foo<T> and Foo<R> may be different type.

playground

Solution

TS uses structural type matching and not nominal. This means that 2 types are compatible if their structure is compatible.

You can read about how matching happens for generics here.

The reason your example works is that from a structure point of view there isn't anything that differentiates between Foo<string> or Foo<number> for instance. This is gonna work in TS

const a: Foo<string> = new Foo<number>()

This is also the reason why your function works, because Foo<TypeA> is compatible with Foo<TypeB> for any TypeA and TypeB.

If you add anything else that use T then the structural compatibility is broken and you get the error you are expecting:

class Foo<T> {
  field: T | null = null

  lift<R>(): Foo<R> {
    const bar = new Foo<T>()
    return bar
  }
}

Type 'Foo<T>' is not assignable to type 'Foo<R>'.
  Type 'T' is not assignable to type 'R'.
    'R' could be instantiated with an arbitrary type which could be unrelated to 'T'.(2322)

You can see this in action here

Answered By - Radu Diță