IsEqual util type in Typescript doesn't work as expected [duplicate]

Question

I write a IsEqual generic type like that :

type IsEqual<T, U> = T extends U 
  ? U extends T 
    ? true
    : false
  : false

type Test1 = IsEqual<number, 3>  // false, ok

type Test2 = IsEqual<true, false> // false, ok

type Test3 = IsEqual<boolean, true> // boolean -> ? 

type Test4 = IsEqual<true, boolean> // boolean -> ? 

What happens here ?

And how to reliably check if a type is the exact same as another ? (not a subtype)

Answer 1

You need to wrap T and U in brackets in the body of the typedef, i.e:

type IsEqual<T, U> = [T] extends [U] 
  ? [U] extends [T]
    ? true
    : false
  : false

Otherwise, TypeScript will distribute the types and evaluate IsEqual<boolean, true> as IsEqual<true, true> | IsEqual<false, true>. Source: this github comment.