type KeysOfUnion<U> = U extends U ? keyof U : never;
See the related article keyof with unions if you haven’t already.
This type is a utility for extracting all keys of a union type.
Counterintuitively to some, keyof (A | B) does not give you keyof A | keyof B, but rather only the keys shared between the members of the union.
Sometimes, you can convert the union to an intersection, and then get the keys of that intersection, but this may break in some cases.
Distributing the union and then using keyof on each member of the union is much more effective and relible.
Here’s an example from the linked article:
type A = { foo: string; bar: number };
type B = { bar: boolean; baz: number };
type Keys = keyof (A | B); // "bar"
Instead, using KeysOfUnion:
type A = { foo: string; bar: number };
type B = { bar: boolean; baz: number };
type Keys = KeysOfUnion<A | B>; // "foo" | "bar" | "baz"