Typescript type guard removes null or undefined union-types

Question

I have an object literal and want to write a type guard for the type of that specific object.

My object looks as follows:

const typeDictator = {
    value: undefined as number | undefined | string,
}

so if I'm not mistaken the type for my object should result in

{value: number | undefined | string}

but whenever I write a type guard for it like this:

function generic<T>(obj: unknown, type: T): obj is T {
    return true;
}

and check the type with

const typedObject = null;

if(generic(typedObject, typeDictator)) {
    typedObject
}

then the type of typedObject is reduced to {value: number | string}.

But why does it remove the undefined or null union-types and does it change anything relevant?

Answer 1

Try adding "strict": true, to your tsconfig. If your tsconfig is too permissive (or missing altogether), then null and undefined will be removed from types.