Helper function eg. is-empty needs to change type

I encountered a really weird issue in TS.

Imagine we have an optional value?: string then simple check if value exists

if (value == null) {
   return;
}
console.log(typeof value); // string

(we don’t care about empty string)

So it means this will ensure the compiler knows that type of optional variable is a string after our check.

If we introduce some helper function let’s say isEmpty

function isEmpty(value: any): boolean {
   return value == null;
}

And now if I create exact same setup as before instead of null check we will use our custom helper

if (isEmpty(value)) {
   return;
}
console.log(typeof value); // string | undefined

How can I modify that helper function in order to let the compiler know that value was already tested for possible undefined?
I know that its possible to use type casting but then it no longer make sense for me to introduce helper functions like this.

What are your ideas?

Thanks

>Solution :

Your function needs a type predicate.

function isEmpty(value: any): value is null {
   return value == null;
}

Playground