In the following example, in the getAdditionalData function, Typescript infers the type of value to never after validating the type through isBarType. I would expect the type to be FooEntity since Entity can be either FooEntity OR BarEntity.

Why is that the case?

I’m aware I can avoid the problematic by doing the opposite validation, i.e. if(isFooType(value)) { return value.additionalData;} but I’m interested to know why Typescript behaves like this.

Link to TS Playground

type BaseEntity = {
    id: string,
    name: string,
}

type FooEntity = BaseEntity & {
    additionalData: string,
}

type BarEntity = BaseEntity & {};

type Entity = FooEntity | BarEntity;

function isFooType(value: Entity): value is FooEntity {
    return value.name === 'foo';
}

function isBarType(value: Entity): value is BarEntity {
    return value.name === 'bar';
}

function getAdditionalData(value: Entity): string {
    if(isBarType(value)) {
        return '';
    }

    // Property 'additionalData' does not exist on type 'never'
    return value.additionalData;
}