gistfile1.txt

type APIWithExtra = {
    hasExtra: true;
    field: string;
    optionalField: string;
}
type APINoExtra = {
    hasExtra: false;
    field: string;
}

type API<T extends boolean> =
    T extends true ? APIWithExtra : APINoExtra;

function getAPI(addExtra: boolean): API<typeof addExtra> {
    if (addExtra) {
        const result : API<typeof addExtra> = {
            hasExtra: true,
            field: "bdbf",
            optionalField: "sda"
        };
        return result;

    } else {
        const result : API<typeof addExtra> = {
            hasExtra: false,
            field: "bdbf"
        };
        return result
    }
}

const a : boolean & number = true;
const x : (typeof a) extends (true | false) ? number : never = 5;
function getAPI2<T extends true | false>(addExtra: T): API<typeof addExtra> {
    if (addExtra) {
        const result : API<typeof addExtra> = {
            hasExtra: true,
            field: "bdbf",
            optionalField: "sda"
        };
        return result;

    } else {
        const result : API<typeof addExtra> = {
            hasExtra: false,
            field: "bdbf"
        };
        return result
    }
}