const group = await fetchGroup(nextId); // (id: string) => Promise<Group>
        //    ^^^^^ 'group' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer. (7022)

interface Group {
    parentId: string | undefined
    //...
}

declare function fetchGroup(id: string): Promise<Group>

async function fetchAncestorGroups(leafId: string): Promise<Group[]> {
    const groups: Group[] = [];
    let nextId: string | undefined = leafId;

    while (nextId) {
        const group = await fetchGroup(nextId);
        //    ^^^^^ 'group' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer. (7022)
        groups.push(group);
        nextId = group.parentId;
    }
    return groups.reverse();
}