Конвертировать рекурсивный объект с двумя типами дочерних к типу с одним

interface Template {
    id: string,
    title: string,
    value: string,
    folderId: string,
    isDeleted: boolean
}

interface Folder {
    id: string,
    title: string,
    childfolders?: Folder[],
    childTemps?: Template[],
    parentFolderId?: string
}

interface TreeData {
    id: string,
    title: string,
    children: TreeData | undefined
}

type $convert = (f: Folder) => TreeData

const convert: $convert = f => {
    let res: TreeData, children: TreeData['children']
    const { id, title, childfolders } = f
    if (undefined !== childfolders) {
        // FIXME: childfolders[0]
        children = convert(childfolders[0])
    }
    res = {
        id,
        title,
        children,
    }
    return res
}