How to add a type to the argument using typescript?

Question

Hi i have code like below

type HierarchyQuery = {
    Hierarchy: {
        data: Array<{
            name: string,
            description: string,
            children: Array<{
                name: string,
                description: string,
            }>
        }>
    } 
}

type HierarchyItem = NonNullable<
    HierarchyQuery['Hierarchy']['data'][0]>;
    

const flattenData = (data: HierarchyItem[] | HierarchyItem) => 
    {
        const result: HierarchyItem[] = [];
        if (isArray(data)) {
            data.forEach((item: HierarchyItem) => {
                result.push(item);
                if (item.children)  {
                   
                    result.push(flattenData(item.children)); 
                    //error here
                });
            }
        }
    }

but i get error when i call flattenData method on item.children.

argument of type {name: string, description: string, children: {}}[] is not assignable to {name: string, description: string, children: {}}

i am not sure how to fix this error. could someone help me with this. thanks.