flatDeep.ts

interface IDocument {
  id: number;
  documents: IDocument[];
}

const DOCUMENTS: IDocument[] = [{
  id: 100,
  documents: [
    {
      id: 1,
      documents: [
        {
          id: 2,
          documents: [
            {
              id: 3,
              documents: [
                {
                  id: 11,
                  documents: [
                    {
                      id: 12,
                      documents: [
                        {
                          id: 13,
                          documents: [
                            {
                              id: 14,
                              documents: [
                                {
                                  id: 15,
                                  documents: [
                                    {
                                      id: 16,
                                      documents: []
                                    }
                                  ]
                                }
                              ]
                            }
                          ]
                        }
                      ]
                    }
                  ]
                }
              ]
            },
            {
              id: 4,
              documents: []
            }
          ]
        }
      ]
    },
    {
      id: 5,
      documents: [
        {
          id: 6,
          documents: []
        }
      ]
    }
  ]
}, {
    id: 200,
    documents: [
      {
        id: 7,
        documents: [
          {
            id: 8,
            documents: [{
              id: 9,
              documents: []
            }]
          }
        ]
      },
      {
        id: 10,
        documents: []
      }
    ]
}]

const flatDeep = <T>(items: T[], key: string) => {
  const innerFlat = (item: T, arr = []): T[] =>
    arr.concat(item, ...(item[key] && item[key].length > 0 ? item[key].map(d => innerFlat(d)) : []));

  return items.map(d => innerFlat(d)).reduce((acc, curr) => [...acc, ...curr]);
};

const flattenedIds = flatDeep<IDocument>(DOCUMENTS, 'documents').map(({ id }) => id);

console.log(flattenedIds);