Recursive Reducer done imperatively

justUseAForLoop.js

function flattenFolderTree(folders, gatheringArray = []) {
  folders.forEach(folder => {
    const {children, ...rest} = folder

    gatheringArray.push(rest)

    if (children && children.length) {
      flattenFolderTree(children, gatheringArray)
    }
  })

  return gatheringArray
}