recursiveReducer2.js

function flattenFolderTree(folders) {
  return folders.reduce((acc, folder) => {
    const {children, ...rest} = folder;

    if (!children || !children.length) {
      return [...acc, rest];
    }

    return [...acc, rest, ...flattenFolderTree(children)];
  }, []);
}

Something I have been considering when using reducers is to expose the reducer itself as the tool.

function flattenFolderTreeReducer(acc, folder) {
  const {children, ...rest} = folder;

  if (!children || !children.length) {
    return [...acc, rest];
  }

  return [...acc, rest, ...children.reduce(flattenFolderTreeReducer, [])];
}

// use it with something like this:
folders.reduce(flattenFolderTreeReducer, [])

The only benefit to this is it would let someone dot chain off the Array.