Object tree mapper, executes map for every node in tree. Src: @mpjme, https://www.youtube.com/watch?v=2jp8N6Ha7tY

mapTree.js

const mapTree = (node,mapper) => {
  return {
    value: mapper(node.value),
    nodes: node.nodes
      ? node.nodes.map( x => 
        mapTree(x, mapper)
      )
      : null
  }
}