function getChildNode (currentNode, targetId) {
  if (currentNode.id === targetId) return currentNode

  for (let child of currentNode.children) {
    if (targetId.indexOf(child.id) !== -1) {
      return getChildNode(child, targetId)
    }
  }
}
Returns the element identified by targetId, which could be the
currentNode itself, one of its children... (and down to any depth)

@param {object} currentNode : The element being inspected
@param {string} targetId : The id of the element expected

@return {object} : The element identified by targetId