code: function nextLeft(node) {
      var children = node.children;
      return children.length && node.isExpand ? children[0] : node.hierNode.thread;
  }
docs: This function is used to traverse the left contour of a subtree (or a subforest).
  It returns the leftmost child of node or the thread of node. The function
  returns null if and only if node is on the highest depth of its subtree.
  
  @param  {module:echarts/data/Tree~TreeNode} node
  @return {module:echarts/data/Tree~TreeNode}