dom traversal with callback option on each node - http://jsbin.com/giluzi/edit

domTraversal.js

// http://jsbin.com/giluzi/edit

const traverseElement = (node,callback) => {
    
    let level = 0;
    
    const traverse = (node) => {
        
      if(!node.nodeName) return;

      level++;

      if(callback) callback(node,level);

      for(let child in node.children){
        traverse(node.children[child]);
      }
        
      level--;
    }
    traverse(node);
}

traverseElement(document.getElementById('root'),
    (node,level) => {
      console.log(level + ': ' + node.nodeName);
    }
);