Как начать понимать спецификации JavaScript?

var t = { 
  header: 'Custom' 
} 
// create trees to drag/drop between 
var tvDragDrop1 = new wijmo.nav.TreeView('#tvDragDrop1', { 
  displayMemberPath: 'header', 
  childItemsPath: 'items', 
  allowDragging: true, 
  dragOver: dragOverBetweenTrees, 
  itemsSource: [t, { 
      header: 'Item 1.1' 
    }, 
    { 
      header: 'Item 1.2' 
    }, 
    { 
      header: 'Item 1.3' 
    }, 
  ] 
}); 
var tvDragDrop2 = new wijmo.nav.TreeView('#tvDragDrop2', { 
  displayMemberPath: 'header', 
  childItemsPath: 'items', 
  allowDragging: true, 
  dragOver: dragOverBetweenTrees, 
  itemsSource: [] 
}); 
 
// allow drag/drop between tvDragDrop1 and tvDragDrop2 
function dragOverBetweenTrees(s, e) { 
  var t1 = e.dragSource.treeView, 
    t2 = e.dropTarget.treeView; 
  if (t1 == tvDragDrop1 || t1 == tvDragDrop2) { 
    if (t2 == tvDragDrop1 || t2 == tvDragDrop2) { 
      e.cancel = false; 
    } 
  } 
} 
 
function getjson() { 
  var nodes = tvDragDrop2.nodes; 
  var data = nodes.map(function recursiveNodes(node) { 
    return node.dataItem; 
  }); 
  console.log(JSON.stringify(data, null, 2)); 
}

.short { 
  min-height: 30px; 
  border: 1px solid red !important; 
}

<!-- latest Wijmo version --> 
<script src="https://cdn.grapecity.com/wijmo/5.latest/controls/wijmo.min.js"></script> 
<script src="https://cdn.grapecity.com/wijmo/5.20181.436/controls/wijmo.nav.min.js"></script> 
<link href="https://cdn.grapecity.com/wijmo/5.latest/styles/wijmo.min.css" rel="stylesheet" /> 
<div class="short" id="tvDragDrop1"></div> 
<div class="short" id="tvDragDrop2"></div> 
 
<input type="button" onclick="getjson()" value="click" />