var nodeArray = "";
var nodesObj = mytree.getChecked();

var i;var length=nodesObj .length;
for (i = 0; i < length; i++) {
nodeArray += nodesObj[i].id + ',';  //允许多选的情况。如果不允许多选，就直接nodesObj[i].id 
}

nodeArray = nodeArray.substring(0, nodeArray.length - 1); 

[{
"text": "To Do", 
"cls": "folder",
"expanded": true,
"children": [{
"text": "Go jogging",
"leaf": true,
"checked": true
},{
"text": "Take a nap",
"leaf": true,
"checked": false
},{
"text": "Climb Everest",
"leaf": true,
"checked": false
}]
},{
"text": "Grocery List",
"cls": "folder",
"children": [{
"text": "Bananas",
"leaf": true,
"checked": false
},{
"text": "Milk",
"leaf": true,
"checked": false
},{
"text": "Cereal",
"leaf": true,
"checked": false
},{
"text": "Energy foods",
"cls": "folder",
"children": [{
"text": "Coffee",
"leaf": true,
"checked": false
},{
"text": "Red Bull",
"leaf": true,
"checked": false
}]
}]
},{
"text": "Remodel Project", 
"cls": "folder",
"children": [{
"text": "Finish the budget",
"leaf": true,
"checked": false
},{
"text": "Call contractors",
"leaf": true,
"checked": false
},{
"text": "Choose design",
"leaf": true,
"checked": false
}]
}]