tree.js

var arr = [
  {id:1, pid:0, name:"SYSTEM"},
  {id:2, pid:1, name:"aa"},
  {id:3, pid:2, name:"aaa"},
  {id:4, pid:2, name:"b"},
  {id:5, pid:0, name:"c"},
  {id:6, pid:5, name:"cc"}
];

function generateTree(arr) {
  var root = {
    name:'root',
    items: [],
    expanded: true
  };
  if (arr.length > 0) {
    var objMap = {};
    arr.forEach(function(item){
      var node = {name: item.name};
      objMap[item.id] = node;
      if(item.pid === 0) {
        root["items"].push(node);
      } else {
        var parent = objMap[item.pid];
        parent["expanded"] = true;
        parent["items"] = parent["items"] || [];
        parent["items"].push(node);
      }
    });
  }
  return [root];
}

var json = JSON.stringify(generateTree(arr), null, 2);

document.getElementById("result").innerHTML = json;