var o = { x: 'y' };
delete o.x; //此时o会成一个慢对象
o.x; //
 
var o = { x: 'y' };
o = null; //应该这样

var a = function() {
 var largeStr = new Array(1000000).join('x');
 return function() {
  return largeStr;
 }
}();

var select = document.querySelector;
var treeRef = select('#tree');
 
//在COM树中leafRef是treeFre的一个子结点
var leafRef = select('#leaf'); 
var body = select('body');
 
body.removeChild(treeRef);
 
//#tree不能被回收入，因为treeRef还在
//解决方法:
treeRef = null;
 
//tree还不能被回收，因为叶子结果leafRef还在
leafRef = null;
 
//现在#tree可以被释放了。

for (var i = 0; i < 90000; i++) {
 var buggyObject = {
  callAgain: function() {
   var ref = this;
   var val = setTimeout(function() {
    ref.callAgain();
   }, 90000);
  }
 }
 
 buggyObject.callAgain();
 //虽然你想回收但是timer还在
 buggyObject = null;
}