function demo(){
alert(typeof a == "undefined");//true
}
function demo(){
alert(a == null);//console error a is not defined
}
function demo(a){
alert(typeof a == "undefined");//true
alert(a == null);//true
}
function demo(){
var a;
alert(typeof a == "undefined");//true
alert(a == null);//true
}
function demo(){
var a=document.getElementById("name").value;//this node exists! but it has no value
alert(a == "");//true
alert(a == null);//false
alert(typeof a == "undefined");//false
}
function demo(){
var a=document.getElementById("name").value;//this node is not here
alert(a == "");//false
alert(a == null);//true
alert(typeof a == "undefined");//false
}
function(){
alert(null == undefined);//true
//使用typeof方法在前面已经讲过，null与undefined的类型是不一样的，所以输出"false"。而===代表绝对等于，在这里null === undefined输出false。
alert(null === undefined);//false
alert(typeof null == typeof undefined); //false
}
javascript 中null，undefined和空的应用
原文：http://my.oschina.net/hcliu/blog/469025