js判断undefined null Nan

留作备忘 

原地址http://www.cnblogs.com/imlucky/archive/2013/05/22/3093355.html

//判断undefined:
var tmp = undefined; 
if (typeof(tmp) == "undefined"){ 
alert("undefined"); 
}

//判断null:
var tmp = null; 
if (!tmp && typeof(tmp)!="undefined" && tmp!=0){ 
alert("null"); 
}

//判断NaN:
var tmp = 0/0; 
if(isNaN(tmp)){ 
alert("NaN"); 
}

//判断undefined和null://
var tmp = undefined; 
if (tmp== undefined) 
{ 
alert("null or undefined"); 
}

var tmp = undefined; 
if (tmp== null) 
{ 
alert("null or undefined"); 
}
//说明：null==undefined

//判断undefined、null与NaN://
var tmp = parseInt("sdfe"); 
if ((!tmp)&&(tmp!=0)) 
{ 
alert("null or undefined or NaN"); 
}