//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");

}