JS中判断null、undefined与NaN的方法

1 var tmp = undefined; 
2 if (typeof(tmp) == "undefined"){ 
3 alert("undefined"); 
4 }

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

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

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

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

var tmp = null; 
if (!tmp) 
{ 
alert("null or undefined or NaN"); 
}