JavaScript에서 정의되지 않은, null, NaN 등의 차이점에 대한 사용 예를 자세히 설명합니다.

var a1;
var a2 = true;
var a3 = 1;
var a4 = "Hello";
var a5 = new Object();
var a6 = null;
var a7 = NaN;
var a8 = undefined;
alert(typeof a);    //显示"undefined"
alert(typeof a1); //显示"undefined"
alert(typeof a2); //显示"boolean"
alert(typeof a3); //显示"number"
alert(typeof a4); //显示"string"
alert(typeof a5); //显示"object"
alert(typeof a6); //显示"object"
alert(typeof a7); //显示"number"
alert(typeof a8); //显示"undefined"

var a1;         //a1的值为undefined
var a2 = null;
var a3 = NaN;
alert(a1 == a2); //显示"true"
alert(a1 != a2); //显示"false"
alert(a1 == a3); //显示"false"
alert(a1 != a3); //显示"true"
alert(a2 == a3); //显示"false"
alert(a2 != a3); //显示"true"
alert(a3 == a3); //显示"false"
alert(a3 != a3); //显示"true"

<script type="text/javascript">
var t1=""
var t2
if (t1===undefined) {document.write("t1 is undefined")}
if (t2===undefined) {document.write("t2 is undefined")}
</script>

<span style="font-size: small;">var tmp = undefined; 
if (typeof(tmp) == "undefined"){ 
alert("undefined"); 
}</span>

<span style="font-size: small;">var tmp = null; 
if (!tmp && typeof(tmp)!="undefined" && tmp!=0){ 
alert("null"); 
}　</span>

<span style="font-size: small;">var tmp = 0/0; 
if(isNaN(tmp)){ 
alert("NaN"); 
}</span>