Javascript Reference - JavaScript undefined Property

The undefined property indicates that a variable has not been assigned a value.

Browser Support


`undefined`	Yes	Yes	Yes	Yes	Yes

Example

The following code shows how to Test if variables are undefined.


<!DOCTYPE html>
<html>
<body>
<button onclick="myFunction()">test</button>
<p id="demo"></p>
<!--from   w  w w  .j  av a  2 s  .  c o  m-->
<script>
function myFunction() {
    var t1 = "myVar";
    var t2;

    if (t1 === undefined) {
        txt1 = "t1 is undefined";
    } else {
        txt1 = "t1 is defined";
    }

    if (t2 === undefined) {
        txt2 = "t2 is undefined";
    } else {
        txt2 = "t2 is defined";
    }

    txt = txt1 + "<br>" + txt2;
    document.getElementById("demo").innerHTML = txt;
}
</script>

</body>
</html>

The code above is rendered as follows: