Search notes:
JavaScript: undefined
By comparing a
variable to
undefined (
=== or
== operator ), it is possible to check if a value was assigned to the variable.
var === undefined returns
true if so and
false otherwise.
Of course, it's possible to assign undefined to a variable in which case the variable becomes undefined again.
Interestingly,
undefined+undefined evaluates to
NaN.
<!DOCTYPE html>
<html>
<head>
<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<title>undefined</title>
<script type="text/javascript">
function main() {
var out = document.getElementById('out');
out.innerHTML = ( typeof(undefined) ) + "<br>"; // undefined
out.innerHTML += ( undefined + 'Foo' ) + "<br>"; // undefinedFoo
out.innerHTML += ( undefined + undefined ) + "<br>"; // NaN
var unassignedVariable;
var assignedVariable = 42;
if (unassignedVariable === undefined) { out.innerHTML += "unassignedVariable === undefined<br>"; } else { out.innerHTML += "unassignedVariable !== undefined<br>"; }
if (unassignedVariable == undefined) { out.innerHTML += "unassignedVariable == undefined<br>"; } else { out.innerHTML += "unassignedVariable != undefined<br>"; }
if (assignedVariable === undefined) { out.innerHTML += "assignedVariable === undefined<br>" ; } else { out.innerHTML += "assignedVariable !== undefined<br>"; }
if (assignedVariable == undefined) { out.innerHTML += "assignedVariable == undefined<br>" ; } else { out.innerHTML += "assignedVariable != undefined<br>"; }
assignedVariable = undefined;
if (assignedVariable === undefined) { out.innerHTML += "assignedVariable === undefined<br>" ; } else { out.innerHTML += "assignedVariable !== undefined<br>"; }
if (assignedVariable == undefined) { out.innerHTML += "assignedVariable == undefined<br>" ; } else { out.innerHTML += "assignedVariable != undefined<br>"; }
}
</script>
</head>
<body onload='main()';>
<div id='out'>
</div>
</body>
</html>