Search notes:
JavaScript: comparing undefined to other values
The following simple
HTML document contains a bit of
JavaScript that compares
undefined to a few values and prints if this comparison is
true or
false.
<!DOCTYPE html>
<html>
<head>
<meta content="text/html;charset=utf-8" http-equiv="Content-Type">
<title>undefined</title>
<script type="text/javascript">
var out;
function evalAndPrint(expr) {
var res = eval(expr);
expr.replace(/&/g, '&');
expr.replace(/</g, '<' );
expr.replace(/g/g, '>' );
if (res) { out.innerHTML += expr + " is true<br>"; } else
{ out.innerHTML += expr + " is false<br>"; }
}
function main() {
out = document.getElementById('out');
evalAndPrint('5 == undefined'); // false
evalAndPrint('5 != undefined'); // true
evalAndPrint('5 > undefined'); // false
evalAndPrint('undefined > undefined'); // false
evalAndPrint('undefined == undefined'); // true
evalAndPrint('undefined === undefined'); // true
evalAndPrint('NaN === undefined'); // true
}
</script>
</head>
<body onload='main()';>
<div id='out'>
</div>
</body>
</html>