Case-sensitivity
Javascript is case-sensitive. The variables, function names, and operators are all case-sensitive. A variable named test is different from a variable named Test.
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<script type="text/javascript">
var a = 'a';
var A = 'A';
document.writeln(a);
document.writeln(A);
</script>
</head>
<body>
</body>
</html>