If you want to compare two values, then you have a choice. If you want to treat an undefined value as being the same as a null value, then you can use the equality operator (==) and rely on JavaScript to convert the types. An undefined variable will be regarded as being equal to a null variable, for example. If you want to differentiate between null and undefined, then you need to use the identity operator (===).
<!DOCTYPE HTML>
<html>
<head>
<title>Example</title>
<script type="text/javascript">
var firstVal = null;
var secondVal;
var equality = firstVal == secondVal;
var identity = firstVal === secondVal;
console.log("Equality: " + equality);
console.log("Identity: " + identity);
</script>
</head>
<body>
This is a simple example
</body>
</html>
The output from this script is as follows:
Equality: true
Identity: false
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