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...
