null Type

The null type has only one value: null. A null value is an empty object pointer. typeof returns "object" for typeof null.

 
<!DOCTYPE html>
<html>
<head>
    <title>Null Example</title>
    <script type="text/javascript">
          
        var aString = null;
        document.writeln(typeof aString);   //"object"
        document.writeln(typeof null);      //"object"
    </script>

</head>
<body>
  
</body>
</html>

Click to view the demo

The value undefined is a derivative of null, so JavaScript defines them to be equal:

 
<!DOCTYPE html>
<html>
<head>
    <title>Null Example</title>
    <script type="text/javascript">
          
        document.writeln(null == undefined);   
              
    </script>

</head>
<body>
  
</body>
</html>

Click to view the demo

We can check for null to see if the variable has been assigned a value:

 
<!DOCTYPE html>
<html>
<head>
    <title>Null Example</title>
    <script type="text/javascript">
          
        var aString = null;
        if(aString == null){
           document.writeln("Not assigned");
        }
        aString = "some value";
        if(aString == null){
           document.writeln("Not assigned");
        }else{
           document.writeln("Assigned");
        }
        
    </script>

</head>
<body>
  
</body>
</html>

Click to view the demo