my.prop == null
In Navigator 3.0 and later, you can distinguish between null and the undefined value with the typeof operator (which is discussed in detail in Chapter 4, Expressions and Operators). This operator returns a string that indicates the data type of any value. We said above that null is actually a object value, and when we use typeof on null, it indicates this by returning the string "object":
type = typeof null; // returns "object"
var new_undefined_variable; type = typeof new_undefined_variable // returns "undefined"