actionscript (flex): how to know whether a property of object exists (or defined)?

I am a Java developer who tries Flex. Here is my problem:

I behave actionScript objects as hashmap but when the object do not have the property it gives exception: No such variable.

Here I expect it gave me null, instead of giving exception. So do you know is there a way to handle it, namely check if the property is defined for object.

trace( obj["2008-02"] ) // gives exception