Getting name of key?

Gidday

I have json data returning from my server, and then I decode it.

In php, I have unique names for the array sent back based on the result of the transaction...

$returnArray['links'] = $links;

$returnArray['msg'] = 'no matches for that code or name';

$returnArray['error'] = '3555';

(only one will ever be sent back at once)

In as3, I want to set up a switch based on the key name of the returned object...

switch (returnedServerData.nameofit-eg-links-or-msg-etc)

...so that I can set up my processing based on whether it's an error, link, msg etc

What do I type after returnedServerData. to access the name of that key?

Thanks guys