anyone know if it's possible to find out what keys are present in a json message?

for example, if the json message is

{
"firstName": "John",
"lastName": "Smith",
"address": {
"streetAddress": "21 2nd Street",
"city": "New York",
"state": "NY",
"postalCode": 10021
},
"phoneNumbers": [
"212 555-1234",
"646 555-4567"
]
}

i need to get firstName, lastName, address, and phoneNumbers.