How to get JSON column names in Javascript

I have the following JSON response:

{
"d": {
    "results": [
        {
            "__metadata": {
                "id": "c55209ae-2af9-4514-ab02-67f222903f6d",
                "uri": "http://ift.tt/1IXDFvS",
                "etag": "\"1\"",
                "type": "SP.Data.LstDoelenListItem"
            },
            "scDoelnaam": "Hoofddoel Afdeling",
            "scDoellevel": 1
        },
        {
            "__metadata": {
                "id": "7e79ab8a-e550-4694-87d4-26d6f5bc6128",
                "uri": "http://ift.tt/1IXDCQL",
                "etag": "\"1\"",
                "type": "SP.Data.LstDoelenListItem"
            },
            "scDoelnaam": "Subdoel 1",
            "scDoellevel": 2
        },
        {
            "__metadata": {
                "id": "c204acca-0362-4bd6-a66b-00236a2320cb",
                "uri": "http://ift.tt/1z9xPxA",
                "etag": "\"1\"",
                "type": "SP.Data.LstDoelenListItem"
            },
            "scDoelnaam": "Subdoel 2",
            "scDoellevel": 2
        },
        {
            "__metadata": {
                "id": "b78b2416-7813-4e64-a920-f9ba377fa36d",
                "uri": "http://ift.tt/1IXDFvU",
                "etag": "\"1\"",
                "type": "SP.Data.LstDoelenListItem"
            },
            "scDoelnaam": "Subdoel 3",
            "scDoellevel": 2
        }
    ]
}

}

I know how to retrieve the values once I know the column names, but how do I get the 2 column names "scDoelnaam" and "scDoellevel" with Javascript and assign them to variables??