Nodejs getting keys from JSON

I am reading a json file in nodejs and want to find a specific key in the json object before doing things with it:

fs.readFile('./output.json', 'utf8', function(err, data) {
        if (err) throw err;
        console.log(Object.keys(data));

});

Problem is, it returns an array of numbers as strings:

[ '0',
  '1',
  '2',
  '3',
  '4'
  ... ]

When my json file has text like this. Why is this happening?

[{
    "id": "vdhbsasqzqnbx2od",
    "message": "On Sun, Jan 1, 2012 at 8:17 AM, Alan Stern <ste...@rowland.harvard.edu> wrote:As Linus pointed out, the real problem here 
}, { ...
}]

via agomez