How to get a value from json data without knowing the key name?

I’m a javascript noob so this might be a dumb question. I’m trying to get data from the wikipedia api (specifically, the url of the main image of the article), but it’s nestled in a bunch of layers, like this:

{
    "batchcomplete": "",
    "query": {
        "normalized": [
            {
                "from": "tortilla",
                "to": "Tortilla"
            }
        ],
        "pages": {
            "51418585": {
                "pageid": 51418585,
                "thumbnail": {
                    "source": "https://upload.wikimedia.org/wikipedia/commons/thumb/e/ef/Mexican_tortilla.jpg/300px-Mexican_tortilla.jpg",
                    "width": 300,
                    "height": 200
                },
                "pageimage": "Mexican_tortilla.jpg"
            }
        }
    }
}

The page id (51418585 in this case) is dynamic, and there’s no way of knowing it beforehand as far as I know. My question is, how can I get the value of "source" without knowing what "51418585" is?

This is my code:

getJSON(url, function(err, data) {
  if (err !== null) {
    alert('Something went wrong: ' + err);
  } else {
    console.log(data[2][0].thumbnail.source);
  }
});

I just want to be able to log the "source" url to the console.
Thanks.

>Solution :

Using the answer in this post: get keys of json-object in JavaScript

You can run Object.keys on the JSON object you retrieve and get to the source like this:

getJSON(url, function(err, data) {
  if (err !== null) {
    alert('Something went wrong: ' + err);
  } else {
    temp = Object.keys(data.query.pages);
    console.log(data.query.pages[temp[0]].thumbnail.source);
  }
});