[javascript] get keys of json-object in JavaScript

I have a json-object in JavaScript and I want to get the used keys in it. My JavaScript-Code looks like this:

var jsonData = [{"person":"me","age":"30"},{"person":"you","age":"25"}];

And I want a loop that alerts me 'person' and 'age', which are the keys of the first object in the json-Array.

The working code

var jsonData = [{person:"me", age :"30"},{person:"you",age:"25"}];_x000D_
_x000D_
for(var obj in jsonData){_x000D_
    if(jsonData.hasOwnProperty(obj)){_x000D_
    for(var prop in jsonData[obj]){_x000D_
        if(jsonData[obj].hasOwnProperty(prop)){_x000D_
           alert(prop + ':' + jsonData[obj][prop]);_x000D_
        }_x000D_
    }_x000D_
}_x000D_
}

var jsonData = [{"person":"me","age":"30"},{"person":"you","age":"25"}];

for(var i in jsonData){
    var key = i;
    var val = jsonData[i];
    for(var j in val){
        var sub_key = j;
        var sub_val = val[j];
        console.log(sub_key);
    }
}

EDIT

var jsonObj = {"person":"me","age":"30"};
Object.keys(jsonObj);  // returns ["person", "age"]

Object has a property keys, returns an Array of keys from that Object

Chrome, FF & Safari supports Object.keys