title: get keys of json-object in JavaScript
body: <p>I have a json-object in JavaScript and I want to get the used keys in it. My JavaScript-Code looks like this:</p>
  
  <pre><code>var jsonData = [{"person":"me","age":"30"},{"person":"you","age":"25"}];
  </code></pre>
  
  <p>And I want a loop that alerts me 'person' and 'age', which are the keys of the first object in the json-Array.</p>