title: How to get all the json keys in array
body: <p>I have a JSON object that has the following structure</p>
  
  <pre><code>{"30-31":[{"msg":"hello","usr":"31"}],
  "33-30":[{"msg":"shah hi","usr":"30"}]}
  </code></pre>
  
  <p>What operation can I perform to get an array like this <code>["30-31", "33-30"]</code>.
  I tried my best using <code>.map</code> etc but with no success, can anybody help?</p>