title: `JSON.stringify` omits custom keys in array
body: <p>I am storing my array as a mixture of array an array and an object. For example assume this one:</p>
  
  <pre><code>let arrObj = [];
  arrObj["x"] = 12;
  arrObj.push(12);
  arrObj["y"] = 15;
  arrObj.push(15);
  
  // result: arrObj = [12, 15, x: 12, y: 15]
  </code></pre>
  
  <p>so that, I can access the value <code>12</code> even using <code>arrObj[0]</code> and <code>arrObj["x"]</code>. And this way, I can do a repeat for on it.</p>
  
  <p>But when I stringify it, the keys <code>x</code> and <code>y</code> get lost:</p>
  
  <pre><code>JSON.stringify(arrObj)
  // result: "[12,15]"
  </code></pre>
  
  <p>How should I maintain those keys and their values?</p>