title: Reference to array returns it's length after push operation
body: <p>I have a map where my keys are <code>strings</code> and values are <code>Array</code>.</p>
  
  <pre><code>let map = new Map();
  
  map.set("a", []);
  
  map.set("a", map.get("a").push(1)); // reference to array returns it's length: 1
  </code></pre>
  
  <p>Is there any reason why the design is like this?</p>