title: Why does ES6 define map.length==0?
body: <p>According to <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map">MDN</a>:</p>
  
  <p><strong>Map.length</strong></p>
  
  <p>The value of the length property is 0.</p>
  
  <p>What is the use case for this? I understand why <code>Map.size</code> is semantically correct. But surely a <code>Map.length</code> that almost always returns the "wrong" answer is a bad idea, especially if there's an oversight migrating code from ES5.  Is there a way to force an error when this is used?</p>