title: What's the difference between ES6 Map and WeakMap?
body: <p>Looking <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Map">this</a> and <a href="https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/WeakMap">this</a> MDN pages it seems like the only difference between Maps and WeakMaps is a missing "size" property for WeakMaps. But is this true? What's the difference between them?</p>