title: ES6 Map: transform values
body: <p>I'm working on a project where I frequently have to transform every value in an ES6 map:</p>
  
  <pre><code>const positiveMap = new Map(
    [
      ['hello', 1],
      ['world', 2]
    ]
  );
  
  const negativeMap = new Map&lt;string, number&gt;();
  for (const key of positiveMap.keys()) {
    negativeMap.set(key, positiveMap.get(key) * -1);
  }
  </code></pre>
  
  <p>Just wondering if there is maybe a better way of doing this?  Ideally a one liner like <code>Array.map()</code>.</p>
  
  <p>Bonus points (not really), if it compiles in typescript!</p>