title: How to use map function to return multiple keys from an array of objects? ES6
body: <p>If I have an array of objects</p>
  
  <pre><code>var bitcoinData = [
  {
    "date": "2013-05-01",
    "txVolume(USD)": 108659660.293,
    "txCount": 52443,
    "marketcap(USD)": 1542820000,
    "price(USD)": 139,
    "exchangeVolume(USD)": 0,
    "generatedCoins": 3575,
    "fees": 36.80599998,
    "activeAddresses": null
  },
  {
    "date": "2013-05-02",
    "txVolume(USD)": 96958519.0041,
    "txCount": 55169,
    "marketcap(USD)": 1292190000,
    "price(USD)": 116.38,
    "exchangeVolume(USD)": 0,
    "generatedCoins": 3425,
    "fees": 54.40791613,
    "activeAddresses": null
  </code></pre>
  
  <p>How can I create an array containing only the date and price of each day by using the map function? </p>