title: Array.map returns null in a redux reducer using arrow functions
body: <p>Consider this array of items:</p>
  <pre><code>items:[
   {
    _id: '111',
    quantity: 3
   },
   {
    _id: '222',
    quantity: 7
   }
  ]
  </code></pre>
  <p>I need to increment the value by 1 by looping using <code>Array#map</code> with CURLY BRACES following the arrow =&gt; (look at the code below), but this make the myNewItems null, and I guess this is because the variable item gets lost along the way and ends up with myNewItems to be null</p>
  <p>This is how I have iterated in a redux reducer</p>
  <pre><code>return {
    ...state,
    myNewItems: items.map((item) =&gt; {
      var itemId = '111'
  
      item._id == itemId ?
        {
          ...item,
          quantity: item.quantity + 1
        } :
        item
    })
  }
  </code></pre>
  <p>Now this makes myNewItems to be null instead of it having updated array of items like this below</p>
  <pre><code>items:[
   {
    _id: '111',
    quantity: 4 //has been incremented by 1
   },
   {
    _id: '222',
    quantity: 7
   }
  ]
  </code></pre>
  <p>How do I get through this?</p>