title: SetState of an array of Objects in React
body: <p>Ok, so I'm so frustrated finding the right solution so I'm posting the problem here. Giving an answer would help me a lot, coz I'm stuck!</p>
  
  <p>the state tree looks like this </p>
  
  <pre><code>this.state = {
        itemList : [{
                      _id : 1234,
                     description : 'This the description',
                     amount : 100
                      }, {
                      _id : 1234,
                     description : 'This the description',
                     amount : 100
                      }],
       }
  </code></pre>
  
  <p>The problems are : </p>
  
  <ol>
  <li>can not update any specific key in the Object of the array according
  to the _id</li>
  <li>The previous state should remain intact</li>
  </ol>