title: Update one of the objects in array, in an immutable way
body: <p>In React's this.state I have a property called <code>formErrors</code> containing the following dynamic array of objects.</p>
  
  <pre><code>[
    {fieldName: 'title', valid: false}, 
    {fieldName: 'description', valid: true},
    {fieldName: 'cityId', valid: false},
    {fieldName: 'hostDescription', valid: false},
  ]
  </code></pre>
  
  <p>Let's say I would need to update state's object having the fieldName <code>cityId</code> to the valid value of <code>true</code>.</p>
  
  <p><strong>What's the easiest or most common way to solve this?</strong></p>
  
  <p>I'm OK to use any of the libraries <a href="https://github.com/kolodny/immutability-helper" rel="noreferrer">immutability-helper</a>, <a href="https://facebook.github.io/immutable-js/" rel="noreferrer">immutable-js</a> etc or ES6. I've tried and googled this for over 4 hours, and still cannot wrap my head around it. Would be extremely grateful for some help.</p>