title: Can a redux reducer return properties of the old state as part of the new state?
body: <p>Let's say I have this state:</p>
  
  <pre><code>state: {
    field1: value1,
    field2: {a: 5, b: 7}
  }
  </code></pre>
  
  <p>If a reducer wants to <strong>update only <code>field1</code></strong>, can the reducer return a new object containing a new <code>field1</code> and the existing object <code>state.field2</code> as <code>field2</code> property of the new returned state? Or does the reducer have to clone field2?</p>