title: How to set only one property in object with useState?
body: <p>I have an object as state in react:</p>
  <pre><code>{course: {…}, userIDs: Array(2), usersNumber: 2}
  </code></pre>
  <p>Where <code>course</code> is such an object:</p>
  <pre><code>price : 100
  promotion : null
  comments : []
  </code></pre>
  <p>What I want to do is add a new object to the <code>comments</code> array and leave all other properties unchanged. How to do it with <code>useState</code> ?</p>