title: How to useEffect only if array length changed?
body: <p>I want update state only when prop "columns" length changed</p>
  
  <pre><code>  useEffect(() =&gt; {
      if (columns.length !== prevColumns.length) {
        // update state
      }
    }, [columns]);
  </code></pre>
  
  <p>How can I do it?</p>