title: "object is not extensible" error trying to add field in map function
body: <p>Im trying to add a new field to items in an array with map: </p>
  
  <pre><code>const newArray = oldArray.map(item =&gt; {
      return (item.newField = 'Something');
  });
  </code></pre>
  
  <p>And Ive tried:</p>
  
  <pre><code>const newArray = oldArray.map(item =&gt; {
      item.newField = 'Something';
      return item;
  });
  </code></pre>
  
  <p>However I get an error:</p>
  
  <pre><code>TypeError: Cannot add property newField, object is not extensible
  </code></pre>