title: Javascript map over two dimensional array
body: <p>I have this array:</p>
  
  <pre><code>rows = [ [ 89, 18, 9 ], [ 1903, 3, 4 ], [ 3, 1, 800 ] ];
  </code></pre>
  
  <p>It should look like this:</p>
  
  <pre><code>[ [ 89, 1903, 3 ], [ 18, 3, 1 ], [ 9, 4, 800 ] ]
  </code></pre>
  
  <p>And the code, that is working, looks like this: </p>
  
  <pre><code>rows[0].map((_, columnIndex) =&gt; rows.map(
              row =&gt; row[columnIndex])
          );
  </code></pre>
  
  <p>How does this work?</p>