title: JavaScript Array#map: index argument
body: <p>My question is about the <code>map</code> method of arrays in JavaScript.</p>
  
  <p>You can pass it a function that takes a second argument, the index of the current element of the array being processed, but... to what purpose? What happens when you do it and what's the difference when you don't?</p>
  
  <p>What would you use this feature for?</p>