title: Javascript: How to find first duplicate value and return its index?
body: <p>I have to find first duplicate value in array and then return its index in variable firstIndex. This has to be done with for loop which should stop after having found first duplicate. I know this is probably pretty simple but I got stuck. So far I have this but it doesn't seem to be working:</p>
  
  <pre><code>var numbers4 = [5, 2, 3, 4, 2, 6, 7, 1, 2, 3];
  var firstIndex = "";
  for (var a = 0; a &lt; numbers4.length; a++) {
      for (var b = a+1; b &lt; numbers4.length; b++) {
          if (numbers4[a] === numbers4[b])
              firstIndex = numbers4.indexOf(numbers4[a]);
              break;
      }
  }
  console.log(firstIndex);
  </code></pre>
  
  <p>Console prints out 1 which is fine because 2 is first duplicate, but when I change numbers in array, the loop doesn't work. Can you advise what can be changed here?</p>
  
  <p>Thanks in advance!</p>