Question

Users will enter various serials in a textarea. Each newline will indicate a new serial. Some requirements/restrictions:

• Leading and trailing white spaces are not allowed.
• White space within a serial is okay.
• Blank serials are not allowed
• I’d prefer to not use JQuery.
• Store duplicates so they can be shown to the user.

Based on my tests I have a working solution. I want to make sure I’m not missing or overlooking anything. My questions are:

• Is there a more efficient ways to check for duplicates?
• Are there any glaring test cases that my solution won’t catch?

Working Example: http://jsbin.com/ivusuj/1/

function duplicateCheck() {
    var output = document.getElementById('Output');
    output.innerHTML = '';
    var duplicateSerials = [];
    var count = 0;
    var textArea = document.getElementById('Serials'); 
    var serials = textArea.value.trim().split(/ *\n */);

    for(var i = 0;i < serials.length;i++){
      var serial = serials[i];

      if(serials.indexOf(serial) != serials.lastIndexOf(serial) && 
         duplicateSerials.indexOf(serial) == -1 && serial !== '') {        
        duplicateSerials.push(serial);
      }  

    }    

  // For testing
  output.innerHTML = '<pre>Serials:\t' + serials.toString() + "<br />" +
                     'Duplicates:\t' + duplicateSerials.toString() + "<br>" +
                     '</pre>';
}

Note: the above is for a client side check. The same check will be performed server side as well to ensure the data is valid.

---

Update

Solution comparison: http://jsbin.com/ivusuj/4/edit

Answer 1

I think you’d get significantly better performance if you used an object to determine which serials you’d seen before. Something closer to this:

var seen = {};
for (var i = 0, j = serials.length; i < j; ++i) {
  var serial = serials[i];
  if (seen.hasOwnProperty(serial)) {
    // Dupe code goes here.
    continue;
  }
  // Can't be a duplicate if we get to this point.
}

Though that won’t work with serials that use periods.