Finds the longest prefix in an array of strings

longestPrefix.js

var longestPrefix = function (arr) {
  var firstString = arr[0],
      prefix      = '';

  for (var i = 0; i < firstString.length; i++) {
    for (var j = 1; j < arr.length; j++) {
      if (arr[j].charAt(i) !== firstString.charAt(i)) {
        return prefix;
      }
    }

    prefix += firstString.charAt(i);
  }

  return prefix;
};