function increment(strint) {
    var length = strint.length;

    if (length === 0) {
      return '1';
    }

    var last = parseInt(strint[length - 1], 10);

    if (last === 9) {
      return increment(strint.slice(0, length - 1)) + '0';
    } else {
      return strint.slice(0, length - 1) + (last + 1);
    }
  }
Increments the given integer represented by a string by one.

@example

  increment('1');  // '2'
  increment('99'); // '100'
  increment('');   // '1'

@param {string} strint
@return {string}
@private