Formats large array of numbers into a string showing any ranges within the original array

example.js

ranger([1,2,3,5,7,8,9]) // "1:3,5,7:9"

ranger.js

(function (root, factory) {

  if (typeof define !== 'undefined' && define.amd) {
    define(factory);
  }
  else if (typeof module === 'object') {
    module.exports = factory();
  }
  else {
    root.ranger = factory();
  }

}(this, function () {

  function isNext (number, nextNumber) {
    return (number + 1) === nextNumber;
  }

  function group (groups, number) {
    var group = groups[groups.length - 1];

    if (!group.length || group.length === 1 && isNext(group[0], number)) {
      group.push(number);
    }
    else if (isNext(group[1], number)) {
      group[1] = number;
    }
    else {
      groups.push([number]);
    }

    return groups;
  }

  function joinGroups (group) {
    return group.join(':');
  }

  function ranger (numbers) {
    return numbers.reduce(group, [[]]).map(joinGroups).join();
  }

  return ranger;

}));