See: http://careercup.com/question?id=5755014750404608

merge-ranges.js

//
// See: http://careercup.com/question?id=5755014750404608
//
// Input: [[1,3], [5,7], [2,4], [6,8]]
// Output: [[1,4], [5,8]] (no specific order)
//

var mergeRanges = (function () {
  
  /**
   * Do these two ranges overlap?
   *
   * @param {array} a
   * @param {array} b
   * @return {boolen}
   */
  var overlaps = function (a, b) {
    return a[0] >= b[0] && a[0] <= b[1]  // a[lo] is in range of b
        || a[1] >= b[0] && a[1] <= b[1]; // a[hi] is in range of b
  };

  /**
   * Merge two ranges, assuming overlap
   *
   * @param {array} a
   * @param {array} b
   * @return {array}
   */
  var merge = function (a, b) {
    return [Math.min(a[0], b[0]), Math.max(a[1], b[1])];
  };

  /**
   * Merge a single range into a list of ranges
   *
   * @param {array[array]} list
   * @param {array} range
   * @param {array[array]}
   */
  var mergeRange = function (list, range) {
    for (var i = 0; i < list.length; i++) {
      if (overlaps(list[i], range)) { // the range overlaps
        var newRange = merge(list[i], range);
        list.splice(i, 1);
        return mergeRange(list, newRange); // recursive call
      }
    }
    list.push(range); // the range doesn't overlap, so just add it to list
    return list;
  };
  
  /**
   * Merge a list of ranges into each other
   *
   * @param {array[array]} ranges
   * @return {array[array]}
   */
  return function (ranges) {
    return ranges.reduce(function (acc, curr) {
      return mergeRange(acc, curr);
    }, []);
  }
  
})();