Marzullo's algorithm in JavaScript based on http://vmaroli2.blogspot.com/2012/11/marzullos-algorithm-in-java.html

marzullo.js

// MarzulloAlgorithm in JavaScript
//
// Not quite what I need which is a list of ranges that contain a specified number.
// 
'use strict';

function Point(name, point, type) {
  this.name = name;
  this.point = point;
  this.type = type;
}

function Range(name, start, end) {
  this.name = name;
  this.start = start;
  this.end = end;
}

function marzullo(ranges) {
  var best = 0;
  var count = 0;
  var beststart = 0;
  var bestend = 0;
  var i = 0;
  var name = "";
  var points = [];

  ranges.forEach(function(r) {
    points.push(new Point(r.name, r.start, -1));
    points.push(new Point(r.name, r.end, 1));
  });

  points.sort(function(a, b) {
    return (a.point - b.point) || (b.type - a.type);
  });

  points.forEach(function(p) {
    count = count - p.type;
    if(best < count) {
      best = count;
      beststart = p.point;
      if (i < points.length-1) {
        bestend = points[i+1].point;
        name = p.name + "," + points[i+1].name;
      }
    }

    i++;
  });

  return new Range(name, beststart, bestend);
}

var ranges = [
  new Range("r1", 10, 40),
  new Range("r2", 30, 50),
  new Range("r3", 35, 45),
];

console.log(marzullo(ranges));