extend.js

function extend(objects) {
  var result = {};

  for (var i = 0; i < objects.length; i++) {
    var object = objects[i];

    for (var key1 in object) {
      if (result.hasOwnProperty(key1)) {
        for (var key2 in object[key1]) {
          result[key1][key2] = object[key1][key2];
        }
      } else {
        result[key1] = object[key1];
      }
    }
  }

  return result;
}

var a = {mrr: {plan1: {a: 1, b: 2}}};
var b = {mrr: {plan2: {a: 2, b: 3, c: 3}}};
var c = {mrr: {plan3: {d: 4}}};

var test = extend([c, extend([a, b])]);
console.log(test);