gistfile1.txt

var list1 = Immutable.fromJS([{a: 10}, {a : 20}]);
var list2 = Immutable.fromJS([{a: 10}, {a : 20}]);

//var list2 = list1.merge(list2);
var list3 = list1.mergeDeep(list2);

var newVal = [{a: 10}, {a : 20}];
var list4 = list1.update(function (value) {
  var nv = Immutable.fromJS(newVal);
  return Immutable.is(list1, newVal) ? value : nv;
});

console.log(list1 === list3);
console.log(list1 === list4);

console.log(list3.toJS());

console.log(Immutable.is(list1, list3));
console.log(Immutable.is(list1, list4));