JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/immutable/3.6.2/immutable.js"></script>

JavaScript

var a = Immutable.List([1,4]);
var b = Immutable.List([2,3,4]); 

console.log(a.concat(b));

function union(left,right){
 var union = {};
 left.forEach(function(x){
  union[x] = undefined;
 });
 right.forEach(function(x){
  union[x] = undefined;
 });
 return Immutable.List(Object.keys(union).map(function(i){ return parseInt(i,10); }));
}

var c = union(a,b);

console.log(c);