const Realms = require("minecraft-realms").Client;

var realms = new Realms("token:$ACCESS_TOKEN:$UUID","1.15.2","$NAME");

//Note 1.15.2 is latest version that provide Realms

//Replace $ACCESS_TOKEN to you access token and $UUID and $NAME. From launcher profiles or Mojang Auth API.
//For more info see https://wiki.vg/Authentication

if (realms.availability == true){
    console.log("Realms available for this user");
}