function idGenerator() {

  this.counter = 1000;

this.generate = function() {

    return (this.counter++).toString(16);

}

}

​

var userIdGenerator = new idGenerator();

var roomIdGenerator = new idGenerator(); 

​

console.log(userIdGenerator.generate());

console.log(userIdGenerator.generate());

​

console.log(roomIdGenerator.generate());

console.log(roomIdGenerator.generate());

​

console.log(userIdGenerator.generate == roomIdGenerator.generate);