seq.js

function LazySeq(head, tail) {
  this.head = head;
  this.tail = tail;
}

LazySeq.prototype.first = function() {
  return this.head;
}

LazySeq.prototype.rest = function() {
  return this.tail();
}

function Cons(head, tail) {
  this.head = head;
  this.tail = tail;
}

Cons.prototype.first = function() {
  return this.head;
}

Cons.prototype.rest = function() {
  return this.tail;
}

function first(seq) {
  return seq.first();
}

function rest(seq) {
  return seq.rest();
}

console.log(first(new LazySeq(1, null)));
console.log(first(new Cons(1, null)));
console.log(first(rest(new Cons(1,new LazySeq(2, function() { return new LazySeq(3, null)})))));