Lazy evaluation in JavaScript

## lazy.js
function delay(expressionAsFunction) {
  var result;
  var isEvaluated = false;

  return function () {
    if (!isEvaluated)
      result = expressionAsFunction();
    return result;
  };
}

function force(promise) {
  return promise();
}

function cons(car, cdr) {
  return [car, cdr];
}

function next(n) {
  return cons(n, delay(function () {return next(n + 1);}));
}

function head(stream) {
  return stream[0];
}

function tail(stream) {
  return force(stream[1]);
}

var stream = next(0);
console.log(stream);
console.log(head(tail(tail(stream))));  //==> 2