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

Except that you never set isEvaluated, so expressionAsFunction is called every time the promise is forced, not just the first time.