LazyMapReduce.js

function Lazy(list, ...steps){
  this.list = list;
  this.steps = steps; 
}

Lazy.prototype.map = function(f){
  return new Lazy(this.list, ...this.steps, iteration 
    => ( for (entry of iteration) f(entry)));
}

Lazy.prototype.takeUntil = function(f){
  return new Lazy(this.list, ...this.steps, function*(iteration){
    for (var entry of iteration){
      yield entry;
      if(f(entry)) break;
    }
  });
}

Lazy.prototype.toArray = function(){
  var steps = this.steps;
  var previousStep = (for (entry of this.list) entry);
  for(var nextStep of steps){
    previousStep = nextStep(previousStep);
  }
  return [for (x of previousStep) x];
}

Lazy.extend = function(name, generator){
  Lazy.prototype[name] = function(f){
    return new Lazy(this.list, ...this.steps, generator.bind(f));
  }
}

new Lazy([1,2,3,4,5,6,7,8,9,10]).map(x => x*x).takeUntil(x => x==25).toArray();

Lazy.extend('skipWhile', function*(iteration){
  var isSkipping = true;
  for(var entry of iteration){
    if(isSkipping && this(entry)) continue;
    isSkipping = false;
    yield entry;
  }
});

new Lazy([1,2,3,4,5,6,7,8,9,10]).skipWhile(x => x<3).map(x => x*x).takeUntil(x => x==25).toArray();//[9, 16, 25]