Wrote an iterator with Steve Kinney at a JS meetup in Denver (following along with the material at http://thedrearlight.com/bwdid-iterables/#/)

my_iterator.js

let MyIterator = function(n) {
  this.n = n;
}

MyIterator.prototype[Symbol.iterator] = function() {
  var i    = -1;
  var n    = this.n;
  var done = false;

  return {
    next:   (something) => {
      if(done) {
        return {next: undefined, done: done}
      } else {
        done = ++i == n;
        return {value: i, done: done};
      }
    },
    throw:  (something)=> { /* idk */ },
    return: (something)=> { /* idk */ },
  }
}

let iter = new MyIterator(4);
for(let val of iter) {
  console.log(val);
}