Node.js Iterator interface

Question

I am reading the MDN guide on Iterators and generators and I implemented the following example :

function Range(low, high){
  this.low = low;
  this.high = high;
}
Range.prototype.__iterator__ = function(){
  return new RangeIterator(this);
};

function RangeIterator(range){
  this.range = range;
  this.current = this.range.low;
}
RangeIterator.prototype.next = function(){
  if (this.current > this.range.high)
    throw StopIteration;
  else
    return this.current++;
};

var range = new Range(3, 5);
for (var i in range) {
  console.log(i);
}

But I get this :

low
high
__iterator__

Where it should have returned :

3
4
5

Is it possible to implement this as expected in Node.js?

Note: I am using node --harmony myscript.js.

** Edit **

I should also note that I am aware that __iterator__ is a Mozilla-feature only. I'd like to know what's the equivalent in Node.js. Or what's the next best thing.

Also, I'd like to not use a generator for this as I want the iteration to be free of any conditional tests.

My conclusion is that the only equivalent is

Range.prototype.each = function (cb) {
  for (var i = this.low; i <= this.high; i++) {
     cb(i);
  }
};

//...
range.each(function (i) {
  console.log(i);
});

Are there alternatives?

Answer 1

I think your code had a bug where you wrote for (var i in range) instead of for (var i of range). The in keyword prints out the object's properties, you want to use of to actually get the iterator values.

Anyway, Node.js now has better harmony support since the question was asked. Here's an updated code sample which works out of the box on Node 4.2. The __iterator__ name was changed to Symbol.iterator, and the StopIteration exception isn't used.

"use strict";

function Range(low, high){
  this.low = low;
  this.high = high;
}
Range.prototype[Symbol.iterator] = function(){
  return new RangeIterator(this);
};

function RangeIterator(range){
  this.range = range;
  this.current = this.range.low;
}
RangeIterator.prototype.next = function(){
  let result = {done: this.current > this.range.high, value: this.current};
  this.current++;
  return result;
};

var range = new Range(3, 5);
for (var i of range) {
  console.log(i);
}

Answer 2

Yes it is.

Generators and iterators are part of ECMAScript 6 / JavaScript 1.7. Node has support for them but you need to activate them, when starting you script.

For example:

node --harmony_generators --harmony_iteration your_script.js

Take a look at this blog post for more information.