io.js 1.0.1 (unstable) is out. ES6!

A team of developers, including some core node.js developers, forked node.js a while ago into a project called io.js. They’ve just released version 1.0.1 (Unstable) and it brings ES6 natively to server-side JavaScript applications.

To test drive, you can download io.js. Be warned, it will completely replace your node and npm system links. Some people have complained about this or reported unexpected behavior after compiling io.js from source.

“Why io.js?” you ask? Joyent was a little stingy with node.js progress (it’s the basis for a huge money-maker for them). The io.js team’s goal is to make io.js more of a community-driven project. It seems to have worked, judging by the progress they’ve made.

Version 1.0.0 of io.js ships with V8, which includes ES6 features well beyond version 3.26.33 that will be shipped with joyent/node@0.12.x.

No more –harmony flag

With io.js, you can now start using some ES6 features natively (e.g. no transpiling using Traceur or 6to5). Here’s the fibonacci example using an ES6 iterator taken from 6to5’s tour:

// fib.js
var fibonacci = { };
fibonacci[Symbol.iterator] = function*() {
  var pre = 0, cur = 1;
  for (;;) {
    var temp = pre;
    pre = cur;
    cur += temp;
    yield cur;

for (var n of fibonacci) {
  // truncate the sequence at 1000
  if (n > 1000) {
  process.stdout.write(n + '\n');

You’ll notice that I’ve had to change how an iterator is constructed and how the output is written. Run this with node fib.js (remember, io.js will overwrite the system node).

While this is excellent news, you’ll need to be aware of what ES6 features are currently supported. For example, io.js does not yet support classes. This won’t work in io.js:

class Animal{
  constructor(legs) {
    switch (legs) {
      case 2:
        this.locomotion = 'bipedal';
      case 4:
        this.locomotion = 'quadrupedal';
  move() {
    process.stdout.writeln("I'm a %j animal!", this.locomotion);
  static getTypeName() {
    return "Animal";

class Bird extends Animal {

class Lion extends Animal {

var bird = new Bird();

var lion = new Lion();

This will, however, work via transpilers:

These examples are on github.

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