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Piotr Mionskowski

Guide to javascript generators

15 June 2014 javascript, nodejs, ES6, and generators

Yield sign For a while now I’ve been hearing and reading about a new exciting features of ES6 like arrow functions, array comprehensions, classes, modules, generators and many more. While many of them are easy to understand generators can take a while to grok. So I thought it would be a good idea to write some samples to master this particular feature.

What are generators

Generators or to be more precise generator functions are functions which can be exited and then reentered with function execution context (variable binding) preserved - a limited version of coroutines if you will. Traditionally they are used to write iterators more easily but of course they have many more interesting use cases.

To run following samples your node version has to support harmony generators. You can check that with node --v8-options | grep harmony. The latest unstable version 0.11.12 supports them.

A simple generator

Here’s a very simple example of generator that yields only two values: when executed it will output:

{value: 1, done: false}
{value: 2, done: false}
{value: undefined, done: true}

A Fibonacci generator

Of course since we have an easy way to create iterators we have to have a way to iterate over them. One approach is to just use a while loop:

but honestly this would be a step back. Fortunately es6 comes with a new kind of for-of loop designed specifically to work with iterators:

for(var current of fibo()){

Iterators and iterables

I’ve mentioned couple of times a word iterator, but what exactly is that? In ES6 iterator is an object that has next method which when called returns and object having 2 properties:

  • value - which represents value returned by iterator
  • done - a boolean indicating whether iterator has done it’s job or not

As we saw in the above generator examples every generator is an implicit iterator thus we can loop through it using new for-of syntax.

There is one more object type it is good to know of - iterable. According to ES6 Wiki an object is iterable if has a internal property named iterator which is a function returning iterator. Unfortunatelly the latests specs aren’t yet implemented in v8 (or incorporated in node.js) so the following example is only my understanding of the specs:

var iterable = {};
iterable[Symbol.iterator] = function(){
    console.log('calling Symbol.iterator')
    return (function *(){
      yield 1;
      yield 2;

for(var item of iterable){

which would print:

calling Symbol.iterator

Passing values to generators

So far we’ve covered the most simplistic usage of generators for those familiar with .NET it reassembles a yield keyword introduced in C# 2.0 in 2005. Since ES6 Generators design was influenced a lot by python generators now wonder we can also pass values to generator functions. You do it by passing value to next function call:

Running it should output something similar to:

How old are you? oracle says:  0.760754773626104
Why? oracle says:  0.36784305213950574
Thank you!

A careful reader will notice that the first question is silently ignored by our oracle generator. This is logical although might be strange at first. The generator function body isn’t executed until we call next for the first time - which is like starting the generator. The call to next will execute function body until it encounters yield - the control flow changes and the function is left to be entered later on - on future next call. Notice that we there is no logical place to store/assign the value passed to first next() call - a value that we pass to first next() call is ignored.

Thanks to @NOtherDev for pointing that out.

Generators error handling

Now that we know how to yield values and receive them inside generators. It’s equally important to be able to handle exceptions gracefully. Since generators are executing synchronously semantics of typical try {} catch {} should be preseved: which would output:

Generator entered, yielding first value
Generator asked for second value - will go bum
[Error: You can only call this generator once, sorry!]

It’s important to note however that if you try to call next method on a generator that has thrown an exception you’ll get another error:;
Error: Generator is already running
    at (native)

Now this was a case when a generator has thrown exception at caller. What if we would like to tell generator that an error occured at caller’s site. Thanks to generator throw function that is easy enough:

This as you may expect will produce:

I will stop when you tell me about error
Got value from you: I will throw in a moment!
Got error from you: This is the end. Goodbye!

Practical generator usage

An obvious use case is of course creating lazy evaluated collections. One can now very easily implement collection operators that are available in LINQ. Consider following examples: which when executed will output (note the order or DEBUG calls):

DEBUG: concat yielding first generator values
DEBUG: reverse will yield 6
DEBUG: filter predicate value true for item 6
DEBUG: take will yield  6 counter 1
DEBUG: select will yield Even number 6
Even number 6
DEBUG: reverse will yield 5
DEBUG: filter predicate value false for item 5
DEBUG: reverse will yield 4
DEBUG: filter predicate value true for item 4
DEBUG: take will yield  4 counter 2
DEBUG: select will yield Even number 4
Even number 4
DEBUG: concat yielding second generator values
At the end

Note the concat function implementation uses yield *otherGenerator() to yield values yielded by other generator which greatly reduces a boilerplate code needed otherwise.

Simplifying asynchronous operations

It may as first be a surprise that generators can streamline asynchronous code - how would synchronously executing yield help in managing asynchronous calls? If you think of the way we write asynchronous code in most of mainstream languages it typically boils down to either:

  • passing a callback function(s) that will be called when the operation completes
  • immediately getting back a promise of result from async function
  • using a library similar to Reactive Extensions - that when it comes to asynchronous code somewhat similar in usage to promises but offers very rich API

Now when you look at async code consuming:

var power = function(number, exponent){
  var deferred = Q.defer();
    deferred.resolve(Math.pow(number, exponent))
  }, 300);
  return deferred.promise;


I immediately translate promise calls in my head to a more natural flow:

var result = _wait_for_async_ power(2,3);

Now since we don’t have async feature like the one C# 5 has (or rather we don’t yet have it) it’s not yet possible to achieve this kind of simplification. Still if you think of how the imaginary _wait_for_async_ keyword would work it seems that it (in case of promise based implememntation):

  • would wait for the promise to complete in a non blocking fashion - possibly leaving function execution context
  • when the promise completes it would return to the exact place from where it was called and return value that it got from promise or throw an exception

Essentially that’s the way yield keyword works - however we still need an umbrella machinery that will take care of calling generator next method so it will return to the original caller as well as it throw method to report exceptions.

As you may have guessed there are already plenty of implementations of this machinery - the one I like most is co. Take a look at an example from their page and enjoy callback free life!