Cover image for Do you know the most powerful feature of JS generators ?

Do you know the most powerful feature of JS generators ?

omenlog profile image Omar E. Lopez ・7 min read

Photo by Amar Yashlaha on Unsplash

In a previous article I described the concept of generator in JS, there was explained the strong relation that exists between Iterators, Iterables and Generators. Now in this post I want to focus on one specific feature that make generators unique inside of JS landscape, this is:

Bidirectional communication

Push and Pull protocols

In order to understand what is bidirectional communication(BC) first Push and Pull as communication protocols, between data producers and consumers should be understood.

With Pull the consumer is who determine when the data is received from the producer. Functions are the simpler example of pull in JS. For any function F is true that it doesn't know when the data will be produced or in another way F doesn't know when it will be executed, the consumer has all responsibility over the F() call to pull some kind of data.

In the other hand with Push protocol the producer has full control over the moment when the data is produced, the consumer doesn't know neither when or how the data is produced.
Promises comply with this definition of Push.
For every promise P a callback should be passed to its then method in order to get the promise data asynchronously, later at some point this callback will be executed when the promise is fulfilled, in this case the callback doesn't know about how the data was produced, the inner implementation of P determine when data is pushed to our callback.

Iterables are another example of Pull and Observables works as a Push mechanism.

Bidirectional communication using generators

BC over generators is based on the fact that they support Pull and Push at the same time, or in other words generators can be at the same time data consumers and data producers.

An example of generator as data producer:

function* producerGen() {
  yield 1;
  yield 2;
  yield 3;

function consumer() {
  const it = producerGen();

  console.log(it.next()); // {done: false, value:1 }
  console.log(it.next()); // {done: false, value:2 }
  console.log(it.next()); // {done: false, value:3 }

In this example producerGen is only acting as producer, the values are consumed inside of consumer function, here we have a pulling happening through our it variable. But a generator can consume data and producing it as well:

function* generator() {
  const dataFromOutSide = yield 1;
  console.log(dataFromOutSide); // 2

function consumer() {
  const it = generator();

  const dataFromGenerator = it.next().value;

  console.log(dataFromGenerator); // 1



Analyzing this piece of code step by step, first iterator it is obtained from generator function.
The first call to it.next() run generator till the point when it reach the yield keyword, at this point the execution of generator is paused and 1 is send to outside, acting generator in its roll as data producer. Then the value emitted from generator is printed and next is called again but passing an argument in the call it.next(2), when next is called with an argument generator execution is resumed, and also the previous yield expression is replaced by the argument used in the call to next, in this example yield 1 will be replaced by 2 so the variable dataFromOutside will receive 2.

bidirectional flow

This gif show the communication flowing in both directions from side to side, so is clear how generator produce and consume data, in fact consumer function is also a producer.

Advantages of Bidirectional Communication

After understand this feature, someone might wonder What are the benefits of bidirectional communication ?, and the answer is:

  • separation of concern
  • inversion of control
  • code easier to test
  • high level of decoupling

As example I'll implement a function two times one using async-await and another using generators, in order to analyze what is gained from bidirectional communication in the generator based implementation.
Suppose a function to get user data that first check if the user is in cache else it request the data from server.

async function getUserData(userId) {
  const userOnCache = await cache.get(`user:${userId}`);

  if (!userOnCache) {
    const userFromBackend = await server.getUser(userId);
    return userFromBackend;

  return userOnCache;

Error handling is not covered for simplicity

Thinking a moment about this function with unit tests in mind first thing to note is that getUserData depends on cache and server, is known that during unit tests should be avoided any call to backend and also any read against cache storage, therefore to test this function in isolation its dependencies should be mocked.
But mocking is a big topic in software development, there are many libraries dedicated to make easier mocks creation and in other hand there are some opinions about mocking as a code smell, besides all of this, developers claiming testing as a difficult task is a fact, mainly in situation when they have a implementation with high level of coupling and therefore should be implemented many mocks, this developers don't enjoy the testing process or worse they decide not to test the code at all.

getUserData can be implemented using some sort of dependency injection making it easier to test but this topic is out of the scope.

After use async-await and conclude that mocks are needed for unit test let's see what happen in the implementation using generators, for this async will be replaced by function* and every await sentence by yield.

function* getUserData(userId) {
  const userOnCache = yield cache.getUser(`user:${userId}`);

  if (!userOnCache) {
    const userFromBackend = yield server.getUser(userId);
    return userFromBackend;

  return userOnCache;

Now getUserData is a generator that will yield promises. Write unit tests for this generator is simple, for example a test for the use case when we don't have user data in cache so we get our user from the server can be:

import { getUserData } from './get-user-data';

it("should get user data from backend when user isn't cached", () => {
  // fake user data
  const userData = { name: 'Jhon', lastName: 'Doe' };

  // get an iterator from generator, remember this iterator will emit promises
  const it = getUserData('user123');

  // run generator til the first yield

  // resume generator execution passing undefined as data

  // resume generator, passing to it userData simulating the server response,
  // also retrieve the next value emitted by it,
  // at this point value came from the return statement
  const { value } = it.next(userData);

  // check that the correct data was returned

This show how easy is to test the code using bidirectional communication. The difference with the first implementation is that with async-await promises are send to JS engine and it will be in charge to resolve them and resume the function execution, that communication between the engine and our code can't be intercepted, so for test the function in isolation its dependencies should be mocked. In other hand generators give full control over the promises yielded by getUserData so they can be intercepted allowing pass to our generator whatever kind of data, indeed getUserData is totally unaware is the promise was resolved or if is fake data being injected.

This test could seem very brittle, coupled to our implementation, because next calls are linked to yield statements of getUserData also for every call to next should be passed manually the correct type of data, having this as a consequence that a little change one the implementation might break the test. For sure this is true this test can be improved, but I'm only showing how powerful BC is, maybe I cover this topic in a future post.

One drawback of generators is that with async functions they can be invoked and the language knows how to execute them, awaiting and resuming promises automatically. The same isn't true for generators, I mean JS doesn't what kind of values generators will produce and what should be done with them, so we as developers are in charge to get data and resume the execution of our generators. But don't worry if we know what type of values will be yielded then we can implement a function that pull values from our generator and resume it automatically.

This idea of write generator runners is not new and is used by some libraries

A siple run function that can execute generators can be:

async function run(iterator) {
  let iteratorResult = it.next();

  while (!iteratorResult.done) {
    const result = await iter.value;
    iteratorResult = it.next(result);

  return iteratorResult.value;

run will receive an iterator, then get the first data using next(), after that it will continue retrieving data from iterator while it isn't done, for every piece of data we await the property value to resume our generator passing the promise result in the next call, by last we return the last value emitted by iterator.

Alt Text

Run can be used like:

run(getUserData('user123')).then((userData) => {});


In summary this post explained very briefly Pull and Push as communication protocols also how bidirectional communication works on generators.
We explored this feature transforming a generators in data producers and consumers. As example the behavior of async-await was reproduced using generators, trying to exemplify how easy is build tests for a generator based implementation. This post isn't a comparative between generators and async-await, both are powerful and I'm really glad that JS support them. Hopefully you understand the basics of BC after this read, in the future post I'll continue writing about it exposing what we can achieve.

Thanks for read.

Posted on by:

omenlog profile

Omar E. Lopez


Developer, lover of JavaScript and interested in functional programming


markdown guide