JavaScript Async Generators Unleashed: Harnessing Asynchronous Power
Tags: javascript, generator, generators, async
Published: 2023-09-15 06:46:16

---
title: JavaScript Async Generators Unleashed: Harnessing Asynchronous Power
published: true
date: 2023-09-15 06:00:11 UTC
tags: javascript, js, generator, generators, async
canonical_url: https://blog.delpuppo.net/javascript-async-generators-unleashed-harnessing-asynchronous-power
cover_image: https://cdn.hashnode.com/res/hashnode/image/upload/v1693134246282/ca146f71-088a-45df-a914-a323c2c8287a.png
series: Javascript Iterators & Generators

---

In the dynamic realm of JavaScript, the ability to efficiently handle asynchronous operations is paramount for building responsive and robust applications. While Async Iterators have already proven their worth in managing asynchronous data streams, JavaScript takes another leap forward with Async Generators. Imagine a world where you can effortlessly produce a continuous stream of asynchronous data while dynamically controlling its flow. This is precisely where Async Generators excel, revolutionizing the way we handle and manipulate asynchronous sequences.

In this blog post, we're diving deep into the captivating world of JavaScript Async Generators. We'll unveil how Async Generators empower developers to create elegant solutions for complex asynchronous scenarios. Whether you're grappling with real-time data synchronization, asynchronous data processing, or any scenario demanding fine-tuned control over data flow, join us on this journey as we unravel the capabilities, nuances, and transformative power of JavaScript Async Generators.

Before we delve into Async Generators, let's recap the basics of asynchronous programming. In JavaScript, asynchronous tasks are crucial for handling time-consuming operations without blocking the main thread. Functions like setTimeout, AJAX requests, and Promises have become commonplace in managing asynchronous operations.

Generators, denoted by `function*` syntax, are a powerful feature introduced in ES6. They allow us to pause and resume the execution of a function, providing a valuable mechanism for lazy evaluation. The `yield` keyword within a generator function lets us yield values individually, pausing the function's execution until the next value is requested. If you want to learn more, there is relative content about them in this series.

Now, imagine combining the magic of asynchronous operations with the versatility of generators. Enter Async Generators! These supercharged functions enable us to work with asynchronous data streams more intuitively and elegantly.

An async generator function is declared using `async function*` syntax. Inside, the `yield` keyword can be combined with `await` to handle asynchronous operations gracefully. This means we can yield promises, fetch data and seamlessly integrate it into our data stream.

The real beauty of async generators lies in their ability to simplify complex asynchronous tasks. They're particularly handy when dealing with data streams from sources like databases, APIs, or even real-time event streams. Imagine processing a continuous stream of sensor data or handling multiple concurrent requests without drowning in callback hell!

Let's take a look at an example

```ts
async function* getUsers(ids: number[]) {
  for (const id of ids) {
    console.log("getUsers next");
    const data = fetch(`https://reqres.in/api/users/${id}`).then(res =>
      res.json()
    );
    yield data;
  }
}
```

Using a `for-of` loop, the function iterates over the IDs and fetches the data for every user on every iteration. However, using the yield, stop the execution and resume it only when the caller asks for the next value.  
Like Async Iterators, also Async Generators can be iterated using a `for-await-of` loop.

```ts
for await (const user of getUsers([1, 2, 3, 4, 5])) {
  console.log(user);
}
```

Error handling and completion are crucial aspects of asynchronous programming. With async generators, we can catch errors using try-catch blocks within the generator function. Plus, we can signal the end of data using the 'return' statement, closing the loop on our data stream.

```ts 
try {
  for await (const user of getUsers([1, 2, 3, 4, 5])) {
    console.log(user);
  }
} catch (err) {
  console.error(err);
}

async function* getUsers(ids: number[]) {
  for (const id of ids) {
    console.log("getUsers next");
    const data = fetch(`https://reqres.in/api/users/${id}`).then(res =>
      res.json()
    );
    if (id > 4) return data; // <-- Return
    yield data;
  }
}
```

Like Generators, also Async Generators can be combined. For instance, we can create an Async Generator `map` function that accepts an Async Generator and a transformation function and returns a new async generator with the transformations.

```ts
async function* map<T, U>(
  asyncIterable: AsyncIterable<T>,
  callback: (value: T) => U
) {
  for await (const item of asyncIterable) {
    yield callback(item);
  }
}
```

Asynchronous programming in JavaScript continues to evolve, and Async Generators are a testament to its dynamic nature. With their power to elegantly manage complex asynchronous tasks, async generators offer a glimpse into the future of streamlined coding. So, fellow developers, embrace this innovative tool and unlock new possibilities in your projects.

I also created a video about Async Generators on my [Youtube channel](https://www.youtube.com/@Puppo_92/) and you can find it below.

{% embed https://youtu.be/0uQvNZ8th08 %}

That's a wrap for today's discussion on JavaScript Async Generators. I hope you found this exploration insightful and inspiring. Remember, staying curious and open to new concepts is the key to mastering your craft in the ever-evolving programming landscape.

If you enjoyed this post, like and share it. And if you have questions, feedback or doubts let me know in the comments.

Until next time, happy coding and thanks for reading 👩💻👨💻

N.B. you can find the code of this post [here](https://github.com/Puppo/javascript-iterators-and-generators/tree/04-async-generators).

{% embed https://dev.to/puppo %}