Understanding Web Workers: What Are They and How Can They Enhance Your Web Apps
Tags: webdev, javascript, beginners, tutorial
Published: 2023-10-08 00:56:58

JavaScript is the language of the web, offering developers a plethora of features to craft exceptional products. Yet, amidst this toolkit, one gem remains relatively underutilized: **web workers**. But what exactly are web workers, and how do they empower us as developers? Let's embark on a journey of discovery in this blog post!

## Challenges of Main Thread Execution
In web development, browsers rely on a single thread to execute JavaScript, which can lead to a common problem. Imagine your web app running smoothly until a heavy JavaScript task suddenly takes over. Because the main thread can handle only one task at a time, it monopolizes the CPU, causing your website to become unresponsive and frustrating users.

## A Solution: Parallel Execution
This blog post addresses this issue by introducing web workers. Web workers enable you to run demanding tasks in parallel with the main thread, ensuring your website remains responsive even during complex operations. Let's delve into web workers and explore how they tackle this challenge.

## Understanding Web Workers: What Are They and How Can They Enhance Your Web Apps
Web workers are JavaScript objects that provide a way to run scripts in the background without affecting the main thread's performance. They are useful for executing tasks that are computationally intensive or time-consuming, ensuring that the user interface remains responsive.
Web workers are a valuable resource, but they can be relatively expensive to create. It's best to reuse them for multiple tasks whenever possible.
One key limitation of web workers is that they cannot access the Document Object Model (DOM) directly, making them ideal for tasks that do not interact with the DOM. To facilitate communication between the main thread and web workers, there are APIs available for seamless data exchange and coordination.
In the world of web workers, there are several types available, each with its specific use cases:

1. **Dedicated Worker**: This type of web worker is widely supported across various browsers and can be used in most web development scenarios.
2. **Shared Worker**: Shared workers are not as universally supported as dedicated workers. Before implementing them, it's essential to check if they are available in your target browsers.
3. **Service Worker**: Service workers are primarily known for their role in creating progressive web apps (PWAs) and handling offline functionality. However, like shared workers, their support varies across browsers, so checking compatibility is crucial.
4. **AudioWorklet**: This worker type is specialized for audio processing and is also subject to varying browser support.

We'll primarily focus on Dedicated Workers, as they are the most widely supported and commonly used web workers in web development.

## Working with Dedicated Workers:
Dedicated Workers offer a straightforward solution for executing scripts concurrently, allowing us to offload resource-intensive tasks from the main thread. Let's delve into a simple example to demonstrate how to use Dedicated Workers effectively:
```javascript
// main.js
const worker = new Worker('worker.js');

// Sending a message to the worker
worker.postMessage('Hello from the main thread!');

// Listening for messages from the worker
worker.onmessage = function (event) {
  console.log(`Received a message from the worker: ${event.data}`);
};
```
In this code snippet, we create a new worker and store it in the `worker` variable. Using the `postMessage()` function, we transmit a message from the main thread to the Dedicated Worker. We've also set up an `onmessage` event listener to capture and log any data emitted by the worker.
Now, let's craft the code for our web worker in a separate JavaScript file (`worker.js`):
```javascript
// worker.js
// Listen for messages from the main thread
self.onmessage = function (event) {
  const messageFromMain = event.data;
  console.log(`Received a message from the main thread: ${messageFromMain}`);

  // Perform some heavy computation
  const result = messageFromMain.toUpperCase();

  // Send the result back to the main thread
  self.postMessage(`Processed message: ${result}`);
};
```
The code is quite self-explanatory. However, you might notice the use of the `self` keyword. In the context of a web worker, `self` refers to an object that points to the worker's context. It's a reliable way to reference the worker context, unlike the `this` keyword, which can behave unpredictably in various situations.
Working with web workers is indeed straightforward. But it's essential to remember one key point: **Do NOT load the worker file in the main thread HTML using a script tag!** Instead, load it dynamically in your JavaScript code using `new Worker(path_to_the_worker_file)`, as demonstrated in the `main.js` file of our example.

### Importing External Scripts in a Web Worker:
In some cases, you might need to import additional scripts within your `worker.js` file. You can achieve this using the `importScripts` function. Here's an example:
```javascript
// worker.js
// Import an external script
importScripts('external-library.js');

// Now you can use functions and objects defined in external-library.js
const result = externalFunction(5); // Call a function from the imported script
console.log(`Result from the external function: ${result}`);
```

Dedicated workers in JavaScript provide a straightforward way to execute scripts concurrently, offloading heavy tasks from the main thread. By creating and communicating with web workers, you can keep your web applications responsive even during complex operations.

## Error Handling in Web Workers:
Web workers provide mechanisms for error handling to ensure robust and reliable execution. There are two main types of errors you might encounter:
### Unhandled Exceptions:
If an unhandled exception occurs within the web worker's code, it can lead to unexpected behavior. To catch such exceptions, you can set up an error event handler within the web worker's script. Here's how you can handle errors:
```javascript
self.addEventListener('error', function (event) {
  console.error('Error occurred in the web worker:', event.message);
  // You can perform custom error handling here
});
```
This event listener allows you to capture and log errors that occur within the web worker's context.
### Message Errors:
Another type of error can occur when you use the `postMessage` method to transfer data between the main thread and the web worker. These errors typically happen when you try to transfer unsupported data types or objects that cannot be cloned using [the structured clone algorithm](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm).
To handle message errors, you can set up a `messageerror` event listener in the main thread. Here's an example:
```javascript
worker.postMessage('Some data'); // Trying to transfer data that might cause an error

worker.addEventListener('messageerror', function (event) {
  console.error('Message error:', event.message);
  // Handle the error gracefully
});
```
The `messageerror` event allows you to detect and manage errors related to data transfer between the main thread and the web worker.
It's essential to implement error handling to ensure your web worker code runs smoothly and reliably, even when exceptions or data-related issues occur.

## Understanding Web Worker Fundamentals
### Threads are immortal unless you kill them! (like vampires🧛)
In the world of web workers, threads don't terminate themselves automatically. Unlike the main thread, which stops when your code reaches the end, web workers keep running indefinitely. To end a web worker thread, you need to explicitly call `self.close()` within the worker's context or use a reference from the main thread using its reference: `worker.terminate()`.
### Memory Usage
Web workers share memory with their parent thread (the main thread). This means that the memory space allocated to a web worker is separate from the main thread but not isolated. Be mindful of how much memory you use in a web worker, as excessive memory consumption can lead to performance issues in your application.
## Messaging with a Dedicated Worker
Communicating with a dedicated worker primarily relies on the `postMessage()` method from the main thread. It allows you to send data between the main thread and the worker. However, it's essential to note that the `message` object isn't passed by reference but by value (it is copied). Sending large amounts of data through messages can result in duplicated data in memory, impacting performance. Therefore, consider the data size and structure when sending messages.
### Transferring Ownership
Transferring ownership of data from the main thread to a web worker is possible to prevent data duplication. However, it's crucial to understand that the data is effectively transferred, not duplicated. For instance, when you transfer an array from the main thread to a web worker, the main thread's reference to that array becomes empty. Still, keep in mind that this feature is available in specific browsers and for specific data types like ArrayBuffer, Blob, and ImageBitmaps.
## One Interesting Use Case of Web Workers:
Imagine your website has lots of images. When the browser loads a webpage with `<img />` tags, it has to download and convert each image into a bitmap before displaying it on the screen. This decoding process happens in the main thread. If, for some reason, it takes a long time, it can lead to an unresponsive website and a poor user experience (what about the developer experience😂). In this scenario, you can harness web workers to fetch and decode the images, and then seamlessly transfer the bitmaps to the main thread. This approach keeps your website responsive and provides a smoother user experience.

## Conclusion
As we conclude our exploration of web workers, remember that they're the hidden gems of web development, waiting to enhance your projects. These tools offer parallelism's power, and our journey has revealed their definition, applications, and best practices. In this ever-evolving web development landscape, web workers are one of your secret weapons. Experiment, innovate, and unleash their potential. Happy coding!