Web Workers are incredibly useful if you need to do some heavy processing without hanging the UI.

This tutorial is aimed at implementing web workers in a create-react-app project without ejecting and avoiding TypeScript errors. If you want to know how to set up a React project you should check out my previous post

Web Workers

In a normal web app, all the JS is running in the main thread, so this means if you execute a very intensive function you could potentially hang the UI until the function has finished, which is not a good user experience. This can be avoided by using web workers because they run scripts in background threads.

Web Workers execute a script from a static file that is separate from your main JS file. Previously, this meant that you would need to eject from create-react-app in order to modify the webpack config and export a separate file. However, with recent developments you can now use web workers in CRA without ejecting!

comlink-loader

There's a great package called comlink-loader that allows you to call functions from a web worker like a class method, and skip the postMessage and onmessage exchange described in the docs.

Install the package:

npm install -D comlink-loader

or 

yarn add comlink-loader

Create the worker

Create a directory called worker and it will contain three files:

• custom.d.ts
• index.ts
• worker.ts

worker.ts is where you will keep the functions that you want executed in a background thread:

/* ./worker/worker.ts */

export function processData(data: string): string {

  // Process the data without stalling the UI

  return data;
}

custom.d.ts is a TypeScript declaration file to avoid compilation errors.

For type safety, add your functions from worker.ts to the class as a method and the return type should be wrapped in a promise.

/* ./worker/custom.d.ts */

declare module 'comlink-loader!*' {
  class WebpackWorker extends Worker {
    constructor();

    // Add any custom functions to this class.
    // Make note that the return type needs to be wrapped in a promise.
    processData(data: string): Promise<string>;
  }

  export = WebpackWorker;
}

index.ts contains the inline loader so you don't have to eject and modify the webpack config

/* ./worker/index.ts */

// eslint-disable-next-line
import Worker from 'comlink-loader!./worker'; // inline loader

export default Worker;

Depending on your linter rules, you may get a build error:

Unexpected '!' in 'comlink-loader!./worker'. Do not use import syntax to configure webpack loader

This is not a problem and the rule can be disabled for the line (see lines 3 and 4 above).

Import into React app

Now the fun part, simply import the worker into your React app and create a new instance to start using it:

/* App.tsx */

import Worker from './worker'

// Create new instance
const instance = new Worker();

const onClick = () => {
  const data = 'Some data';

  return new Promise(async resolve => {

    // Use a web worker to process the data
    const processed = await instance.processData(data);

    resolve(processed);
  });
};

Conclusion

It is quite simple to implement a web worker into your React app, and it will be a major improvement to the user experience if your app does a lot of heavy processing.

If you have any suggestions or questions feel free to leave a comment.