promise-worker-bi

A small (~2 kB min+gz) and performant library for communicating with web workers and shared workers, using promises. Post a message from the browser to the worker, get a promise that resolves to the response. Post a message from the worker to the browser, get a promise that resolves to the response. And with shared workers, you can either broadcast to all browser tabs or send a message to a specific tab.

This is based on promise-worker which only allows you to send messages from the browser to the worker, not in reverse. This library allows both, using the exact same API, and has additional support for shared workers.

Usage

Install:

yarn add promise-worker-bi

Inside your main bundle:

// main.js
import { PWBHost } from "promise-worker-bi";

const worker = new Worker("worker.js");
const promiseWorker = new PWBHost(worker);

// Only needed if you send messages from the worker to the host
promiseWorker.register((message) => {
  return "pong2";
});

try {
  const response = await promiseWorker.postMessage("ping");
  // handle response 'pong'
} catch (error) {
  // handle error
}

Inside your worker.js bundle:

// worker.js
import { PWBWorker } from "promise-worker-bi";

const promiseWorker = new PWBWorker();

// Only needed if you send messages from the host to the worker
promiseWorker.register((message) => {
  return "pong";
});

try {
  const response = await promiseWorker.postMessage("ping2");
  // handle response 'pong2'
} catch (error) {
  // handle error
}

Notice that except for initialization of the promiseWorker object, the API is identical in the browser and in the worker. Either one can initiate a message. In all of the subsequent examples, promiseWorker initialization is omitted, so you can put the two blocks of code respectively in the worker and browser, or in the browser and worker.

Message format

The message you send can be any object, array, string, number, etc. - anything that is serializable by the structured clone algorithm:

await promiseWorker.postMessage({
  hello: "world",
  answer: 42,
  "this is fun": true,
});

promiseWorker.register((message) => {
  console.log(message); // { hello: 'world', answer: 42, 'this is fun': true }
});

Promises

The registered handler can return either a Promise or a normal value:

promiseWorker.register(async () => {
  "much async, very promise";
});

const message = await promiseWorker.postMessage(null);
console.log(message): // 'much async, very promise'

Error handling

Any thrown errors or asynchronous rejections during a response will be propagated as a rejected promise. For instance:

promiseWorker.register((message) => {
  throw new Error("naughty!");
});

try {
  await promiseWorker.postMessage("whoops");
} catch (error) {
  console.log(error.message); // 'naughty!'
}

But what about errors in the worker that are not during a response? (Errors in the host, you can handle as you would normally do.)

For a Web Worker, we could just use the normal error event and be fine. But for a Shared Worker, that is not the case, because browsers seem to handle errors inside Shared Workers differently (currently Firefox seems to send the normal error event to all hosts, but Chrome does not).

Therefore, promise-worker-bi includes a unified API that works in Web Workers and Shared Workers. Include some code like this in your host:

promiseWorker.registerError((error) => {
  console.log("Error inside worker!", err);
});

That will work if worker is a Web Worker or a Shared Worker, but there is one important difference. For a Shared Worker, it will only fire in the first host, to prevent duplicate errors from reaching an error log (assuming you're logging these errors somewhere).

Source maps

If you use a source map for your worker script, this will not be reflected in the stack traces from promise-worker-bi (either in promiseWorker.registerError in response to promiseWorker.postMessage). However, you can still use your source map to translate the file/line/column numbers. Clever third-party error reporting services like Bugsnag do this automatically!

Multi-type messages

If you need to send messages of multiple types to the worker, just add some type information to the message you send:

const response1 = await promiseWorker.postMessage({
  type: "en",
});

const response2 = await promiseWorker.postMessage({
  type: "en",
});

promiseWorker.register((message) => {
  if (message.type === "en") {
    return "Hello!";
  } else if (message.type === "fr") {
    return "Bonjour!";
  }
});

Shared Workers

Shared Workers are like Web Workers, but multiple tabs of your app can share the same worker process. In this model, you have one copy of promise-worker-bi running inside your Shared Worker and one copy running in each tab the user opens of your application. Sending messages from a tab to the Shared Worker is the same as above. But when sending messages from the Shared Worker, it's different because there are potentially multiple tabs. Currently, promise-worker-bi supports either broadcasting a message to all tabs or sending a message to one specific tab.

Here's an example:

// main.js
import { PWBHost } from "promise-worker-bi";

const worker = new SharedWorker("worker.js");
const promiseWorker = new PWBHost(worker);

promiseWorker.register((message) => {
  console.log(message);
});

// setTimeout is just to give you enough time to open main.js and main2.js in
// two separate tabs.
setTimeout(async () => {
  const response = await promiseWorker.postMessage("broadcast");
  console.log("Echoed response:", response);
}, 1000);

// main2.js
import { PWBHost } from "promise-worker-bi";

const worker = new SharedWorker("worker.js");
const promiseWorker = new PWBHost(worker);

promiseWorker.register((message) => {
  console.log(message);
});

// setTimeout is just to give you enough time to open main.js and main2.js in
// two separate tabs.
setTimeout(() => {
  const reponse = await promiseWorker.postMessage("just this tab");
  console.log("Echoed response:", response);
}, 2000);

// worker.js
import { PWBWorker } from "promise-worker-bi";

const promiseWorker = new PWBWorker();

promiseWorker.register((message, hostID) => {
  if (message === "broadcast") {
    promiseWorker.postMessage("to all tabs");
  } else {
    promiseWorker.postMessage(`hello host ${hostID}`, hostID);
  }

  return message;
});

Then open main.js and main2.js in two browser tabs. The message sent from main.js ('broadcast') will result in worker.js sending a message to both tabs, but the message sent in main2.js will result in a message sent only to that one tab. So if you look in the consoles in your two tabs, you will see this in the first tab:

to all tabs
Echoed response: broadcast

And this in the second tab:

to all tabs
hello host 1
Echoed response: just this tab

(If you open main2.js first, "hello host 1" will instead be "hello host 0".)

Transferable objects

It is possible to transfer certain types of objects from host to worker or from worker to host. The syntax is similar to how it works with a normal postMessage, where an array of the objects to be transferred is added as an additional parameter.

One tricky thing - because promiseWorker.postMessage already has a second argument for specifying an optional hostID value fo sending a message from a shared worker to a specific host, the transferables argument is the third argument:

const buffer = new ArrayBuffer(1);
promiseWorker.postMessage(buffer, hostID, [buffer]);

From host to worker, the hostID argument doesn't do anything, so in most situations (host -> worker, web worker -> host, shared worker -> all hosts) you can pass undefined as the second argument:

const buffer = new ArrayBuffer(1);
promiseWorker.postMessage(buffer, undefined, [buffer]);

If you want to respond to a message by sending a transferable object back, such as if you want to send an object a worker for processing and then return it to the host, you can do it like this, in either the worker or host:

promiseWorker.register(async (message) => {
  const processedMessage = await processMessage(message);

  return {
    message: processedMessage,
    _PWB_TRANSFER: [processedMessage.someTransferableProprty],
  };
});

That _PWB_TRANSFER property is used internally by promise-worker-bi under the assumption that no normal object in your application would have a property with that name.

Browser support

In theory it should work in:

• Chrome
• Firefox
• Safari 8+
• IE 10+
• Edge
• iOS 8+
• Android 4.4+

There used to be automated tests in various browsers via zuul and Sauce Labs, but it suffered bit rot and I could no longer get it to work consistently (PRs welcome!). Currently, yarn test uses Karma to run tests in whatever versions of Chrome and Firefox you have installed.

API

Main bundle

new PWBHost(worker: Worker | SharedWorker)

Create a new instance of PWBHost, using the given worker.

• worker - the Worker, SharedWorker or PseudoWorker to use.

promiseWorker.registerError((error: Error) => void)

This should only be called in the browser, not in a worker.

When an error in your web/shared worker process occurs that is not directly in response to a promiseWorker.postMessage call, it will be sent to the callback you provide here to promiseWorker.registerError.

Although normally stack traces are not sent from the worker to the main thread, promise-worker-bi magically works around this problem so you will see a stack trace in error.

Worker bundle

new PWBWorker()

Create a new instance of PWBWorker.

Both bundles

promiseWorker.register((message: any, hostID?: number) => any)

Register a message handler wherever you will be receiving messages: in the worker, in the browser, or both. Your handler consumes a message and returns a promise or value.

The hostID parameter is only defined inside a shared worker, in which case it is a unique number identifying the host that the message came from.

The return value of the callback function (shown above as any) must resolve to something structured cloneable.

If you want to transfer an object in the return value of the callback, then the return type should be { message: any, _PWB_TRANSFER: Transferable[] } rather than just any.

promiseWorker.postMessage(message: any, hostID?: number, transfer?: Transferable[]): Promise<any>

Send a message to the browser or worker and return a Promise. message must resolve to something structured cloneable.

The hostID parameter is only meaningful when sending a message from a shared worker. If you leave it out, it will send the message to all hosts. If you include it, it will send the message only to that specific host. You can get the hostID from the promiseWorker.register function described above.