Presenting a tool to create an asynchronous webworker version of a library

A library webworker wrapper

By: David
Tags: webworker

The premise

Imagine you have a library, consisting of an object with a bunch of methods. Some or all of those are heavy, and will lock up the thread for a while.

var myHeavyLib = {
  aHeavyMethod: function(arg1,arg2){
    // extremely heavy computing left out here
    return arg1 + arg2;
  anotherHeavyMethod: function(arg1,arg2){
    // extremely heavy computing left out here
    return arg1 + arg2;
  // etc

If we're using this library in a web app, this won't be a nice user experience. We should fix that by delegating the heavy lifting to a Web worker.

This situation happened to me very recently, and instead of just webworkerifying my library, I generalised prematurely and made a tool to webworkerify any library!

Meet WorkerWrapper

I made a command line tool I call WorkerWrapper. Feed it a library...

...and it will generate two files. First off an async version of the library. This is a very small file that gives you an object containing the same method names as the library, but they now return promises that resolves when a webworker gets the result from the real method.

var asyncVersion = {
  aHeavyMethod: function(arg1,arg2){
    // wrapping magic left out here
    return promise;
  anotherHeavyMethod: function(arg1,arg2){
    // wrapping magic left out here
    return promise;
  // etc

The async version then spawns web workers from the worker version, which is really just the library with a web worker facade on top.


Say our heavy lib is at lib/heavylib.js.

We would then include WorkerWrapper as a dependency in package.json, and add a script using the workerwrap command that WorkerWrapper exposes:

  "dependencies": {
    "workerwrapper": "github:krawaller/workerwrapper"
  "scripts": {
    "wrap": "workerwrap lib/heavylib.js"

The command just takes one parameter, namely the relative path to the library to be wrapped.

Now heavylib_worker.js and heavylib_async.js will be created next to heavylib.js. I consume heavylib_async.js in my app.

However! In order to allow setup with the worker file, heavylib_async.js doesn't export the wrapped library directly, but a function!

module.exports = function(pathToWorkerFile, numberOfWorkers){

    // ...wrapping magic here...

    return wrappedLib;

You call this function with a relative path to where you've placed the worker file, as well as the number of parallel webworkers you want spun up.

See it in action

In the live app below I've wrapped this silly library:

var heavyLib = {
  aHeavyMethod: function(arg1,arg2){
    console.log("aHeavyMethod called in lib");
    for(var a=0; a<70000; a++){
      for(var b=0; b<70000; b++){
    return arg1 + arg2;

On my machine the method takes around 2 seconds. Experiment with hitting the single-worker and multi-worker versions multiple times in quick succession, and you'll see the benefits of parallel workers!

You can run the demo in a standalone tab here, and the demo source code is here.

Under the hood of the wrapper

You can peruse the full source code for the tool here, but in short, the script doing the wrapping works with two template files.

Here's the one for the async version:

module.exports = function(pathToLib, nbrOfWorkers){

  nbrOfWorkers = nbrOfWorkers || 1;

  var workerListeners = {};
  var freeWorkers = [];
  var busyWorkers = [];
  var nextCallId = 0;

  function freeUpWorker(worker){
    busyWorkers = busyWorkers.filter(function(w){ return w !== worker; });

  function requestWorker(){
    var worker = (freeWorkers.length ? freeWorkers : busyWorkers).shift();
    return worker;

  function workerMessageHandler(e){
    var resultid =[0];
    var result =[1];
    delete workerListeners[resultid];

  function libMethod(method){
    var args =;
    return new Promise(function(resolve,reject){
      var callid = ++nextCallId;
      var worker = requestWorker();
      workerListeners[callid] = resolve;

  for(var i=0; i<nbrOfWorkers; i++){
    var worker = new Worker(pathToLib);
    worker.onmessage = workerMessageHandler;

  return LIB_METHODS.reduce(function(mem,method){
    mem[method] = libMethod.bind(null,method);
    return mem;


Notice the LIB_METHODS at the bottom, which the build script replaces with an array of the methods in the library to be wrapped:

  pathToLibDir + '/'+libName+'_async.js',
  wrapperTemplate.replace('LIB_METHODS', JSON.stringify(Object.keys(lib)))

The template to the worker file is much simpler:

var lib = require('PATH_TO_LIB');

onmessage = function(e){
  var method =[0];
  var callid =[1];
  var args =[2];
  var result = lib[method].apply(lib,args);

The script uses webpack to bundle the library into the template.

fs.writeFileSync(pathToTempFile, workerTemplate.replace('PATH_TO_LIB', pathToLib));

var compiler = webpack({
  entry: pathToTempFile,
  output: {
    path: pathToLibDir,
    filename: libName + '_worker.js'
  resolve: {
    extensions: [".js"]
  var message = stats.toString("errors-only") || 'Webworker file created at '+pathToLibDir+'/'+libName + '_worker.js';

Refer to the source code for the full truth.

Wrapping up

Pun very much intented.

Is a stand-alone tool to do this really needed? Likely not, but I had fun, and it felt good to be able to remove all the wrapping code from the repo of the library I was working on.

Which is also the library I should immediately go back to work on, if I want to keep my job.

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