https://gist.github.com/developit/65a2212731f6b00a8aaa55d70c594f5c revised at line 33

workerize-pool.js

/**
 * Copyright 2018 Google Inc. All Rights Reserved.
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *     http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*  import Worker from 'workerize-loader!./worker';
 *  import WorkerPool from './worker-pool';
 *  let pool = new WorkerPool(Worker, 4);
 *  for (let i=10; i--; ) pool.doThing();
 */
export default (workerFactory, size) => new WorkerPool(workerFactory, size);

class WorkerPool {
  constructor(workerFactory, size) {
    let worker = (this.workerFactory = workerFactory)();
    this.poolSize = size || 4;
    this.used = 1;
    this.pool = [worker];
    this.jobs = [];
    for (let i in worker) if (worker.hasOwnProperty(i) && typeof worker[i]==='function') {
      this[i] = this._method(i);  // proxy all methods available on the worker
    }
  }
  _method(name) {
    return (...args) => this._queueJob(name, args);
  }
  _queueJob(method, args) {
    return new Promise((y, n) => {
      this.jobs.push({ method, args, y, n });
      this._nextJob();
    }) 
  }
  _nextJob() {
    let worker = this.pool.pop();
    if (!worker) {
      if (this.used >= this.poolSize) return;
      this.used++;
      worker = this.workerFactory();
    }
    const job = this.jobs.shift();
    if (!job) return;
    worker[job.method](...job.args).then(job.y).catch(job.n).finally(() => {
      this.pool.push(worker);
      this._nextJob();
    });
  }
}