Web worker async task

BackgroundTask.js

/*
 * This is a simple example of web worker
 * Background task is a class which creates a new thread.
 * 'fn' is a heavy CPU utilization function
 * 'start' method accepts only one argument (a message for web worker)
 * Restrictions:
 * - fn must be a pure function. It does not have an access to any outer scope vars.
 * - fn must not be native function.
 * - fn must not be bound to any 'this'
 */

// Main class
class BackgroundTask {
    constructor(fn) {
        this.fn = fn;
        this.blob = null;
        this.worker = null;
    }

    start(message) {
        this.blob = new Blob([
            `self.onmessage=${this.fn.toString()}`
        ], {type: 'application/javascript'});
        this.worker = new Worker(URL.createObjectURL(this.blob));
        this.worker.postMessage(message);
        return new Promise((resolve) => {
            this.worker.onmessage = ({data}) => {
                resolve(data);
                this.worker.terminate();
            };
        })
    }
}

// Heavy CPU Fibonacci example
new BackgroundTask(function fibonacci(msg) {
   const fib = (n) => n < 1 ? 0
        : n <= 2 ? 1
        : fib(n - 1) + fib(n - 2);
   this.postMessage(fib(+msg.data));
})
    .start('50')
    .then((data) => {
        console.log(data);
    });