Web Wroker returning a Promise. Sort of .Net Task.Run

PromiseWorkerHelper.js

class PromiseWorkerHelper{
    constructor(){
        //one shot object, we create it and invoke its run method just once
        this.alreadyCalled = false;

        //this.worker = new Worker("WebWorkerCode.js");
        this.worker = this._createWorkerFromString();

        //this is executed when the worker posts a message
        this.worker.onmessage = (msg) => this.resolveFunc(msg.data);
        this.worker.onerror = (msg) => this.rejectFunc(msg); 
        this.resolveFunc = undefined;
        this.rejectFunc = undefined;
    }

    _createWorkerFromString(){
        let workerOnmessageHandler = function(e){
            console.log('Message received from main script');
            let fnSt = e.data[0];
            console.log("fnSt: " + fnSt);
            let args = e.data[1];
            
            //we need the () trick for eval to return the function
            let fn = eval("(" + fnSt + ")");
            let workerResult = fn(...args);
            console.log('Posting result back to main script: ' + workerResult);
            postMessage(workerResult);
        
        };

        let str = "onmessage = " + workerOnmessageHandler.toString() + ";";
        let blob = new Blob([str], {type: 'application/javascript'});
        return new Worker(URL.createObjectURL(blob));

    }

    run(fn, ...args){
        if (this.alreadyCalled){
            throw "already used once";
        }
        this.alreadyCalled = true;

        let pr = new Promise((resolve, reject) => {
            this.resolveFunc = resolve;
            this.rejectFunc = reject;
        });

        this.worker.postMessage([fn.toString(), args]);
        return pr;

    }
}

useIt.js

function longFormatting(txt, formatCharacter){
    console.log("starting longFormatting");
    let res = formatCharacter + txt + formatCharacter;
    start = Date.now();
    while(Date.now() - start < 1000){}
    console.log("finishing longFormatting");
    return res;
}

function main(){
    document.getElementById("launchCalculationBt").addEventListener("click", async () => {
        console.log("Main.js, inside button handler");

        let txt = "hello";
        let formatCharacter = "---";

        //this does not work, I can not post a funtion to a webWorker, I have to stringify it
        // //trap in a closure the arguments to the function
        // let longFormattingClosure = () => {
        //     return longFormatting(txt, formatCharacter);
        // };
        // let promise = new PromiseWorkerHelper().run(longFormattingClosure);
        
        let promise = new PromiseWorkerHelper().run(longFormatting, txt, formatCharacter);
        
        let res = await promise;

        console.log("Main.js, result: " + res);

    });
}

window.addEventListener("load", main, false);