build.hxml

-main MyWorkerScript
-js myworkerscript.js

--next
-main MyWorkerTest
-js myworkertest.js

MyWorkerScript.hx

using WorkerScript;

class MyWorkerScript extends WorkerScript {
   
  public override function onMessage(e){ 
  	trace("on message triggered"); 
        postMessage("ola");
  }
  
  static function main(){
     new MyWorkerScript().export();
  }

}

MyWorkerTest.hx

class MyWorkerTest {
  public static function main(){
    var worker = new Worker("myworkerscript.js");
    worker.postMessage("coucou");
  }
}

Worker.hx

@:native("Worker")

extern class Worker {
   public function new( script : String ) : Void;
   public function postMessage( m : Dynamic ) : Void;
   public function addEventListener( type : Dynamic , cb : Dynamic -> Void ) : Void;
}

WorkerScript.hx

class WorkerScript {
        public function new(){}
	public function onMessage( e : Dynamic ) : Void { }
        public inline function postMessage( m : Dynamic ) : Void {
              untyped __js__("self.postMessage( m )");
        }
        public static function export( script : WorkerScript ){
             untyped __js__("self.onmessage = script.onMessage");
        } 
}