Multithreading on javascript

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Javascript

threads

prototype

Hello.
I want to give multi threading ability to a javascript prototype. I mean i have a funtion in my object ,when i call it, it generates a thread or better to say web worker. basic things done, but when i receive message from web worker, I cant access 'this' variable that refers to current object ,therefore its no way to access current object stuffs.
Here is my code:

var worker = new Worker('mi.js');
        this.p = 1;
        worker.postMessage({ test:1});
        worker.onmessage = function (e) {
            console.log("RETURNED : " + e.data); 
           //i cant access p on current object.
            this.p += 2;
            console.log(this.p.toString());      
            worker.terminate();
        };

Thank you