[kobezzza]

var code = 'postMessage(123);';
var blob = new Blob([code],{type: "text/javascript"});
var ref = URL.createObjectURL(blob);
var worker = new Worker(ref);
worker.onmessage = function (oEvent) {
    alert("data from worker: " + oEvent.data);
};
URL.revokeObjectURL(ref);

Удобнее использовать toString у функций.

function foo() {
    ...
}

var blob = new Blob([foo.toString()], {type: "text/javascript"});