Actually postMessage can post not only String, but any Object, e.g.
myWorker.postMessage({
m1: 'msg1',
m2: 'msg2'
});
And in your worker:
onmessage = function (event) {
//will be msg1
console.log(event.data.m1);
//will be msg2
console.log(event.data.m2);
};