test.js

var blob = new Blob([
    "onmessage = function(e) { postMessage('msg from worker'); }"],
    {type:"application/javascript"}
);

// Obtain a blob URL reference to our worker 'file'.
var blobURL = window.URL.createObjectURL(blob);

var worker = new Worker(blobURL);
worker.onmessage = function(e) {
  var p = document.createElement('p');
  p.textContent = e.data;
  document.body.appendChild(p);
  console.log(e.data);
  // e.data == 'msg from worker'
};
worker.postMessage(""); // Start the worker.

Read the code again, all the worker is doing is onmessage = function(e) { postMessage('msg from worker'); }". The dom manipulation is done in the main context where it belongs.