webworker

main.js

/*
Question: I need to access to the web worker's onmessage data somehow
*/
var worker = new Worker('worker.js');

var file = {
  createReadStream: function() {
    worker.postMessage(options); // some sort of options
  
    worker.onmessage = function(e) {
      // this has to be outside?
      // can't make async to sync so..
      var stream = e.data; 
    }
    // has to return stream HERE
  }
};

worker.js

self.onmessage = function(e) {
  var options = e.data;
  var stream = getStreamObjectSomehow(options);
  postMesage(stream);
};

// try
window.stream = e.data

Use bind to bind your onmessage callback to the file object (this), create an instance variable to store the object there, or if you're using something like Backbone, trigger an event and pass the data along with that event. The listener listening to file will hear about it when it's ready, no guessing needed.

Here's another workaround, using the Promise object.

var worker = new Worker('worker.js');

var file = {
  createReadStream: function() {
    worker.postMessage(options); // some sort of options

    // sorta impossible to make an async function synchronous, so.... promises, promises, promises...
    var promise = new Promise(function(resolve, reject) {
      worker.onmessage = function(e) {
        var stream = e.data;
        resolve(stream)
      };
      worker.onerror = function(e) {
        reject(e.data)
      }
    });
    // return a Promise object, instead of a stream
    return promise;
  }
};
file.createReadStream().then(function(stream) { console.log(stream) }).catch(function(err) { console.log(err) });