Javascript – How to create a Web Worker from a string


How can I use create a Web worker from a string (which is supplied via a POST request)?

One way I can think of, but I'm not sure how to implement it, is by creating a data-URI from the server response, and passing that to the Worker constructor, but I've heard that some browsers don't allow this, because of the same origin policy.

MDN states the uncertainty about the origin policy around data URI's:

Note: The URI passed as parameter of the Worker constructor must obey the same-origin policy. There is currently disagreement among browsers vendors on whether data URIs are of the same-origin or not; Gecko 10.0 (Firefox 10.0 / Thunderbird 10.0) and later do allow data URIs as a valid script for workers. Other browsers may disagree.

Here's also a post discussing it on the whatwg.

Best Solution


  • blob: for Chrome 8+, Firefox 6+, Safari 6.0+, Opera 15+
  • data:application/javascript for Opera 10.60 - 12
  • eval otherwise (IE 10+)

URL.createObjectURL(<Blob blob>) can be used to create a Web worker from a string. The blob can be created using the BlobBuilder API deprecated or the Blob constructor.


// URL.createObjectURL
window.URL = window.URL || window.webkitURL;

// "Server response", used in all examples
var response = "self.onmessage=function(e){postMessage('Worker: ';}";

var blob;
try {
    blob = new Blob([response], {type: 'application/javascript'});
} catch (e) { // Backwards-compatibility
    window.BlobBuilder = window.BlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
    blob = new BlobBuilder();
    blob = blob.getBlob();
var worker = new Worker(URL.createObjectURL(blob));

// Test, used in all examples:
worker.onmessage = function(e) {
    alert('Response: ' +;


Web workers are supported in the following browsers source:

  • Chrome 3
  • Firefox 3.5
  • IE 10
  • Opera 10.60
  • Safari 4

This method's support is based on the support of the Blob API and the URL.createObjectUrl method. Blob compatibility:

  • Chrome 8+ (WebKitBlobBuilder), 20+ (Blob constructor)
  • Firefox 6+ (MozBlobBuilder), 13+ (Blob constructor)
  • Safari 6+ (Blob constructor)

IE10 supports MSBlobBuilder and URL.createObjectURL. However, trying to create a Web Worker from a blob:-URL throws a SecurityError.

Opera 12 does not support URL API. Some users may have a fake version of the URL object, thanks to this hack in browser.js.

Fallback 1: data-URI

Opera supports data-URIs as an argument to the Worker constructor. Note: Do not forget to escape special characters (Such as # and %).

// response as defined in the first example
var worker = new Worker('data:application/javascript,' +
                        encodeURIComponent(response) );
// ... Test as defined in the first example


Fallback 2: Eval

eval can be used as a fallback for Safari (<6) and IE 10.

// Worker-helper.js
self.onmessage = function(e) {
    self.onmessage = null; // Clean-up
// Usage:
var worker = new Worker('Worker-helper.js');
// `response` as defined in the first example
// .. Test as defined in the first example