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Before reading this please ensure you fully understand the concept of Web Workers.

Operative is a small JS utility (~1.8k gzipped) for seamlessly creating Web Worker scripts. Its features include:

  • Seamless API Authoring
  • Producing debuggable Worker Blobs
  • Providing console interface for simple logging
  • Degrading where Worker/Blob support is lacking

Why Operative?

Utilising unabstracted Workers can be cumbersome and awkward. Having to design and implement message-passing contracts and having to setup event listeners yourself is time-consuming and error-prone. Operative takes care of this stuff so you can focus on your code.

Before you get excited:

Even with Operative you are still subject to the constraints of Web Workers, i.e.

  • No DOM/BOM Access.
  • No synchronous communication with parent page.
  • An entirely separate execution context (no shared scope/variables).
  • Limitations on data transfer depending on browser.

And it won't make things uniformly faster or less burdensome on the UI. Operative will fall-back to using iframes in older browsers, which gives you no non-blocking advantage.

Non-blob worker support (i.e. for IE10) requires that you have a same-origin copy of Operative (this means you can't solely rely on CDNs or elsewhere-hosted scripts if you want to support IE10).

Browser Support

Operative 0.4.4 has been explicitly tested in:

  • Chrome 14, 23, 29, 37, 42
  • Firefox 3, 10, 18, 23, 32
  • IE 8, 9, 10 (Windows 7)
  • IE 11 (Windows 10)
  • Opera 25 (Mac)
  • Opera 10.6 (Windows 7)
  • Safari 5.1 (Windows 7)
  • Safari 6, 8 (Mac)
  • Safari (iPad Air, iOS 8)
  • Safari (iPhone 4S, iOS 5.1)

Support for Workers, with varying degrees of support for Transferables and Blobs:

  • FF 17+
  • Chrome 7+
  • Safari 4+
  • Opera 11+
  • IE10+

Note: Operative has not been tested in non-browser envs

Quick install

# bower
bower install operative
# or npm
npm install operative

Or just grab the built JS file from dist/, also available here (0.4.4):

Creating an Operative Module

An Operative module is defined as an object containing properties/methods:

var calculator = operative({
	add: function(a, b, callback) {
		callback( a + b );

This would expose an asynchronous API:

calculator.add(1, 2, function(result) {
	result; // => 3

The add() function will run within a worker. The value it returns is handled by operative and forwarded, asynchronously to your callback function in the parent page.

Notice that the exposed add method requires its last argument to be a callback. The last argument passed to an operative method must always be a callback. All preceding arguments are passed into the worker itself.

NOTE: It's important to note that the Operative code is not executed in-place. *It's executed within a Worker. You won't be able to access variables surrounding the definition of your operative:


var something = 123;

var myWorker = operative({
	doStuff: function() {
		something += 456;

(the something variable won't exist within the Worker)

Instead you can do:

var myWorker = operative({
	something: 123,
	doStuff: function() {
		this.something += 456;

Need to iterate 10,000,000,000 times? No problem!

var craziness = operative({

	doCrazy: function(cb) {

		for (var i = 0; i < 10000000000; ++i);

		cb('I am done!');


craziness.doCrazy(function(result) {
	// Console outputs: Craziness: 14806.419ms
	result; // => "I am done!"

Degraded Operative

Operative degrades in this order:

(higher is better/cooler)

  • Full Worker via Blob & Structured-Cloning (Ch13+, FF8+, IE11+, Op11.5+, Sf5.1+)
  • Full Worker via Eval & Structured-Cloning (IE10)
  • Full Worker via Blob & JSON marshalling (???)
  • Full Worker via Eval & JSON marshalling (Sf4)
  • No Worker: Regular JS called via iframe (older browsers)

Operative will degrade in environments with no Worker or Blob support. In such a case the code would execute as regular in-place JavaScript. The calls will still be asynchronous though, not immediate.

If you are looking to support this fully degraded state (honestly, only do it if you have to) then you'll also need to avoid utilising Worker-specific APIs like importScripts.

No Worker-Via-Blob Support

Operative supports browsers with no worker-via-blob support (e.g. IE10, Safari 4.0) via eval, and it requires operative.js or operative.min.js to be its own file and included in the page via a <script> tag. This file must be on the same origin as the parent page.

If you're bundling Operative with other JS, you'll have to have an additional (same-origin!) operative.js and specify it before creating an operative module via operative.setSelfURL('path/to/operative.js') (this'll only generate a request where the aforementioned support is lacking). Due to the usage of eval in these cases it is recommended to debug your operatives in more capable browsers.

Operative API Documentation

  • {Function} operative: A global function which creates a new Operative module with the passed methods/properties. Note: Non-function properties must be basic objects that can be passed to JSON.stringify.
  • Pass an object of methods, e.g. operative({ method: function() {...} ... });
  • Or pass a single function, e.g. operative(function() {}) (in which case a single function is returned)
  • Either signature allows you to pass dependencies as a second param: operative(..., ['dep1.js', 'dep2.js']).
  • {Boolean} operative.hasWorkerSupport: A boolean indicating whether both Blob and Worker support is detected.
  • {Function} operative.setSelfURL: Allows you to set the URL of the operative script. Use this if you want IE10 & Safari 4/5 support and you're not including operative by the conventional <script src="operative.js"></script>.
  • {Function} operative.setBaseURL: Allows you to set the URL that should be prepended to relative dependency URLs.

Creating an Operative:

To create an operative module pass an object of methods/properties:

var myOperative = operative({
	doX: function(a, b, c, callback) {
		// ...
	doY: function(a, b, c, callback) {
		// ...

Or a single function to create a singular operative:

var myOperative = operative(function(a, b, c, callback) {
	// Send the result to the parent page:

// Example call:
myOperative(1, 2, 3, function() { /*callback*/ });

Returning results

The most simple way to use operative is to pass in a callback function when calling an operative function and within the operative method call the callback with your result:

var combine = operative(function(foo, bar, callback) {
	callback(foo + bar);

combine('foo', 'bar', function() {
	// This callback function will be called with
	// the result from your operative function.
	result; // => 'foobar'

Return via Promises

If you don't pass a callback when calling an operative method, operative will assume you want a Promise. Note that operative will reference operative.Promise and will expect it to be a native Promise implementation or compliant polyfill. Operative does not come bundled with a Promise implementation.

var combine = operative(function(foo, bar) {
	// Internally, use a Deferred:
	var deferred = this.deferred();

	// A deferred has two methods: fulfill & reject:

	if (foo !== 'foo') {
		// Error (Rejection)
		deferred.reject('foo should be "foo"!');
	} else {
		// Success (Filfillment)
		deferred.fulfill(foo + bar);

// Usage externally:
var promise = combine('foo', 'bar');

promise.then(function(value) {
	// Fulfilled
}, function(err) {
	// Rejected

NOTE: Operative will only give you a promise if you don't pass a callback and if operative.Promise is defined. By default operative.Promise will reference window.Promise (native implementation if it exists).

Declaring dependencies

Operative accepts a second argument, an array of JS files to load within the worker ( or in its degraded state, an Iframe ):

// Create interface to call lodash methods inside a worker:
var lodashWorker = operative(function(method, args, cb) {
		_[method].apply(_, args)
}, [

lodashWorker('uniq', [[1, 2, 3, 3, 2, 1, 4, 3, 2]], function(output) {
	output; // => [1, 2, 3, 4]

Declared dependencies will be loaded before any of your operative's methods are called. Even if you call one from the outside, that call will be queued until the context (Worker/iFrame) completes loading the dependencies.

Note: Each dependency, if not an absolute URL, will be loaded relative to the calculated base URL, which operative determines like so:

var baseURL = (
	location.protocol + '//' +
	location.hostname +
	(location.port?':'+location.port:'') +
).replace(/[^\/]+$/, '');

To override at runtime use:

// Ensure it ends in a '/'

// To retrieve the current Base URL:

Destroying an operative

To terminate the operative (and thus its worker/iframe):


(terminate is aliased to the now-deprecated destroy)

Testing & Building

Special thanks to BrowserStack for providing free testing!

$ # grab dependencies
$ npm install

$ # install grunt globally if you don't have it...
$ npm install -g grunt-cli

$ # test
$ grunt test

$ # do everything + build dist:
$ grunt


  • 0.4.6 (19 Jan 2017)
  • Fix uncloneable native Error obj issue (see #44 & #45)
  • 0.4.5
  • Fix error Uncaught ReferenceError: hasTransferSupport is not defined (see #43)
  • 0.4.4 (27 Apr 2015)
  • Reverted to a global variable to fix undefined errors in bundles
  • 0.4.3 (26 Apr 2015)
  • Fixed self-url setting (see #36)
  • Improved readme
  • 0.4.2 (25 Apr 2015)
  • Added support for CommonJS
  • 0.4.0 (10 Apr 2015)
  • Removed deprecated async() method in favor of callbacks or promises
  • Refactor / Restructure code to make maintenance a bit easier
  • Use mocha_phantomjs to setup mocha testing via grunt
  • 0.4.0-rc1
  • Refactor test suites (use mocha instead of jasmine and fix various flakey specs).
  • Deprecate deferred.fulfil() (worker context promise API) in favour of deferred.resolve() (alias for fulfil still exists).
  • Introduce Transfers API (#23).
  • Fix #18.
  • Retain callbacks (allowing them to be called again and again -- a la Events). See #15.
  • Introduce small benchmarks suite
  • 0.3.2 (7 Jul 2014) AMD Support + Align correctly with ES6 Promise API (PRs 21 and 22 -- thanks Rich!)
  • 0.3.1 (27 Apr 2014) Improved release flow via PR #20.
  • 0.3.0 (21 Sep 2013) API: terminate aliased to destroy (deprecating the latter). See Issue #14.
  • 0.2.1 (30 Jul 2013) Fix worker-via-eval support (Safari 4/5, IE8/9)
  • 0.2.0 (29 Jul 2013) See #10
  • Dependency Loading (initially suggested in #8)
  • Deprecate direct returning in favour of a callback passed to each operative invocation.
  • Fallback to IFrame (to provide safer sandbox for degraded state)
  • 0.1.0 (25 Jul 2013) Support Promises (from Issue #3) if they're provided by a native Promise implementation or compliant polyfill. Also added support for operative(Function) which returns a single function.
  • 0.0.3 (18 Jul 2013) Support for asynchronous returning from within operative methods (via this.async()).
  • 0.0.2 (12 Jul 2013) Improved browser support: IE10 support via eval, degraded JSON-marshalling etc.
  • 0.0.1 (11 Jul 2013) Initial



🐕 Seamlessly create Web Workers







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