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    Generator based control flow goodness for nodejs and the browser, using promises, letting you write non-blocking code in a nice-ish way.

    Why a Fork

    This is the same pacakage as normal co yet it has domain support. Node has problems with domains and Promises, that is fixed in this version.

    Co v4

    co@4.0.0 has been released, which now relies on promises. It is a stepping stone towards the async/await proposal. The primary API change is how co() is invoked. Before, co returned a "thunk", which you then called with a callback and optional arguments. Now, co() returns a promise.

    co(function* () {
      var result = yield Promise.resolve(true);
      return result;
    }).then(function (value) {
    }, function (err) {

    If you want to convert a co-generator-function into a regular function that returns a promise, you now use co.wrap(fn*).

    var fn = co.wrap(function* (val) {
      return yield Promise.resolve(val);
    fn(true).then(function (val) {

    Platform Compatibility

    co@4+ requires a Promise implementation. For versions of node < 0.11 and for many older browsers, you should/must include your own Promise polyfill.

    When using node 0.10.x and lower or browsers without generator support, you must use gnode and/or regenerator.

    When using node 0.11.x, you must use the --harmony-generators flag or just --harmony to get access to generators.

    Node v4+ is supported out of the box, you can use co without flags or polyfills.


    $ npm install co

    Associated libraries

    Any library that returns promises work well with co.

    • mz - wrap all of node's code libraries as promises.

    View the wiki for more libraries.


    var co = require('co');
    co(function *(){
      // yield any promise
      var result = yield Promise.resolve(true);
    co(function *(){
      // resolve multiple promises in parallel
      var a = Promise.resolve(1);
      var b = Promise.resolve(2);
      var c = Promise.resolve(3);
      var res = yield [a, b, c];
      // => [1, 2, 3]
    // errors can be try/catched
    co(function *(){
      try {
        yield Promise.reject(new Error('boom'));
      } catch (err) {
        console.error(err.message); // "boom"
    function onerror(err) {
      // log any uncaught errors
      // co will not throw any errors you do not handle!!!


    The yieldable objects currently supported are:

    • promises
    • thunks (functions)
    • array (parallel execution)
    • objects (parallel execution)
    • generators (delegation)
    • generator functions (delegation)

    Nested yieldable objects are supported, meaning you can nest promises within objects within arrays, and so on!


    Read more on promises!


    Thunks are functions that only have a single argument, a callback. Thunk support only remains for backwards compatibility and may be removed in future versions of co.


    yielding an array will resolve all the yieldables in parallel.

    co(function* () {
      var res = yield [
      console.log(res); // => [1, 2, 3]


    Just like arrays, objects resolve all yieldables in parallel.

    co(function* () {
      var res = yield {
        1: Promise.resolve(1),
        2: Promise.resolve(2),
      console.log(res); // => { 1: 1, 2: 2 }

    Generators and Generator Functions

    Any generator or generator function you can pass into co can be yielded as well. This should generally be avoided as we should be moving towards spec-compliant Promises instead.


    co(fn*).then( val => )

    Returns a promise that resolves a generator, generator function, or any function that returns a generator.

    co(function* () {
      return yield Promise.resolve(true);
    }).then(function (val) {
    }, function (err) {

    var fn = co.wrap(fn*)

    Convert a generator into a regular function that returns a Promise.

    var fn = co.wrap(function* (val) {
      return yield Promise.resolve(val);
    fn(true).then(function (val) {




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