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co v4.6.0

generator async control flow goodness


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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.

Co v4

co@4.0.0 has been released, which now relies on promises. It is a stepping stone towards ES7 async/await. 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 0.11.x or greater, you must use the --harmony-generators flag or just --harmony to get access to generators.

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

io.js 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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