co

What is co?

The co npm package is a generator based flow-control utility for Node.js and the browser, making it easier to work with asynchronous JavaScript operations. It allows you to use generators to yield any function that returns a Promise. It can be used to simplify callback or promise-based code, especially in the context of async/await patterns.

What are co's main functionalities?

Sequential Execution

This feature allows for sequential execution of asynchronous tasks. The code sample demonstrates how you can use co to run promises in sequence using a generator function, which yields a promise that resolves to true.

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

Error Handling

This feature demonstrates how co can be used for error handling in asynchronous operations. The code sample shows a generator function yielding a promise that gets rejected, and the error is caught and logged.

co(function* () {
  try {
    yield Promise.reject(new Error('Oops!'));
  } catch (err) {
    console.error(err.message);
  }
});

Parallel Execution

This feature showcases how co can handle parallel execution of promises. The code sample illustrates a generator function yielding an array of promises, which co runs in parallel, and then logs the array of results.

co(function* () {
  var res = yield [
    Promise.resolve(1),
    Promise.resolve(2),
  ];
  return res;
}).then(function (value) {
  console.log(value);
});

Other packages similar to co

bluebird

Bluebird is a fully featured promise library with focus on innovative features and performance. It provides utilities for working with promises including but not limited to parallel execution, which is similar to what co offers. However, Bluebird does not use generator functions.

async

Async is a utility module which provides straight-forward, powerful functions for working with asynchronous JavaScript. Although it does not use promises or generators in the same way as co, it offers similar functionalities in terms of controlling the flow of asynchronous operations.

q

Q is a promise library for JavaScript which provides a toolset for creating and composing asynchronous promises. It is similar to co in that it helps manage asynchronous operations, but it does not utilize generator functions for flow control.

README

co

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) {
  console.log(value);
}, function (err) {
  console.error(err.stack);
});

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.

Installation

$ 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.

Examples

var co = require('co');

co(function *(){
  // yield any promise
  var result = yield Promise.resolve(true);
}).catch(onerror);

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];
  console.log(res);
  // => [1, 2, 3]
}).catch(onerror);

// errors can be try/catched
co(function *(){
  try {
    yield Promise.reject(new Error('boom'));
  } catch (err) {
    console.error(err.message); // "boom"
 }
}).catch(onerror);

function onerror(err) {
  // log any uncaught errors
  // co will not throw any errors you do not handle!!!
  // HANDLE ALL YOUR ERRORS!!!
  console.error(err.stack);
}

Yieldables

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!

Promises

Read more on promises!

Thunks

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.

Arrays

yielding an array will resolve all the yieldables in parallel.

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

Objects

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 }
}).catch(onerror);

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.

API

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) {
  console.log(val);
}, function (err) {
  console.error(err.stack);
});

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) {

});

License

MIT