tj/co

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) {
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 0.10.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.

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) {

});