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# co
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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](https://github.com/lukehoban/ecmascript-asyncawait).
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.
```js
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*)`.
```js
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](https://github.com/TooTallNate/gnode) and/or [regenerator](http://facebook.github.io/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](https://github.com/normalize/mz) - wrap all of node's code libraries as promises.
View the [wiki](https://github.com/visionmedia/co/wiki) for more libraries.
## Examples
```js
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!](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise)
### 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
`yield`ing an array will resolve all the `yieldables` in parallel.
```js
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 `yieldable`s in parallel.
```js
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 `Promise`s instead.
## API
### co(fn*).then( val => )
Returns a promise that resolves a generator, generator function,
or any function that returns a generator.
```js
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`.
```js
var fn = co.wrap(function* (val) {
return yield Promise.resolve(val);
});
fn(true).then(function (val) {
});
```
## License
MIT
[npm-image]: https://img.shields.io/npm/v/co.svg?style=flat-square
[npm-url]: https://npmjs.org/package/co
[travis-image]: https://img.shields.io/travis/tj/co.svg?style=flat-square
[travis-url]: https://travis-ci.org/tj/co
[coveralls-image]: https://img.shields.io/coveralls/tj/co.svg?style=flat-square
[coveralls-url]: https://coveralls.io/r/tj/co
[downloads-image]: http://img.shields.io/npm/dm/co.svg?style=flat-square
[downloads-url]: https://npmjs.org/package/co
[gitter-image]: https://badges.gitter.im/Join%20Chat.svg
[gitter-url]: https://gitter.im/tj/co?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge