---
title: co
---

[co]: https://github.com/tj/co
[thunkify]: https://github.com/visionmedia/node-thunkify
[unyield]: https://github.com/MatthewMueller/unyield

[co] allows you to use generators to manage async flow.

### Generator → promise

A generator can `yield` a thunk or promise. Using `co()` will immediately invoke the block inside it.

```js
co(function * () {
  yield Promise.resolve(true)
}).then(...)
```

### Generator() → Promise()

Use `co.wrap()`. Most of the time, you'll be using co.wrap.

```js
var fn = co.wrap(function * (val) {
  return yield Promise.resolve(val)
})

fn().then(...)
```

### Generator() → Node callback()

Use [unyield]. (You can [thunkify] this later)

```js
var get = unyield(function * () {
})

get(function (err, res) { ... })
```

### Node callback() → Thunk()

Use [thunkify]. You can yield this.

```js
var readFile = thunkify(fs.readFile)

co(function * () {
  var data = yield readFile('index.txt', 'utf-8')
})
```