![contra.png][logo]

    

> Asynchronous flow control with a functional taste to it

λ aims to stay small and simple, while powerful. Inspired by [async][1] and [lodash][2]. Methods are implemented individually and not as part of a whole. That design helps when considering to export functions individually. If you need all the methods in async, then stick with it. Otherwise, you might want to check λ out!

Feature requests will be considered on a case-by-case basis.

#### Quick Links

- CHANGELOG
- Comparison with async
- Browser Support
- License

#### API

Flow Control

- λ.waterfall
- λ.series
- λ.concurrent

Functional

- λ.each
- λ.each.series
- λ.map
- λ.map.series
- λ.filter
- λ.filter.series

Uncategorized

- λ.queue
- λ.emitter
- λ.curry

Install

Install using npm or bower. Or get the [source code][3] and embed that in a

`

The only reason contra isn't published as λ directly is to make it easier for you to type.

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API

These are the asynchronous flow control methods provided by λ.

λ.waterfall(tasks, done?)

Executes tasks in series. Each step receives the arguments from the previous step.

- tasks Array of functions with the (...results, next) signature
- done Optional function with the (err, ...results) signature

`js
λ.waterfall([
function (next) {
next(null, 'params for', 'next', 'step');
},
function (a, b, c, next) {
console.log(b);
// <- 'next'
next(null, 'ok', 'done');
}
], function (err, ok, result) {
console.log(result);
// <- 'done'
});
`

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λ.concurrent(tasks, cap?, done?)

Executes tasks concurrently. Results get passed as an array or hash to an optional done callback. Task order is preserved in results. You can set a concurrency cap, and it's uncapped by default.

- tasks Collection of functions with the (cb) signature. Can be an array or an object
- cap Optional concurrency level, used by the internal queue
- done Optional function with the (err, results) signature

`js
λ.concurrent([
function (cb) {
setTimeout(function () {
cb(null, 'boom');
}, 1000);
},
function (cb) {
cb(null, 'foo');
}
], function (err, results) {
console.log(results);
// <- ['boom', 'foo']
});
`

Using objects

`js
λ.concurrent({
first: function (cb) {
setTimeout(function () {
cb(null, 'boom');
}, 1000);
},
second: function (cb) {
cb(null, 'foo');
}
}, function (err, results) {
console.log(results);
// <- { first: 'boom', second: 'foo' }
});
`

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λ.series(tasks, done?)

Effectively an alias for λ.concurrent(tasks, 1, done?).

Executes tasks in series. done gets all the results. Results get passed as an array or hash to an optional done callback. Task order is preserved in results.

- tasks Collection of functions with the (next) signature. Can be an array or an object
- done Optional function with the (err, results) signature

`js
λ.series([
function (next) {
setTimeout(function () {
next(null, 'boom');
}, 1000);
},
function (next) {
next(null, 'foo');
}
], function (err, results) {
console.log(results);
// <- ['boom', 'foo']
});
`

Using objects

`js
λ.series({
first: function (next) {
setTimeout(function () {
next(null, 'boom');
}, 1000);
},
second: function (next) {
next(null, 'foo');
}
}, function (err, results) {
console.log(results);
// <- { first: 'boom', second: 'foo' }
});
`

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λ.each(items, cap?, iterator, done?)

Applies an iterator to each element in the collection concurrently.

- items Collection of items. Can be an array or an object
- cap Optional concurrency level, used by the internal queue
- iterator(item, key?, cb) Function to execute on each item
- item The current item
- key Optional, array/object key of the current item
- cb Needs to be called when processing for current item is done
- done Optional function with the (err) signature

`js
λ.each({ thing: 900, another: 23 }, function (item, cb) {
setTimeout(function () {
console.log(item);
cb();
}, item);
});
// <- 23
// <- 900
`

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λ.each.series(items, iterator, done?)

Effectively an alias for λ.each(items, 1, iterator, done?).

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λ.map(items, cap?, iterator, done?)

Applies an iterator to each element in the collection concurrently. Produces an object with the transformation results. Task order is preserved in the results.

- items Collection of items. Can be an array or an object
- cap Optional concurrency level, used by the internal queue
- iterator(item, key?, cb) Function to execute on each item
- item The current item
- key Optional, array/object key of the current item
- cb Needs to be called when processing for current item is done
- done Optional function with the (err, results) signature

`js
λ.map({ thing: 900, another: 23 }, function (item, cb) {
setTimeout(function () {
cb(null, item * 2);
}, item);
}, function (err, results) {
console.log(results);
<- { thing: 1800, another: 46 }
});
`

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λ.map.series(items, iterator, done?)

Effectively an alias for λ.map(items, 1, iterator, done?).

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λ.filter(items, cap?, iterator, done?)

Applies an iterator to each element in the collection concurrently. Produces an object with the filtered results. Task order is preserved in results.

- items Collection of items. Can be an array or an object
- cap Optional concurrency level, used by the internal queue
- iterator(item, key?, cb) Function to execute on each item
- item The current item
- key Optional, array/object key of the current item
- cb Needs to be called when processing for current item is done
- err An optional error which will short-circuit the filtering process, calling done
- keep Truthy will keep the item. Falsy will remove it in the results
- done Optional function with the (err, results) signature

`js
λ.filter({ thing: 900, another: 23, foo: 69 }, function (item, cb) {
setTimeout(function () {
cb(null, item % 23 === 0);
}, item);
}, function (err, results) {
console.log(results);
<- { another: 23, foo: 69 }
});
`

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λ.filter.series(items, iterator, done?)

Effectively an alias for λ.filter(items, 1, iterator, done?).

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λ.queue(worker, cap=1)

Used to create a job queue.

- worker(job, done) Function to process jobs in the queue
- job The current job
- done Needs to be called when processing for current job is done
- cap Optional concurrency level, defaults to 1 (serial)

Returns a queue you can push or unshift jobs to. You can pause and resume the queue by hand.

- push(job, done?) Array of jobs or an individual job object. Enqueue those jobs, continue processing (unless paused). Optional callback to run when each job is completed
- unshift(job, done?) Array of jobs or an individual job object. Add jobs to the top of the queue, continue processing (unless paused). Optional callback to run when each job is completed
- pending Property. Jobs that haven't started processing yet
- length Short-hand for pending.length, only works if getters can be defined
- pause() Stop processing jobs. Those already being processed will run to completion
- resume() Start processing jobs again, after a pause()
- on('drain', fn) Execute fn whenever there's no more pending _(or running)_ jobs and processing is requested. Processing can be requested using resume, push, or unshift

`js
var q = λ.queue(worker);

function worker (job, done) {
console.log(job);
done(null);
}

q.push('job', function () {
console.log('this job is done!');
});

q.push(['some', 'more'], function () {
console.log('one of these jobs is done!');
});

q.on('drain', function () {
console.log('all done!');
// if you enqueue more tasks now, then drain
// will fire again when pending.length reaches 0
});

// <- 'this job is done!'
// <- 'one of these jobs is done!'
// <- 'one of these jobs is done!'
// <- 'all done!'
`

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λ.emitter(thing={}, options={})

Augments thing with the event emitter methods listed below. If thing isn't provided, an event emitter is created for you. Emitter methods return the thing for chaining.

- thing Optional. Writable JavaScript object
- emit(type, ...arguments) Emits an event of type type, passing any ...arguments
- emitterSnapshot(type) Returns a function you can call, passing any ...arguments
- on(type, fn) Registers an event listener fn for type events
- once(type, fn) Same as on, but the listener is discarded after one callback
- off(type, fn) Unregisters an event listener fn from type events
- off(type) Unregisters all event listeners from type events
- off() Unregisters all event listeners

The emitterSnapshot(type) method lets you remove all event listeners before emitting an event that might add more event listeners which shouldn't be removed. In the example below, thing removes all events and then emits a 'destroy' event, resulting in a 'create' event handler being attached. If we just used thing.off() after emitting the destroy event, the 'create' event handler would be wiped out too _(or the consumer would have to know implementation details as to avoid this issue)_.

`js
var thing = λ.emitter();

thing.on('foo', foo);
thing.on('bar', bar);
thing.on('destroy', function () {
thing.on('create', reinitialize);
});

var destroy = thing.emitterSnapshot('destroy');
thing.off();
destroy();
`

The emitter can be configured with the following options, too.

- async Debounce listeners asynchronously. By default they're executed in sequence.
- throws Throw an exception if an error event is emitted and no listeners are defined. Defaults to true.

`js
var thing = λ.emitter(); // also, λ.emitter({ foo: 'bar' })

thing.once('something', function (level) {
console.log('something FIRST TROLL');
});

thing.on('something', function (level) {
console.log('something level ' + level);
});

thing.emit('something', 4);
thing.emit('something', 5);
// <- 'something FIRST TROLL'
// <- 'something level 4'
// <- 'something level 5'
`

Returns thing.

Events of type error have a special behavior. λ.emitter will throw if there are no error listeners when an error event is emitted. This behavior can be turned off setting throws: false in the options.

`js
var thing = { foo: 'bar' };

λ.emitter(thing);

thing.emit('error', 'foo');
<- throws 'foo'
`

If an 'error' listener is registered, then it'll work just like any other event type.

`js
var thing = { foo: 'bar' };

λ.emitter(thing);

thing.on('error', function (err) {
console.log(err);
});

thing.emit('error', 'foo');
<- 'foo'
`

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λ.curry(fn, ...arguments)

Returns a function bound with some arguments and a next callback.

`js
λ.curry(fn, 1, 3, 5);
// <- function (next) { fn(1, 3, 5, next); }
`

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Comparison with async

[async][1]|λ
---|---
Aimed at Noders|Tailored for browsers
Arrays for [some][5], collections for [others][6]|Collections for everyone!
apply|curry
parallel|concurrent
parallelLimit|concurrent
mapSeries|map.series
More _comprehensive_|More _focused_
~29.6k (minified, uncompressed)|~2.7k (minified, uncompressed)

λ isn't meant to be a replacement for async. It aims to provide a more focused library, and a bit more consistency.

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Browser Support

![Browser Support](https://ci.testling.com/bevacqua/contra)

If you need support for one of the legacy browsers listed below, you'll need contra.shim.js.

- IE < 10
- Safari < 6
- Opera < 16

`js
require('contra/shim');
var λ = require('contra');
`

`html



`

The shim currently clocks around ~1.2k` minified, uncompressed.

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License

MIT

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[logo]: https://raw.github.com/bevacqua/contra/master/resources/contra.png
[1]: https://github.com/caolan/async
[2]: https://github.com/lodash/lodash
[3]: https://github.com/bevacqua/contra/tree/master/src/contra.js
[4]: https://github.com/bevacqua
[5]: https://github.com/caolan/async#maparr-iterator-callback
[6]: https://github.com/caolan/async#paralleltasks-callback