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tunnckoCore/each-promise

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Summary

Maintainability
Test Coverage
<h1 align="center">{%= name %}
  <a href="https://www.npmjs.com/package/{%= name %}"><img src="https://img.shields.io/npm/v/{%= name %}.svg" alt="npm version"></a>
  <a href="https://www.npmjs.com/package/{%= name %}"><img src="https://img.shields.io/npm/l/{%= name %}.svg" alt="npm version"></a>
  <a href="https://www.npmjs.com/package/{%= name %}"><img src="https://img.shields.io/npm/dm/{%= name %}.svg" alt="npm downloads monthly"></a>
  <a href="https://www.npmjs.com/package/{%= name %}"><img src="https://img.shields.io/npm/dt/{%= name %}.svg" alt="npm downloads total"></a>
  <br>
<img src="https://cdn.jsdelivr.net/emojione/assets/svg/1f300.svg" width="256" height="256" alt="Each Promise - Async control flow library"><br>
Asynchronous control flow library
</h1>

> {%= description %}

[![codeclimate][codeclimate-img]][codeclimate-url] 
[![codestyle][standard-img]][standard-url] 
[![linux build][travis-img]][travis-url] 
[![windows build][appveyor-img]][appveyor-url] 
[![codecov][coverage-img]][coverage-url] 
[![dependency status][david-img]][david-url]

## Table of Contents
<!-- toc -->

## Install
Install with [npm](https://www.npmjs.com/)

```
$ npm install {%= name %} --save
```

or install using [yarn](https://yarnpkg.com)

```
$ yarn add {%= name %}
```

## Usage
> For more use-cases see the [tests](./test.js)

```js
const {%= varname %} = require('{%= name %}')
const arr = [
  123,
  'foo',
  () => 456,
  Promise.resolve(567)
  false,
  () => Promise.resolve(11)
]

{%= varname %}
  .serial(arr)
  .then((res) => {
    console.log(res) // => [123, 'foo', 456, 567, false, 11]
  })
```

## Background
You may think why this exists, what is this for, why not Sindre's microlibs like [p-map][], [p-map-series][], [p-settle][], [p-each-series][] or [p-reduce][].

### Why not "promise fun"?
They do their jobs okey, but in some cases they don't. And that's the my case. I need control over _"fast fail"_ behavior, also known as _"settle"_ or _"bail"_. I need serial and parallel iteration, but parallel with concurrency too. They requires node v4, and uses native Promise constructor. I believe in that we should not use modern things if we don't need them, it is just syntax sugar. This package is written in way that works in node versions below v4 and also you can pass custom Promise constructor through [options.Promise](#options) if you want.

- node@4 required
- no hooks system
- no settle / fail-fast / bail
- no custom Promise
- no real and meaningful tests
- concurrency control

**[back to top](#readme)**

### Why not separate libs?
Why not separate `.serial` and `.parallel` into own libs like Sindre did? Because the main core logic and difference is absolutely in just 2-3 lines of code and one `if` check. The main thing is that `parallel` uses `for` loop with `concurrency` combination, and `series` does not use loops, but recursive function calls.

For free you get hooks system. And really it cost nothing. It just able to be done, because the structure of the code and because I need such thing.

- node v0.10 and above
- custom Promise constructor
- real settle / fail fast
- hook system, through options
- very stable and well tested with real tests
- concurrency control

**[back to top](#readme)**

## API
{%= apidocs('index.js') %}

## Options
> You have control over everything, through options.

* `Promise` **{Function}**: custom Promise constructor to be used, defaults to native
* `mapper` **{Function}**: function to apply to each item in `iterable`, see [item section](#item)
* `settle` **{Boolean}**: if `false` stops after first error (also known as _"fail-fast"_ or _"bail"_), default `true`
* `flat` **{Boolean}**: result array to contain only values, default `true`
* `concurrency` **{Number}**: works only with `.parallel` method, defaults to `iterable` length
* `start` **{Function}**: on start hook, see [hooks section](#hooks)
* `beforeEach` **{Function}**: called before each item in `iterable`, see [hooks section](#hooks)
* `afterEach` **{Function}**: called after each item in `iterable`, see [hooks section](#hooks)
* `finish` **{Function}**: called at the end of iteration, see [hooks section](#hooks)
* `context` **{Object}**: custom context to be passed to each `fn` in `iterable`
* `args` **{Array}**: custom argument(s) to be pass to `fn`, given value is arrayified

**[back to top](#readme)**

## Hooks
> You can do what you want between stages through hooks - start, before each, after each, finish.

* `start` **{Function}**: called at the start of iteration, before anything
* `beforeEach` **{Function}**: passed with `item, index, arr` arguments
  + `item` is an object with `value`, `reason` and `index` properties, see [item section](#item)
  + `index` is the same as `item.index`
  + `arr` is the iterable object - array or object
* `afterEach` **{Function}**: passed with `item, index, arr` arguments
  + `item` is an object with `value`, `reason` and `index` properties, see [item section](#item)
  + `index` is the same as `item.index`
  + `arr` is the iterable object - array or object
* `finish` **{Function}**: called at the end of iteration, see [finish hook section](#finish-hook)

**[back to top](#readme)**

## Item
> That object is special object, that is passed to `beforeEach` and `afterEach` hooks, also can be found in `result` object if you pass `opts.flat: false` option. And passed to `opts.mapper` function too.

* `item.value` resolved/rejected promise value, if at `beforeEach` hook it can be `function`
* `item.reason` may not exist if `item.value`, if exist it is standard Error object
* `item.index` is number, order of "executing", not the order that is defined in `iterable`

**[back to top](#readme)**

## Finish hook
> This hooks is called when everything is finished / completed. At the very end of iteration. It is passed with `err, result` arguments where:

* `err` is an Error object, if `opts.settle: false`, otherwise `null`
* `result` is always an array with values or [item objects](#item) if `opts.flat: false`

**[back to top](#readme)**

{% if (verb.related && verb.related.list && verb.related.list.length) { %}
## Related
{%= related(verb.related.list, {words: 18}) %}
{% } %}

## Contributing
Pull requests and stars are always welcome. For bugs and feature requests, [please create an issue](https://github.com/{%= repository %}/issues/new).  
Please read the [contributing guidelines](CONTRIBUTING.md) for advice on opening issues, pull requests, and coding standards.  
If you need some help and can spent some cash, feel free to [contact me at CodeMentor.io](https://www.codementor.io/tunnckocore?utm_source=github&utm_medium=button&utm_term=tunnckocore&utm_campaign=github) too.

**In short:** If you want to contribute to that project, please follow these things

1. Please DO NOT edit [README.md](README.md), [CHANGELOG.md](CHANGELOG.md) and [.verb.md](.verb.md) files. See ["Building docs"](#building-docs) section.
2. Ensure anything is okey by installing the dependencies and run the tests. See ["Running tests"](#running-tests) section.
3. Always use `npm run commit` to commit changes instead of `git commit`, because it is interactive and user-friendly. It uses [commitizen][] behind the scenes, which follows Conventional Changelog idealogy.
4. Do NOT bump the version in package.json. For that we use `npm run release`, which is [standard-version][] and follows Conventional Changelog idealogy.

Thanks a lot! :)

## Building docs
Documentation and that readme is generated using [verb-generate-readme][], which is a [verb][] generator, so you need to install both of them and then run `verb` command like that

```
$ npm install verbose/verb#dev verb-generate-readme --global && verb
```

_Please don't edit the README directly. Any changes to the readme must be made in [.verb.md](.verb.md)._

## Running tests
Clone repository and run the following in that cloned directory

```
$ npm install && npm test
```

## Author
{%= includeEither('authors', 'author') %}
+ [codementor/tunnckoCore](https://codementor.io/tunnckoCore)

## Logo
The logo is [Cyclone Emoji](https://cdn.jsdelivr.net/emojione/assets/svg/1f300.svg) from [EmojiOne.com](http://emojione.com/). Released under the [CC BY 4.0](http://emojione.com/licensing/) license.

## License
{%= copyright({ start: 2016, linkify: true, prefix: 'Copyright', symbol: '©' }) %} {%= licenseStatement %}

***

{%= include('footer') %}  
_Project scaffolded using [charlike][] cli._

{%= reflinks(verb.reflinks) %}

[downloads-url]: https://www.npmjs.com/package/{%= name %}
[downloads-img]: https://img.shields.io/npm/dt/{%= name %}.svg

[codeclimate-url]: https://codeclimate.com/github/{%= repository %}
[codeclimate-img]: https://img.shields.io/codeclimate/github/{%= repository %}.svg

[travis-url]: https://travis-ci.org/{%= repository %}
[travis-img]: https://img.shields.io/travis/{%= repository %}/master.svg?label=linux

[appveyor-url]: https://ci.appveyor.com/project/tunnckoCore/{%= name %}
[appveyor-img]: https://img.shields.io/appveyor/ci/tunnckoCore/{%= name %}/master.svg?label=windows

[coverage-url]: https://codecov.io/gh/{%= repository %}
[coverage-img]: https://img.shields.io/codecov/c/github/{%= repository %}/master.svg

[david-url]: https://david-dm.org/{%= repository %}
[david-img]: https://img.shields.io/david/{%= repository %}.svg

[standard-url]: https://github.com/feross/standard
[standard-img]: https://img.shields.io/badge/code%20style-standard-brightgreen.svg