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flags
====

A simple feature flagging library.

100 lines of code, no dependencies. Written in ES6+.

[![Build Status](https://travis-ci.org/reddit/flags.svg?branch=master)](https://travis-ci.org/reddit/flags)

__flags__ lets you quickly switch features on or off based on a context. This is
useful to test features for specific users (such as flagging on new functionality
in a web application by reading the response context), dark-launching code,
and a/b testing.

```javascript
// Import it!
import Flags from '@r/flags';

// Set up your experiment config!
const config = {
  loggedoutSearch: { loggedin: false },
  oldDesign: false,
  newListingStyle: { users: ['ajacksified'] },
};

const feature = new Flags(config);

// Add rules!
feature.addRule('loggedin', function(val) { return this.loggedin === val; });
feature.addRule('users', function(names) { return names.includes(this.username); });

// Get whatever blob of data you'll use to determine your experiment truthiness
const userData = {
  loggedin: true,
  username: 'ajacksified',
};

// For ease of use, build a context-bound flags instance. (Alternatively, you
// could call `feature.on(rule, context)` instead.)
const featureContext = feature.withContext(userData);

// Build your UI with a Flags context bound to userdata!

// false (loggedin is true, but the config wants false)
if (featureContext.enabled('loggedoutSearch')) {
  searchControl = <LoggedOutSearch />;
  console.log('show the logged out search');
}

// false (the config says it's always false)
if (featureContext.enabled('oldDesign')) {
  layout = <OldLayout />;
}

// true (the username is in the users array)
if (featureContext.enabled('newListingStyle')) {
  listing = <Listing2 />;
}

// Get the list of enabled featues:
const enabled = featureContext.allEnabled();
// => ['newListingStyle'];

// Or disabled:
const disabled = featureContext.allDisabled();
// => ['loggedoutSearch', 'oldDesign'];
```

Rules and Configuration
-----------------------

__flags__ configuration and rules are very simple:

* Define a name for your features; such as `'loggedoutSearch'` or `'oldDesign'` as above. These
  will be the basis if your config and your feature flags for later on.
* Define the rules upon which your feature will be on or off. Above, we implement
  a boolean check (shoes) and an array check (username).
* Check the flag - either by sending it in (`feature.enabled('feature', { data } )`) or
  by hanging on to a context-bound instance (
  `featureContext = feature.withContext({ data }); featureContext.enabled('feature')`)

Of note: if a rule is defined in configuration but it is not implemented, it is
assumed to be false.

Three special "pseudo-rules" implement boolean operators: `and`, `or`, and
`not`, so that rules may stay simple and general and be combined in useful
ways. For example:

```javascript
const config = {
  friend: {
    and: [
      { or: [ { type: 'cyborg' }, { type: 'human' } ] },
      { not: { role: 'supervillain' } }
    ]
  }
};
```

You can also parse large objects to handle things such as environment variables.
To do so, you should have configuration in the format `feature_name`. `feature_`
will be stripped from the key and lowercased:

```javascript
import Flags from flags;

const config = Flags.parseConfig({
  something: 'wut',
  feature_flippers: { names: ['bob', 'jane'] }
});

// => { flippers: { names: ['bob', 'jane'] } };
```

You can supply your own function, too, and customize both key and value. In
this example, we'll expect things to start with `f_` instead of `feature_`.

```javascript
import Flags from flags;

// => { flippers: { names: ['bob', 'jane'] } };
```

You can also retrieve a list of all currently enabled or disabled rules for a
given context by calling `feature.allEnabled` or `feature.allDisabled`. These
can be called with a context passed in, or else will use a bound context.

Sometimes, for testing, it's nice to clone an instance that doesn't use the
original rules and configuration by reference. You can call
`flagsinstance.clone()` to return a new flags instance with shallow copies of
the config, rules, and context.

Development
-----------

__flags__ is an ES6 library. Take a look at the `.babelrc` file to see what
presets we're using. To import it in ES5, use
`var Flags = require ('@r/flags').default;`.

To get started:

* With node installed,
* Fork __flags__
* Clone your fork to your machine
* Run `npm install` to install dev dependencies (there are no regular
  dependencies; dependencies are for testing and running.)
* Write features, and run `npm test` and `npm run lint`. Feature changes should
  have tests! For ease of use, install `eslint` for your favorite editor.
* Check out `CONTRIBUTING.md` for more detailed info.

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