knobs: A JavaScript repository from prestonp

Knobs

Easy feature toggles

Install

npm install knobs

Usage

Define a list of features, each feature supports

name (required) - string for the feature name
env - a boolean environment variable for configuring the features. Takes precedence over the default value.
default - a boolean or function

var features = [
  { "name": "autorecovery", "env": "FEATURE_AUTORECOVERY", "default": false },
  { "name": "dynamic_scaling", "env": "FEATURE_DYNAMIC_SCALING", "default": false },
  { "name": "holiday_promo", "default": function(user) { return user.id % 2; } }
];
var Knobs = require('Knobs').default;
var knobs = new Knobs(features);

process.env['FEATURE_DYNAMIC_SCALING'] = true;
knobs.enabled('autorecovery'); // false
knobs.enabled('dynamic_scaling'); // true
knobs.enabled('holiday_promo', { id: 5 }); // true
knobs.enabled('holiday_promo', { id: 4 }); // false

Using Knobs allows for easy A/B testing. You could schedule future releases or slowly roll out by using a computed value:

knobs.load([
  {
    'name': 'foo',
    'default': function() {
      return Math.floor(Math.random()*10) % 2;
    }
  },
  {
    'name': 'bar',
    'default': function() {
      return new Date() > '02-12-2016';
    }
  }
]);

knobs.enabled(foo); // enabled for approx half of users
knobs.enabled(bar); // enabled after a certain date

Or you can manually override things with environment variables:

script.js

knobs.load({ name: 'foo', env: 'FEATURE_FOO', default: false });
console.log('Foo Enabled -', knobs.enabled('foo'));

Running without specified flags

$ node script
Foo Enabled - false

Running with flag on

$ FEATURE_FOO=true node script
Foo Enabled - true

API

constructor(features)

Create a new Knobs instance with a list of features.

.load

Load in a list of features.

.set(name, val)

Set the value of a feature. val can be a boolean or a function.

.enable(name)

Alias for .set(name, true)

.disable(name)

Alias for .set(name, false)

.enabled(name, [...args])

Returns whether a feature is enabled. Accepts optional parameters if the feature is defined by a computed value function.

.disabled(name, [...args])

Inverse of .enabled

Events

Knobs is an event emitter and emits on certain methods.

'load' event

When features are loaded, the "load" event is emitted with the list of features.

knobs.load(require('./features.json'));
knobs.on('load', (features) => { ... });

'change:[name]' event

Emitted when a feature changes via .set, .enable or .disable with the new feature value.

knobs.enable('launch redesign');
knobs.on('change:launch redesign', (val) => { ... });

License

MIT

prestonp/knobs