feature-helper

This library is a helper to abstract all different sources of feature flag services on your application. Regardless of any feature flag source, format, or type, it can help you connect to one API in order to work with feature flags in runtime.

The library is written in TypeScript and is compatible for use with JavaScript as well.

##Installation

yarn add @bills/feature-helper

or

npm i @bills/feature-helper --save

Usage

TypeScript syntax - You can import the interfaces as well

import { FeatureFlagHelper, IFeatureSource, TFetcher, TTransformer } from '@bills/feature-helper';

EcmaScript 2015 syntax

import { FeatureFlagHelper } from '@bills/feature-helper';

EcmaScript5 syntax

var FeatureHelper = require('@bills/feature-helper');
var FeatureFlagHelper = FeatureHelper.FeatureFlagHelper;

Pass your feature flag sources to the constructor method in an array. The Flag sources need to implement the IFeatureSource interface.

interface IFeatureSource<T> {
    sourceName: string;
    fetcher: TFetcher<T>;
    transformer?: TTransformer<T>;
}

fetcher - TFetcher<T>

type TFetcher<T> = () => Promise<T>;

The Fetcher Function is a function that can asynchronously retrieve all feature flag data. This may be a remote call, a database read, etc... What it is does not matter to the library. All that matters is that is returns a promise, and the type that gets resolved in the promise is of generic type T. Type T is important for the Transformer function, explained below.

transformer - TTransfomer<T>

type TTransformer<T> = (featureObj: T) => TFeatures;
type TFeatures = IFeature[];
interface IFeature {
    name: string;
    source: string;
    value: any;
}

The Transformer Function takes the result of the fetcher call (the type was generic T, remember?), and translates it into an array of IFeatures. IFeature is pretty simple: name of the feature flag, the source it came from, and the value. Most likely, the value will be a boolean, but some feature flags do have other types such as string, number, or object.

So...

Construct the class like the following

const featureSource = {
  name: 'Local Properties',
  fetcher: async () => { /*...*/ },
  transfrormer: (response: T) => { /*...*/ }
};

const FeatureFlags = new FeatureFlagHelper(featureSource);

NOTE: Validation

The IFeatureSource fields are extremely important and they will be validated at the constructor. If they are invalid, the constructor will throw an Error. You can check it by calling FeatureFlagHelper.isValidFeatureSource(mySource) and retrieve a boolean.

Usage cont'd...