How To Add Feature Flags Into Your Javascript Applications

Feature flags are a great way to prevent stale branches by regularly shipping features in your code without officially enabling them. A feature flag let’s you turn code on and off, in the case of features, a feature flag means you can regularly merge branches and release them.

While there are many different ways you can approach this, one of my favourite and most simple approaches is a features.json file in your application.

It can be something as simple as a JSON file of properties and boolean values.

{
    "feature1": true,
	"feature2": false
}

Or in the case of something more flexible and less straightforward it can be something like this where we can specify specific roles who are allowed to use a certain feature.

{
    "feature1": {
        "roles:" ["admin", "editor"]
        "enabled": true
    },
    "feature2": {
        "enabled": false
    }
}

In your code, you import the features.json file and then reference the features. Depending on your framework or environment, this will look different.

import features from './features.json';

const FeatureOne = features.feature1 ? import ('./features/feature-one') : null;

If you were to do this in a framework like Angular or Aurelia, you would create a service or middleware of some kind which compares the route against the property in your features file and reacts accordingly.

import features from './features.json';

export class FeatureService {
    getFeature(key) {
        return features?.[key] ?? null;
    }
}

There are a few feature flag services out there which provide SDK’s and ways to turn features on and off, do A/B testing, but for most use-cases, you don’t need anything else other than a JSON file and a little code to wire it all up.

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