Feature flags, along with other kinds of development flags, are an essential part of building and managing digital products. Feature flags (sometimes called feature toggles) control who can see features in specific environments for your app or site.

Feature flags are defined by a key identifier, flag type, and flag value, which gets included in your code.

Feature flags let you control user access to specific features as they are being built. They keep untested features out of the hands of your customers and safe from accidental discovery.

A simple application of a feature flag is to create a flag when you start working on a feature, then remove the flag when the feature is ready for release.

There’s many benefits to using feature flags.

• Engineers can keep their in-progress work confined to a development environment
• Allows for more secure feature testing prior to release
• Enables more granular release control, i.e. for continuous delivery
• Gives you the ability to target users with specific features

Not all feature flags are the same. There are a few different types that can be used for different reasons. Kinde currently offers:

• Boolean - operate like an on/off switch using true | false values. Ideal for showing and hiding features or switching functions on and off.
• String - to pass configuration values, e.g. colour, and content such as label text, etc. Say if you want to test if a green or a red button gets more clicks.
• Integer - for updating numeric values using whole numbers only. Decimal numbers will be trimmed after the decimal point. The integer range is -2147483648 to +2147483647.

The value of a feature flag is applied in a hierarchy, where flags are created at the business level, and values are inherited in the environments and organizations.

However, you can allow overrides to be applied in either environments or organizations as part of the flag definition.

You’re building a new major feature such as ‘notifications’ and you want to make sure that nobody can use it until development is complete. You set the feature value to false for everyone, to ensure it is hidden. But because you want to test it later, you also make sure that the value can be overriden for your testing environment.

Once you’ve finished building and you’re ready to test, you change the value to true for the testing environment to enable your internal QA team to look at it.

When it’s finished and bug-free, you can go back and change the value to true so all environments and organizations can access it. Released!

You want to test out some new button names in your app. You create a string type flag called buy-button where the value can be overridden for different environments and organizations.

Instead of making changes in production, test them behind a flag to select organizations or cohorts before you decide what works best. For example, you could test if buy it now or add to cart is a more effective call to action.

Another use for string flags is to apply a unique color scheme for each different environment you work in. Developers frequently switch between production, staging, and local environments as they work, and with string flags it’s easy to make these visually distinct.

In his own project, one of Kinde’s founders created the heading-color string flag to distinguish each of his three environments: yellow for local, orange for staging, and pink for production.

Less used for releasing features than for controlling various elements in your app or site, integer flags enable more granular control in certain situations.

For example, say you want to set the number of items you display on a page in your online store, you can use an integer flag to create a page limit. You might set the display limit to 12 for mobile devices, and to 24 for desktop.

Integer flags can also be used to set other limits, such as limiting the number of concurrent requests to an API endpoint, or to limit the number of password attempts before locking a user out. The advantage of using a flag for this is being able to edit the value and deploy changes quickly and easily, without major code updates.