Toggle is a feature flag that lets developers enable and disable new functionality at runtime. It can be used to manage specific features or entire feature sets. When toggles are implemented well they can make the difference between a successful or unsuccessful product launch. They should be labelled clearly and use visual cues to indicate their state. Toggles can be used to implement many types of user interface functionality such as filtering, highlighting, and sorting. Toggles are commonly found in options menus, settings screens, and other places that allow users to choose between two or more alternatives. For example, the Caps Lock and Num Lock keys on a keyboard are toggles that control the function of those buttons.

Toggles can be used to manage complex or sensitive settings and can provide a great deal of flexibility in the application. However, it is important to understand that they can also be confusing if not labelled well and if the application uses inconsistent visual cues. Toggles should always be labeled clearly, and the UI must clearly indicate their state, for instance, by using a different color to distinguish the active from inactive states. Additionally, designers should consider the societal and cultural implications of the colors they select for toggle states, such as red, which may have negative associations in some cultures.

Another common use for toggles is to enable a test version of the application to be used for testing and debugging. This allows development teams to work with a more stable and predictable environment while allowing the ability for key stakeholders to be able to review or modify the implementation of a new feature without interrupting other production activities. Toggle configuration typically lives side by side with release code and can be re-configured at runtime through an API or other mechanism.

It is a good practice for teams to test with the expected production toggle configuration flipped On as well as the fallback configuration flipped Off. This helps to ensure that any future releases don’t accidentally break existing functionality or cause regressions in previously working functionality. A simple and effective way to do this is to use an automated test runner that supports dynamic in-memory re-configuration of specific service instances, such as a cloud-based CI/CD system. Other more sophisticated methods exist to support this type of functionality, but they can be expensive and complicated to implement. For that reason it is a best practice to limit the use of this type of dynamic configuration to feature flags that are intended for use in a live production environment such as Canary Releases or Champagne Brunch features.