A toggle is a control used to quickly switch between two possible states. They are ideal for settings or preferences that can be applied immediately after the user “flips the switch”.

In software a toggle is typically a Boolean variable that’s set to either on or off. The value of the variable is changed whenever the toggle is flipped, and that change causes the feature to execute differently in the codebase. When the toggle is flipped back to off the behavior returns to the default.

Most UI tools support one or more toggles, and many of them use visual cues like color and movement to make it clear to users what the toggle is doing. A well designed toggle should feel natural and unobtrusive, allowing the user to easily understand what effect each state will have. It should also be able to respond to input immediately, without any delays or other types of lag.

Using toggles with complex, multi-step actions is often dangerous. A toggle should only be used for very simple actions that can be executed in a few seconds. When the toggle is being used for a longer process such as submitting a form or uploading a file it’s generally better to replace it with another type of control such as a checkbox.

If a toggle is used to implement a feature that will have a significant impact on the end user then it’s usually necessary to include an explanation with the control. This can help the user understand what will happen and how to manage their expectations in the event that a toggle is flipped in production.

Savvy teams consider the inventory of Feature Toggles to be an operating cost and seek to keep that stock low by regularly removing toggles that are no longer needed. Some teams even put expiration dates on toggles to ensure that they get removed before they become a problem.

For Feature Toggles that are being used for A/B testing it’s often possible to track the behavior of different cohorts by implementing a Toggle Router. This allows us to consistently send different cohorts down different codepaths at runtime, and to compare the results.

Toggle Routers can be implemented with a wide variety of technologies, from simple commenting which doesn’t allow for dynamic re-configuration through to more sophisticated approaches that utilize a database or existing application DB. In most cases however the more sophisticated methods come with a trade off of complexity and overhead.

Once a toggle is configured in the codebase it’s important to test the configuration that will eventually be live in production. This includes testing the toggle with its default state flipped off as well as the toggle in its on state. To prevent regressions from being introduced it’s sometimes useful to also test the toggle with its fallback state flipped on. This will give you a chance to catch bugs that would be otherwise difficult to reproduce.