Toggle is a small UI element that allows users to update preferences, settings, and other types of information. When used correctly, toggles should enforce a binary state and provide immediate results. They can be used in a variety of ways and should always be easy for users to understand. Toggles should be used sparingly, and when they are, should be accompanied by direct labels that describe the current state.

A toggle is a switch that has two positions, on and off, and can be pressed or pulled to change its state. It’s used in many devices and computer programs and can be thought of as a digital version of the classic hardware switch used on electrical switches and light fixtures. The word also has a figurative meaning: It can mean to alternate back and forth, like when we “toggle” between the caps lock and the numeric keypad when typing on our keyboards or the way we shift between screens as we video chat with friends at different locations.

Historically the term toggle has been associated with fasteners and other devices that use a rod to bind things together. It has more recently been applied to the UI component that serves the same function.

There are a number of ways to implement a toggle, ranging from a simple comment through to the use of a preprocessor’s #ifdef feature which is typically only viable when the toggle is used at relatively small scale. Once a toggle reaches a certain size or complexity however, changing configuration via static files becomes unwieldy and maintaining consistency across multiple deployments is difficult. In many cases it’s preferable to move the toggle configuration into some type of centralized store, typically an existing application DB.

In addition to the benefits cited above using a centralized store for toggle configuration also makes it easy to run experiments on features. In this case a Toggle Router can be created to consistently send a given user down one codepath or the other based upon their cohort and the results of an A/B test. This is often referred to as Canary Release or Champagne Brunch.

While this type of dynamic re-configuration of a toggle is very useful, it does require some level of engineering to manage. For this reason many organizations choose to go even further than this and build out some form of admin UI which allows product managers, system operators and testers to view and modify feature flags and their toggle configuration in an easily understood format.