BehaviorSubject

BehaviorSubject is a specialized subject in reactive programming that combines the capabilities of an observer and an observable with an added memory of the latest value. It stores the most recent value emitted and immediately provides that value to any new subscriber. When creating a BehaviorSubject, you must supply an initial value; this value becomes the current value until a new one is emitted.

On subscription, a BehaviorSubject immediately emits its current value to the subscriber, then continues to emit any subsequent values as they are produced by next(). The current value can be retrieved synchronously via a getValue() method. BehaviorSubject participates in the standard observable lifecycle: if the subject completes or errors, new subscribers will receive the terminal notification instead of a value.

Common use cases include representing state that changes over time and should be observable by multiple parts of an application, such as UI state, form input, authentication status, or feature flags. It is useful when components need to start with a meaningful initial value and always have access to the latest state upon subscription.

Compared with ReplaySubject, BehaviorSubject stores only the latest value rather than buffering multiple values. This makes it a good fit for state-like streams where subscribers should observe the most recent state immediately upon subscribing, while still supporting multicasting and reactive updates. BehaviorSubject is a part of RxJS and related reactive libraries implement similar constructs.