fromEvent

fromEvent is a function in RxJS that creates an observable from events emitted by a given event target. It attaches a listener for a specified event type on the target (typically via addEventListener) and emits each event object to subscribers. The listener is removed automatically when the subscription is disposed, helping to prevent memory leaks.

Usage and behavior: fromEvent(target, eventName) returns an observable that emits events of the requested type. For example, fromEvent(document, 'click') creates an observable of click events on the document. This pattern also works with other EventTarget-compatible objects, such as elements, window, or custom objects that implement the standard event binding interface. An optional third argument can alter what is emitted: a selector function can map the raw event to a custom value, so fromEvent(target, eventName, selector) may emit transformed values. Additionally, an options argument can configure addEventListener behavior, such as capture, passive, or once, for example fromEvent(element, 'scroll', { passive: true }).

Notes: fromEvent is commonly used for integrating imperative DOM events into reactive streams, enabling composition with operators like map, filter, debounceTime, and switchMap. The exact overloads and supported targets can vary between RxJS versions, so consult the version-specific documentation for precise signatures and capabilities. Overall, fromEvent provides a concise, memory-safe way to bridge traditional event handling with reactive programming.