switchMap

switchMap is an operator used in reactive programming, notably in RxJS. It maps each value from a source observable into an inner observable and subscribes to the most recent inner observable, unsubscribing from any previously active inner observer.

Mechanism: for every emission from the source, it applies a projection function to produce an inner observable. It then subscribes to that inner observable and emits its values to downstream subscribers. If a newer source value arrives before the current inner observable completes, the previous inner subscription is unsubscribed and its work is canceled. If the inner errors, the error is forwarded downstream and the sequence terminates unless handled.

When to use: switchMap is especially useful when only the latest result matters, such as type-ahead search, live filtering, or replacing in-flight API calls with newer user input. It contrasts with mergeMap (flatMap), which subscribes to all inner observables concurrently; with concatMap, which processes inner observables in sequence, waiting for each to complete before starting the next; and with exhaustMap, which ignores new source values while an inner observable is active.

Example (conceptual): source$.pipe(switchMap(value => httpRequest(value))). This setup triggers a new HTTP request for each new value but cancels any in-flight request from earlier values, delivering only the latest response when it arrives. switchMap thus helps manage rapid streams of events by prioritizing the most recent downstream result.