combineLatest

CombineLatest is an operator in reactive programming libraries that subscribes to multiple source observables and emits values whenever any of the sources emits, using the latest value from each input. It typically requires that every input has produced at least one value before emitting, and after that initial synchronization, any new emission from any input triggers a new combined value composed of the latest values from all inputs.

The emitted value is usually the result of applying a combining function to the latest values from all inputs. Many implementations provide a projection function to map the latest values to a desired form; without a projection, the operator often emits a collection (such as an array or tuple) of the latest inputs.

Library variations exist but share core behavior. In RxJS, combineLatest accepts multiple sources and an optional projection function, emitting the latest values as an array or the projection result. In RxJava and other ports, the concept is the same: it subscribes to all sources and emits whenever any source emits, using the latest values from each input. The exact syntax may differ across libraries, but the semantics are aligned.

Compared with other operators, combineLatest differs from zip, which waits for a complete set of values from each input before emitting and completes after a finite number of emissions. It also differs from withLatestFrom, which combines a primary source with the latest values from others only when the primary emits, and does not emit on emissions from the other inputs alone.

Common use cases include combining streams of user input with server data, coordinating multiple UI events, and performing real-time validation that depends on several sources. Edge behavior follows standard reactive rules: errors propagate, and completion typically occurs when all inputs complete.