📄 Custom RxJS operators 3/7: "firstMatching operator"

#angular #rxjs #firstMatching

For most use cases, the task can be accomplished at least in two ways. However, there is a subtle difference when the source stream completes without having emitted a value that meets requirements. The detailed explanation can be found in one of my RxJS hints. The operator in question allows to choose a given strategy to handle such scenario based on the required argument.

operator

import { of, MonoTypeOperatorFunction, pipe, first, filter, take } from 'rxjs';

function firstMatching<T>(
  predicateFn: (v: T) => boolean,
  required: boolean = false
): MonoTypeOperatorFunction<T> {
  return required ? first(predicateFn) : pipe(filter(predicateFn), take(1));
}