mergeTakeOne.ts

import {combineLatest, ObservableInput, OperatorFunction} from 'rxjs';
import {map, mergeMap, take} from 'rxjs/operators';

// if you need some extra values from other observables - preferably synchronous like ngrx state
//
// Example:
//     this.action.pipe(
//         ofType(myActionType),
//         tap((myActionType) => {}),                        // <-- only has the action value
//         mergeTakeOne(this.store.select(myStoreValue), /* andAnotherObservable$, andEvenMoreObservables$ */),
//         tap(([myActionType, myStoreValue]) => {}),        // <-- has both, the action value and the value from the store observable
//

export function mergeTakeOne<SOURCE, MERGE extends ObservableInput<any>[]>(
    ...toBeMerged$: [...MERGE]
): OperatorFunction<SOURCE, [SOURCE, ...{[P in keyof MERGE]: MERGE[P] extends ObservableInput<infer U> ? U : never}]> {
    return mergeMap((source) =>
        combineLatest(toBeMerged$).pipe(
            take(1),
            map((source2): any => [source, ...source2]),
        ),
    );
}