📄 Custom RxJS operators 7/7: "toLatestFrom operator"

#angular #rxjs #toLatestFrom

The built-in withLatestFrom operator allows to add data from supplementary streams to the source stream. However, sometimes the source stream is just a trigger to perform a certain action for which data from supplementary streams is needed. As a result, the array of elements in the output stream contains a dummy first element. A better solution is to neglect the value from the trigger.

operator

function toLatestFrom<T, D1>(d1$: ObservableInput<D1>): OperatorFunction<T, D1>;
function toLatestFrom<T, D1, D extends unknown[]>(
  d1$: ObservableInput<D1>,
  ...data$: [...ObservableInputTuple<D>]
): OperatorFunction<T, [D1, ...D]>;

function toLatestFrom<D1, D extends unknown[]>(
  d1$: ObservableInput<D1>,
  ...data$: [...ObservableInputTuple<D>]
) {
  return pipe(
    withLatestFrom(d1$, ...data$),
    map(([_, ...data]) => (data.length === 1 ? data[0] : data))
  );
}