`flatMap`

Maps each signal to a Stream and flattens all inner streams concurrently.

Signature

function flatMap<T extends Signal, R extends Signal>(
  project: (signal: T, index: number) => Stream<R>
): Operator<T, R>

Also exported as mergeMap.

Example

import { singlePointer } from "cereb";
import { flatMap, session } from "cereb/operators";

singlePointer(element)
  .pipe(
    session(),
    flatMap((signal) => fetchRelatedData(signal.value.x, signal.value.y))
  )
  .on((result) => {
    console.log(result.value);
  });

How It Works

When a signal arrives, flatMap creates a new inner stream and subscribes to it immediately—without canceling any existing inner streams. All inner streams run concurrently, and their signals are merged into the output stream.

Source:   ──a──────b──────c──▶
              \        \        \
Inner:         ─A1─A2─▶  ─B1─▶   ─C1─C2─▶

Output:   ────A1─A2──B1───C1─C2─▶

Use Cases

Parallel Data Fetching

Fetch data for each pointer session without waiting for previous requests:

singlePointer(element)
  .pipe(
    session(),
    flatMap((signal) => fetchLocationData(signal.value.x, signal.value.y))
  )
  .on((data) => {
    renderMarker(data.value);
  });

Concurrent Sound Effects

Play multiple sounds simultaneously:

keyboard(window)
  .pipe(
    flatMap((signal) => playSound(signal.value.key))
  )
  .on(() => {});

Note

If you need to cancel previous operations when new signals arrive, use switchMap instead. If you need to ignore new signals until the current operation completes, use exhaustMap.