Toast queue — notifications one at a time

Pattern

The server returned 5 errors at once. You're not going to show 5 overlapping toasts in a stack! Show them one at a time: the first for three seconds, then the second, then the third. A queue.

The problem it solves

An imperative queue — an array + setInterval + a "currently showing" flag. It's easy to get a "stuck" flag after destroy and dropped messages.

Operators and why they matter

• concatMap — subscribes to the next toast only after the previous one completes. That's the queue.
• delay inside the inner stream — simulates the toast display time (3 seconds).
• map — builds a display event for the container.

Gotchas

• mergeMap — all toasts show in parallel. A stack of errors.
• switchMap — a new toast replaces the previous one. Messages are lost.
• exhaustMap — new notifications are ignored while the current one shows. Also lost.