Computed and collections

Two things that layer on top of a plain path: a value derived from other paths, and a list with the operations a list actually needs.

Computed

store.computed('$/summary/services/down', {
  from: ['$/services/list'],
  select: (values) => (values['$/services/list'] as Service[]).filter((s) => s.status === 'down').length,
});

A computed defined as data uses the tiny select language - count, sum, first, last, join:, , not, concat - so a computed path can live in a manifest rather than in code:

store.computed('$/summary/alerts', { from: ['$/alerts/list'], select: 'count' });

Collections

const log = store.collection<LogLine>('$/logs/lines');

log.append(line);
log.cap(500);                                  // keep the last 500
log.update((l) => l.id === id, { level: 'error' });
log.remove((l) => l.level === 'debug');

cap is the one that matters for a log tail: without it a long-running application grows until it stops.