Lesson 15 — Watchers

Run code when a value changes

watch(source, callback) calls the callback every time the source ref changes, giving you the new value (and the old one). Use it for side effects — saving, logging, fetching — rather than for values you display, which belong in computed.

watch(query, (now, before) => {
  console.log("search changed from", before, "to", now)
})

Why it matters: "save the draft whenever it changes" and "reload when the filter changes" are watchers.