docs / Runtime

Scheduler

When writes actually reach the DOM: flushSync, tick, and what batches for you.

Writes are batched. Several cell writes in one turn become one update pass, so a render is never half-applied. That is almost always invisible — and occasionally not, which is what these two are for.

Call	Does
flushSync()	Applies every pending write immediately and synchronously. Returns once the DOM is up to date.
tick()	Yields to the microtask queue so pending updates land, without forcing a synchronous flush mid-expression.

app/probe/page.vsk

									
										component Probe() {
  const &[n] = track(0)

  // Two writes, then read the DOM: without flushSync the element still shows 0.
  const probe = () => {
    n++
    n++
    flushSync()
    const el = document.querySelector('#probe')
    console.log(el ? el.textContent : 'not on the server')
  }

  <span id="probe">{n}</span>
  <button onClick={probe}>run</button>
}

Do not reach for flushSync by reflex. It forces a synchronous DOM pass, which costs a layout flush and can hide ordering bugs. Reach for it when you genuinely must observe the DOM right after a write — measuring, focusing, or driving an imperative API.

On the server there is no scheduler to flush: SSR renders once, in order, and flushSync() is a no-op. The example above still renders — it simply logs nothing, because document does not exist.