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Svelte Debouncing Guide

Debouncing delays a function until calls have stopped for a configured amount of time. Each new call restarts the timer. With the default settings, only the most recent call executes, using its arguments.

Use debouncing when intermediate calls can be discarded and the final value is what matters. Search inputs, form validation, autosave, and resize handling are common examples.

How debouncing works

The timeline below shows calls arriving in bursts. Every call resets the timer. The final call in each burst executes after three ticks of inactivity.

plaintext
Debouncing (wait: 3 ticks)
Timeline: [1 second per tick]
Calls:        ⬇️  ⬇️  ⬇️  ⬇️  ⬇️     ⬇️  ⬇️  ⬇️  ⬇️               ⬇️  ⬇️
Executed:     ❌  ❌  ❌  ❌  ❌     ❌  ❌  ❌  ⏳   ->   ✅     ❌  ⏳   ->   ✅
             [================================================================]
                                                       ^ Executes here after
                                                         3 ticks of no calls

             [Burst of calls]     [More calls]   [Wait]      [New burst]
             No execution         Resets timer   Execute     Reset and execute

Only the latest call in each burst executes. All earlier calls are discarded.

Debouncing is intentionally lossy. If every operation must run, use queuing instead.

When to use debouncing

Choose debouncing when:

  • You want to wait until activity stops.
  • Only the latest arguments matter.
  • Repeating the operation for every event would waste work.
  • A short delay is acceptable.

Choose another utility when:

  • Work should run at a steady interval while activity continues. Use throttling.
  • A fixed number of calls may run within a time window. Use rate limiting.
  • Every operation must eventually run. Use queuing.
  • Several items should be processed together. Use batching.
  • You need to await a result, handle errors, retry, or abort in-flight work. Use async debouncing.

Using debouncing in Svelte

The adapter provides two levels of debouncing API:

  • createDebouncedSignal and createDebouncedValue delay state or a changing value.
  • createDebouncer exposes lifecycle methods, dynamic options, callbacks, and selected state.

Create utilities during component initialization. In .svelte or .svelte.ts files, use runes for reactive inputs. Signal and value helpers return getters; call a getter where the template reads its value.

The snippets use application functions such as saveDraft and updateSearchResults. Supply those functions in your component.

Debounced callback

Use createDebouncer(...).maybeExecute when an event should invoke a debounced side effect:

ts
import { createDebouncer } from '@tanstack/svelte-pacer'

const search = createDebouncer(
  (query: string) => updateSearchResults(query),
  { wait: 500 },
).maybeExecute
function onInput(event: Event) {
  search((event.target as HTMLInputElement).value)
}

Keep the utility instance when the component also needs cancel() or flush(). Its bound maybeExecute method can be passed directly as an event handler.

Debounced state and values

Use createDebouncedSignal when Pacer should own the delayed state, or createDebouncedValue when a value already changes elsewhere:

ts
import { createDebouncedValue } from '@tanstack/svelte-pacer'

let query = $state('')
const [debouncedQuery] = createDebouncedValue(() => query, { wait: 500 })
// Read debouncedQuery() in the template.

Instance API

ts
import { createDebouncer } from '@tanstack/svelte-pacer'

const debouncer = createDebouncer(saveDraft, { wait: 500 }, (state) => ({
  isPending: state.isPending,
}))
// Bind !debouncer.state.isPending to the Save now button's disabled attribute.
function saveNow() {
  debouncer.flush()
}

The bound maybeExecute() method returns void. The synchronous adapter does not retain return values or catch errors. Handle errors inside a trailing callback, or use async debouncing when the caller needs a Promise result.

The following timing and control snippets use the debouncer instance created in your component. Run those operations from event handlers or other application code.

Execution timing

The leading and trailing options control which edge of the wait period may execute.

leadingtrailingBehavior
falsetrueWait for inactivity, then execute the most recent call. This is the default.
truefalseExecute the first call immediately. Later calls do not execute and restart the wait period.
truetrueExecute the first call immediately. If another call arrives during the wait period, execute the most recent call at the trailing edge.
falsefalseDo not execute any calls.
ts
debouncer.setOptions({
  wait: 1000,
  leading: true,
  trailing: true,
})

debouncer.maybeExecute('first') // Executes immediately.
debouncer.maybeExecute('second')
debouncer.maybeExecute('latest') // Executes after 1 second of inactivity.

With both edges enabled, a single call executes only on the leading edge. A trailing execution occurs only when another call arrives during the wait period.

No maximum wait

createDebouncer does not provide a maxWait option. A continuous stream of calls can keep postponing a trailing execution indefinitely. Use throttling when work must continue at a bounded interval while calls are still arriving.

Controlling pending work

The instance API distinguishes between executing, canceling, and resetting pending work.

Flush

flush() immediately executes the pending trailing call with the most recent arguments. It does nothing when no trailing call is pending.

ts
debouncer.setOptions({ wait: 1000 })

debouncer.maybeExecute('draft')
debouncer.flush() // Executes saveDraft('draft') now.

Cancel

cancel() clears the pending timeout without executing the function. It also allows a leading call to execute immediately the next time maybeExecute() is called.

ts
debouncer.maybeExecute('discarded draft')
debouncer.cancel()

Reset

reset() restores the debouncer's state counters and flags to their defaults. It does not clear an already scheduled timeout. Call cancel() first when you need to discard pending work and reset state.

ts
debouncer.cancel()
debouncer.reset()

Configuring behavior at runtime

Use setOptions() to change options after construction:

ts
debouncer.setOptions({
  wait: 1000,
  leading: true,
  trailing: false,
})

A new wait value applies when the next call schedules a timeout. It does not reschedule a timeout that is already pending. Calling maybeExecute() again clears the old timeout and schedules a new one using the current options.

Enabling and disabling

Set enabled to false to prevent execution. Disabling a debouncer through setOptions() also cancels its pending call.

ts
debouncer.setOptions({
  wait: 500,
  enabled: false,
})

debouncer.maybeExecute('ignored')
debouncer.setOptions({ enabled: true })
debouncer.maybeExecute('saved')

The enabled and wait options may also be functions that receive the debouncer instance:

ts
debouncer.setOptions({
  enabled: (debouncer) => debouncer.store.state.executionCount < 10,
  wait: (debouncer) => (debouncer.store.state.executionCount === 0 ? 300 : 500),
})

Observing executions

Use onExecute for a side effect after the wrapped function runs. The callback receives the executed arguments followed by the debouncer instance.

ts
debouncer.setOptions({
  wait: 500,
  onExecute: (args, debouncer) => {
    console.log('Saved arguments:', args)
    console.log('Execution count:', debouncer.store.state.executionCount)
  },
})

Reactive options

Pass an options factory or property getters to read reactive settings. Local options override provider defaults, and option changes retain the same utility instance.

ts
let wait = $state(500)
const debouncer = createDebouncer(saveDraft, () => ({ wait }))

Svelte lifecycle

Destroying the Svelte component cancels pending work and disposes reactive effects and subscriptions. Providing onUnmount replaces the default cleanup, so a custom callback must perform every required lifecycle action. Flushing during teardown can run callbacks after the component has begun to be destroyed.

Reactive state

Pass a selector to subscribe to the state your component reads. Without a selector, selected state is an empty object. Read the selection through debouncer.state. The core store remains available even when you do not select state for rendering.

Option functions and lifecycle callbacks receive the public utility instance. Reading .store.state in those callbacks is supported. Component rendering should read the selected adapter state.

Child subscriptions

A child can select state without subscribing the utility owner. The child subscription cleans up when its own scope or component is destroyed:

svelte
<debouncer.Subscribe selector={(state) => ({ isPending: state.isPending })}>
  {#snippet children({ isPending })}
    <span>{isPending}</span>
  {/snippet}
</debouncer.Subscribe>

To restore selected state that your app has persisted, pass a partial snapshot through initialState. It is merged with the defaults. Restore only durable fields; pending timers are not restored.

  • isPending: Whether a trailing execution is waiting.
  • executionCount: How many times the wrapped function has executed.
  • lastArgs: The arguments recorded by the most recent trailing-enabled call. Check isPending before treating them as pending work.
  • status: 'disabled', 'idle', or 'pending'.

See the Svelte API reference for adapter signatures and the adapter guide for provider and helper return shapes.

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