# Preact 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.

```text
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](./queuing.md) 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](./throttling.md).
- A fixed number of calls may run within a time window. Use [rate limiting](./rate-limiting.md).
- Every operation must eventually run. Use [queuing](./queuing.md).
- Several items should be processed together. Use [batching](./batching.md).
- You need to await a result, handle errors, retry, or abort in-flight work. Use [async debouncing](./async-debouncing.md).

## Choose an API

- `useDebouncedCallback` for a stable debounced event handler
- `useDebouncedState` or `useDebouncedValue` for delayed Preact state
- `useDebouncer` for lifecycle methods and selected state

Use the callback API for event handlers, the state or value API for delayed UI state, and the instance API when you need `cancel()`, `flush()`, selected state, or dynamic options.

## Preact example

```tsx
import { useDebouncedCallback, useDebouncer } from '@tanstack/preact-pacer'

function SearchBox() {
  const search = useDebouncedCallback(runSearch, { wait: 300 })
  const debouncer = useDebouncer(saveDraft, { wait: 500 }, (state) => ({
    isPending: state.isPending,
  }))

  return (
    <>
      <input onChange={(event) => search(event.currentTarget.value)} />
      <button
        onClick={() => debouncer.flush()}
        disabled={!debouncer.state.isPending}
      >
        Save now
      </button>
    </>
  )
}
```

The focused snippets later in this guide use `useDebouncer` and assume they run inside a component or another hook.

## Execution timing

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

| `leading` | `trailing` | Behavior                                                                                                                               |
| --------- | ---------- | -------------------------------------------------------------------------------------------------------------------------------------- |
| `false`   | `true`     | Wait for inactivity, then execute the most recent call. This is the default.                                                           |
| `true`    | `false`    | Execute the first call immediately. Later calls do not execute and restart the wait period.                                            |
| `true`    | `true`     | Execute the first call immediately. If another call arrives during the wait period, execute the most recent call at the trailing edge. |
| `false`   | `false`    | Do not execute any calls.                                                                                                              |

```ts
const debouncer = useDebouncer(saveDraft, {
  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

`useDebouncer` does not provide a `maxWait` option. A continuous stream of calls can keep postponing a trailing execution indefinitely. Use [throttling](./throttling.md) 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
const debouncer = useDebouncer(saveDraft, { 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
const debouncer = useDebouncer(saveDraft, {
  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
const debouncer = useDebouncer(saveDraft, {
  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
const debouncer = useDebouncer(saveDraft, {
  wait: 500,
  onExecute: (args, debouncer) => {
    console.log('Saved arguments:', args)
    console.log('Execution count:', debouncer.store.state.executionCount)
  },
})
```

## Preact lifecycle

The adapter cancels pending work when its owner is destroyed. Providing `onUnmount` replaces that default cleanup, so a custom callback must perform every required lifecycle action. When custom cleanup flushes work, remember that user callbacks can run while the component is being destroyed.

## Reactive state

The adapter subscribes only to the state returned by the selector argument. Without a selector, the adapter state is empty. Create the utility at the top level of a component or another hook and select only fields used by the view:

```ts
const debouncer = useDebouncer(saveDraft, { wait: 500 }, (state) => ({
  isPending: state.isPending,
  executionCount: state.executionCount,
}))

console.log(debouncer.state.isPending, debouncer.state.executionCount)
```

Option functions and lifecycle callbacks receive the underlying public utility instance. The `.store.state` reads inside those callbacks in the examples above are supported. Rendering code should read the selected adapter state shown here.

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 [Preact API reference](../reference/index.md) for adapter signatures and the public core reference for complete option and state types.