# asyncThrottle

# Function: asyncThrottle()

```ts
function asyncThrottle<TFn>(fn, initialOptions): (...args) => Promise<ReturnType<TFn> | undefined>;
```

Defined in: [async-throttler.ts:627](https://github.com/TanStack/pacer/blob/main/packages/pacer/src/async-throttler.ts#L627)

Creates an async throttled function that limits how often the function can execute.
The throttled function will execute at most once per wait period, even if called multiple times.
If called while executing, it will wait until execution completes before scheduling the next call.

Async vs Sync Versions:
The async version provides advanced features over the sync throttle function:
- Returns promises that can be awaited for throttled function results
- Built-in retry support via AsyncRetryer integration
- Abort support to cancel in-flight executions
- Cancel support to prevent pending executions from starting
- Comprehensive error handling with onError callbacks and throwOnError control
- Detailed execution tracking (success/error/settle counts)
- Waits for ongoing executions to complete before scheduling the next one

The sync throttle function is lighter weight and simpler when you don't need async features,
return values, or execution control.

What is Throttling?
Throttling limits how often a function can be executed, allowing only one execution within a specified time window.
Unlike debouncing which resets the delay timer on each call, throttling ensures the function executes at a
regular interval regardless of how often it's called.

Configuration Options:
- `wait`: Time window in milliseconds during which the function can only execute once (required)
- `leading`: Execute immediately when called (default: true)
- `trailing`: Execute on the trailing edge of the wait period (default: true)
- `enabled`: Whether the throttler is enabled (default: true)
- `asyncRetryerOptions`: Configure retry behavior for executions

Error Handling:
- If an `onError` handler is provided, it will be called with the error and throttler instance
- If `throwOnError` is true (default when no onError handler is provided), the error will be thrown
- If `throwOnError` is false (default when onError handler is provided), the error will be swallowed
- Both onError and throwOnError can be used together - the handler will be called before any error is thrown
- The error state can be checked using the underlying AsyncThrottler instance

State Management:
- Uses TanStack Store for reactive state management
- Use `initialState` to provide initial state values when creating the async throttler
- Use `onSuccess` callback to react to successful function execution and implement custom logic
- Use `onError` callback to react to function execution errors and implement custom error handling
- Use `onSettled` callback to react to function execution completion (success or error) and implement custom logic
- The state includes error count, execution status, last execution time, and success/settle counts
- State can be accessed via the underlying AsyncThrottler instance's `store.state` property
- When using framework adapters (React/Solid), state is accessed from the hook's state property

## Type Parameters

### TFn

`TFn` *extends* [`AnyAsyncFunction`](../type-aliases/AnyAsyncFunction.md)

## Parameters

### fn

`TFn`

### initialOptions

[`AsyncThrottlerOptions`](../interfaces/AsyncThrottlerOptions.md)\<`TFn`\>

## Returns

```ts
(...args): Promise<ReturnType<TFn> | undefined>;
```

Attempts to execute the throttled function. The execution behavior depends on the throttler options:

- If enough time has passed since the last execution (>= wait period):
  - With leading=true: Executes immediately
  - With leading=false: Waits for the next trailing execution

- If within the wait period:
  - With trailing=true: Schedules execution for end of wait period
  - With trailing=false: Drops the execution

### Parameters

#### args

...`Parameters`\<`TFn`\>

### Returns

`Promise`\<`ReturnType`\<`TFn`\> \| `undefined`\>

### Example

```ts
const throttled = new AsyncThrottler(fn, { wait: 1000 });

// First call executes immediately
await throttled.maybeExecute('a', 'b');

// Call during wait period - gets throttled
await throttled.maybeExecute('c', 'd');
```

## Example

```ts
const throttled = asyncThrottle(async (value: string) => {
  const result = await saveToAPI(value);
  return result; // Return value is preserved
}, {
  wait: 1000,
  onError: (error) => {
    console.error('API call failed:', error);
  }
});

// This will execute at most once per second
// Returns the API response directly
const result = await throttled(inputElement.value);
```