p-limit/index.d.ts

Run multiple promise-returning & async functions with limited concurrency

export type LimitFunction = {
	/**
	The number of promises that are currently running.
	*/
	readonly activeCount: number;

	/**
	The number of promises that are waiting to run (i.e. their internal `fn` was not called yet).
	*/
	readonly pendingCount: number;

	/**
	Get or set the concurrency limit.
	*/
	concurrency: number;

	/**
	Discard pending promises that are waiting to run.

	This might be useful if you want to teardown the queue at the end of your program's lifecycle or discard any function calls referencing an intermediary state of your app.

	Note: This does not cancel promises that are already running.

	When `rejectOnClear` is enabled, pending promises are rejected with an `AbortError`.
	This is recommended if you await the returned promises, for example with `Promise.all`, so pending tasks do not remain unresolved after `clearQueue()`.
	*/
	clearQueue: () => void;

	/**
	Process an iterable of inputs with limited concurrency.

	The mapper function receives the item value and its index.

	This is a convenience function for processing inputs that arrive in batches. For more complex use cases, see [p-map](https://github.com/sindresorhus/p-map).

	@param iterable - An iterable containing an argument for the given function.
	@param mapperFunction - Promise-returning/async function.
	@returns A promise equivalent to `Promise.all(Array.from(iterable, (item, index) => limit(mapperFunction, item, index)))`.
	*/
	map: <Input, ReturnType> (
		iterable: Iterable<Input>,
		mapperFunction: (input: Input, index: number) => PromiseLike<ReturnType> | ReturnType
	) => Promise<ReturnType[]>;

	/**
	@param fn - Promise-returning/async function.
	@param arguments - Any arguments to pass through to `fn`. Support for passing arguments on to the `fn` is provided in order to be able to avoid creating unnecessary closures. You probably don't need this optimization unless you're pushing a *lot* of functions.

	Warning: Avoid calling the same `limit` function inside a function that is already limited by it. This can create a deadlock where inner tasks never run. Use a separate limiter for inner tasks.

	@returns The promise returned by calling `fn(...arguments)`.
	*/
	<Arguments extends unknown[], ReturnType>(
		function_: (...arguments_: Arguments) => PromiseLike<ReturnType> | ReturnType,
		...arguments_: Arguments
	): Promise<ReturnType>;
};

/**
Run multiple promise-returning & async functions with limited concurrency.

@param concurrency - Concurrency limit. Minimum: `1`. You can pass a number or an options object with a `concurrency` property.
@returns A `limit` function.

@example
```
import pLimit from 'p-limit';

const limit = pLimit(1);

const input = [
	limit(() => fetchSomething('foo')),
	limit(() => fetchSomething('bar')),
	limit(() => doSomething())
];

// Only one promise is run at once
const result = await Promise.all(input);
console.log(result);
```

@example
```
import pLimit from 'p-limit';

const limit = pLimit({concurrency: 1});
```
*/
export default function pLimit(concurrency: number | Options): LimitFunction;

export type Options = {
	/**
	Concurrency limit.

	Minimum: `1`.
	*/
	readonly concurrency: number;

	/**
	Reject pending promises with an `AbortError` when `clearQueue()` is called.

	Default: `false`.

	This is recommended if you await the returned promises, for example with `Promise.all`, so pending tasks do not remain unresolved after `clearQueue()`.
	*/
	readonly rejectOnClear?: boolean;
};

/**
Returns a function with limited concurrency.

The returned function manages its own concurrent executions, allowing you to call it multiple times without exceeding the specified concurrency limit.

Ideal for scenarios where you need to control the number of simultaneous executions of a single function, rather than managing concurrency across multiple functions.

@param function_ - Promise-returning/async function.
@return Function with limited concurrency.

@example
```
import {limitFunction} from 'p-limit';

const limitedFunction = limitFunction(async () => {
	return doSomething();
}, {concurrency: 1});

const input = Array.from({length: 10}, limitedFunction);

// Only one promise is run at once.
await Promise.all(input);
```
*/
export function limitFunction<Arguments extends unknown[], ReturnType>(
	function_: (...arguments_: Arguments) => PromiseLike<ReturnType>,
	options: Options
): (...arguments_: Arguments) => Promise<ReturnType>;