p-limit
Run multiple promise-returning & async functions with limited concurrency
Works in Node.js and browsers.
Install
npm install p-limit
Usage
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);
API
pLimit(concurrency) default export
Returns a limit function.
concurrency
Type: number | object
Minimum: 1
Concurrency limit.
You can pass a number or an options object with a concurrency property.
rejectOnClear
Type: boolean
Default: false
Reject pending promises with an AbortError when clearQueue() is called.
This is recommended if you await the returned promises, for example with Promise.all, so pending tasks do not remain unresolved after clearQueue().
import pLimit from 'p-limit';
const limit = pLimit({concurrency: 1});
limit(fn, ...args)
Returns the promise returned by calling fn(...args).
fn
Type: Function
Promise-returning/async function.
args
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.
limit.map(iterable, mapperFunction)
Process an iterable of inputs with limited concurrency.
The mapper function receives the item value and its index.
Returns a promise equivalent to Promise.all(Array.from(iterable, (item, index) => limit(mapperFunction, item, index))).
If the iterable throws during iteration, the returned promise rejects with that error. Mapper functions that were already scheduled still run, and their results are discarded.
This is a convenience function for processing inputs that arrive in batches. For more complex use cases, see p-map.
limit.activeCount
The number of promises that are currently running.
limit.pendingCount
The number of promises that are waiting to run (i.e. their internal fn was not called yet).
limit.clearQueue()
Discard pending promises that are waiting to run.
This might be useful if you want to tear down 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().
limit.concurrency
Get or set the concurrency limit.
limitFunction(fn, options) named export
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.
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);
fn
Type: Function
Promise-returning/async function.
options
Type: object
concurrency
Type: number
Minimum: 1
Concurrency limit.
rejectOnClear
Type: boolean
Default: false
Reject pending promises with an AbortError when clearQueue() is called.
This is recommended if you await the returned promises, for example with Promise.all, so pending tasks do not remain unresolved after clearQueue().
limitedFunction.clearQueue()
Discard pending calls that have not started yet.
When rejectOnClear is enabled, pending promises are rejected with an AbortError. Otherwise, their promises remain unresolved.
This does not cancel calls that are already running.
Recipes
See recipes.md for common use cases and patterns.
FAQ
How is this different from the p-queue package?
This package is only about limiting the number of concurrent executions, while p-queue is a fully featured queue implementation with lots of different options, introspection, and ability to pause the queue.
Related
- p-throttle - Throttle promise-returning & async functions
- p-debounce - Debounce promise-returning & async functions
- p-map - Run promise-returning & async functions concurrently with different inputs
- p-all - Run promise-returning & async functions concurrently with optional limited concurrency
- More…
