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p-map

Map over promises concurrently

Useful when you need to run promise-returning & async functions multiple times with different inputs concurrently.

This is different from Promise.all() in that you can:

  • Control the concurrency
  • Decide whether or not to stop iterating when there's an error
  • Stop iterating at any point (like break in for loops)

Install

$ npm install p-map

Usage

import pMap from 'p-map';
import got from 'got';

const sites = [
	getWebsiteFromUsername('https://sindresorhus'), //=> Promise
	'https://avajs.dev',
	'https://github.com'
];

const mapper = async site => {
	const {requestUrl} = await got.head(site);
	return requestUrl;
};

const result = await pMap(sites, mapper, {concurrency: 2});

console.log(result);
//=> ['https://sindresorhus.com/', 'https://avajs.dev/', 'https://github.com/']

Breaking from iteration

import pMap from 'p-map';
import got from 'got';

const numbers = Array.from({length: 2000}).map((_, index) => index + 1);
//=> [1, 2, ..., 1999, 2000]

const mapper = async number => {
	if (number !== 404) {
		const {transcript} = await got(`https://xkcd.com/${number}/info.0.json`).json();
		if (/unicorn/.test(transcript)) {
			console.log('Found a XKCD comic with an unicorn:', number);
			return pMap.stop();
		}
	}
};

await pMap(numbers, mapper, {concurrency: 50});
//=> Found a XKCD comic with an unicorn: 948

API

pMap(input, mapper, options?)

Returns a Promise that is fulfilled when all promises in input and ones returned from mapper are fulfilled, or rejects if any of the promises reject. The fulfilled value is an Array of the fulfilled values returned from mapper in input order.

input

Type: Iterable<Promise | unknown>

Iterated over concurrently in the mapper function.

mapper(element, index)

Type: Function

Expected to return a Promise or value.

options

Type: object

concurrency

Type: number (Integer)
Default: Infinity
Minimum: 1

Number of concurrently pending promises returned by mapper.

stopOnError

Type: boolean
Default: true

When set to false, instead of stopping when a promise rejects, it will wait for all the promises to settle and then reject with an aggregated error containing all the errors from the rejected promises.

pMap.stop(options?)

Creates a special value that should be returned from mapper in order to immediately stop iteration.

options

Type: object

value

Type: any
Default: undefined

Value to provide as result for this iteration.

ongoingMappings

Type: object

Options to configure what pMap must do with any concurrent ongoing mappings at the moment stop is called.

collapse

Type: boolean
Default: false

Whether or not to remove all holes from the result array (caused by pending mappings).

fillWith

Type: any
Default: undefined

Value to use as immediate result for pending mappings, replacing holes from the result array.

This option is ignored if collapse is set to true.

p-map for enterprise

Available as part of the Tidelift Subscription.

The maintainers of p-map and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. Learn more.

Related

  • p-all - Run promise-returning & async functions concurrently with optional limited concurrency
  • p-filter - Filter promises concurrently
  • p-times - Run promise-returning & async functions a specific number of times concurrently
  • p-props - Like Promise.all() but for Map and Object
  • p-map-series - Map over promises serially
  • p-queue - Promise queue with concurrency control
  • More…