p-map
v7.0.3
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Map over promises concurrently
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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 and also decide whether or not to stop iterating when there's an error.
Install
npm install p-map
Usage
import pMap from 'p-map';
import got from 'got';
const sites = [
getWebsiteFromUsername('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/']
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.
pMapIterable(input, mapper, options?)
Returns an async iterable that streams each return value from mapper
in order.
import {pMapIterable} from 'p-map';
// Multiple posts are fetched concurrently, with limited concurrency and backpressure
for await (const post of pMapIterable(postIds, getPostMetadata, {concurrency: 8})) {
console.log(post);
};
input
Type: AsyncIterable<Promise<unknown> | unknown> | Iterable<Promise<unknown> | unknown>
Synchronous or asynchronous iterable that is iterated over concurrently, calling the mapper
function for each element. Each iterated item is await
'd before the mapper
is invoked so the iterable may return a Promise
that resolves to an item.
Asynchronous iterables (different from synchronous iterables that return Promise
that resolves to an item) can be used when the next item may not be ready without waiting for an asynchronous process to complete and/or the end of the iterable may be reached after the asynchronous process completes. For example, reading from a remote queue when the queue has reached empty, or reading lines from a stream.
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
.
backpressure
Only for pMapIterable
Type: number
(Integer)
Default: options.concurrency
Minimum: options.concurrency
Maximum number of promises returned by mapper
that have resolved but not yet collected by the consumer of the async iterable. Calls to mapper
will be limited so that there is never too much backpressure.
Useful whenever you are consuming the iterable slower than what the mapper function can produce concurrently. For example, to avoid making an overwhelming number of HTTP requests if you are saving each of the results to a database.
stopOnError
Only for pMap
Type: boolean
Default: true
When true
, the first mapper rejection will be rejected back to the consumer.
When false
, instead of stopping when a promise rejects, it will wait for all the promises to settle and then reject with an AggregateError
containing all the errors from the rejected promises.
Caveat: When true
, any already-started async mappers will continue to run until they resolve or reject. In the case of infinite concurrency with sync iterables, all mappers are invoked on startup and will continue after the first rejection. Issue #51 can be implemented for abort control.
signal
Only for pMap
Type: AbortSignal
You can abort the promises using AbortController
.
import pMap from 'p-map';
import delay from 'delay';
const abortController = new AbortController();
setTimeout(() => {
abortController.abort();
}, 500);
const mapper = async value => value;
await pMap([delay(1000), delay(1000)], mapper, {signal: abortController.signal});
// Throws AbortError (DOMException) after 500 ms.
pMapSkip
Return this value from a mapper
function to skip including the value in the returned array.
import pMap, {pMapSkip} from 'p-map';
import got from 'got';
const sites = [
getWebsiteFromUsername('sindresorhus'), //=> Promise
'https://avajs.dev',
'https://example.invalid',
'https://github.com'
];
const mapper = async site => {
try {
const {requestUrl} = await got.head(site);
return requestUrl;
} catch {
return pMapSkip;
}
};
const result = await pMap(sites, mapper, {concurrency: 2});
console.log(result);
//=> ['https://sindresorhus.com/', 'https://avajs.dev/', 'https://github.com/']
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 forMap
andObject
- p-map-series - Map over promises serially
- More…