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Map over promises concurrently

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

This is different fromPromise.all() in that you can control the concurrency and also decide whether or not to stop iterating when there's an error.

npm install p-map

importpMapfrom'p-map';importgotfrom'got';constsites=[getWebsiteFromUsername('sindresorhus'),//=> Promise'https://avajs.dev','https://github.com'];constmapper=asyncsite=>{const{requestUrl}=awaitgot.head(site);returnrequestUrl;};constresult=awaitpMap(sites,mapper,{concurrency:2});console.log(result);//=> ['https://sindresorhus.com/', 'https://avajs.dev/', 'https://github.com/']

Returns aPromise that is fulfilled when all promises ininput and ones returned frommapper are fulfilled, or rejects if any of the promises reject. The fulfilled value is anArray of the fulfilled values returned frommapper ininput order.

Returns an async iterable that streams each return value frommapper in order.

import{pMapIterable}from'p-map';// Multiple posts are fetched concurrently, with limited concurrency and backpressureforawait(constpostofpMapIterable(postIds,getPostMetadata,{concurrency:8})){console.log(post);};

Type:AsyncIterable<Promise<unknown> | unknown> | Iterable<Promise<unknown> | unknown>

Synchronous or asynchronous iterable that is iterated over concurrently, calling themapper function for each element. Each iterated item isawait'd before themapper is invoked so the iterable may return aPromise that resolves to an item.

Asynchronous iterables (different from synchronous iterables that returnPromise 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.

Type:Function

Expected to return aPromise or value.

Type:object

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

Number of concurrently pending promises returned bymapper.

Only forpMapIterable

Type:number(Integer)
Default:options.concurrency
Minimum:options.concurrency

Maximum number of promises returned bymapper that have resolved but not yet collected by the consumer of the async iterable. Calls tomapper 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.

Only forpMap

Type:boolean
Default:true

Whentrue, the first mapper rejection will be rejected back to the consumer.

Whenfalse, instead of stopping when a promise rejects, it will wait for all the promises to settle and then reject with anAggregateError containing all the errors from the rejected promises.

Caveat: Whentrue, 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.

Only forpMap

Type:AbortSignal

You can abort the promises usingAbortController.

importpMapfrom'p-map';importdelayfrom'delay';constabortController=newAbortController();setTimeout(()=>{abortController.abort();},500);constmapper=asyncvalue=>value;awaitpMap([delay(1000),delay(1000)],mapper,{signal:abortController.signal});// Throws AbortError (DOMException) after 500 ms.

Return this value from amapper function to skip including the value in the returned array.

importpMap,{pMapSkip}from'p-map';importgotfrom'got';constsites=[getWebsiteFromUsername('sindresorhus'),//=> Promise'https://avajs.dev','https://example.invalid','https://github.com'];constmapper=asyncsite=>{try{const{requestUrl}=awaitgot.head(site);returnrequestUrl;}catch{returnpMapSkip;}};constresult=awaitpMap(sites,mapper,{concurrency:2});console.log(result);//=> ['https://sindresorhus.com/', 'https://avajs.dev/', 'https://github.com/']

• 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 - LikePromise.all() but forMap andObject
• p-map-series - Map over promises serially
• More…