Reduce a list of values using promises into a promise for a value

Usage no npm install needed!

<script type="module">
  import pReduce from 'https://cdn.skypack.dev/p-reduce';



Reduce a list of values using promises into a promise for a value

Useful when you need to calculate some accumulated value based on async resources.


$ npm install p-reduce


import pReduce from 'p-reduce';
import humanInfo from 'human-info'; // Not a real module

const names = [
    getUser('sindresorhus').then(info => info.name),
    'Addy Osmani',
    'Pascal Hartig',
    'Stephen Sawchuk'

const totalAge = await pReduce(names, async (total, name) => {
    const info = await humanInfo(name);
    return total + info.age;
}, 0);

//=> 125


pReduce(input, reducer, initialValue?)

Returns a Promise that is fulfilled when all promises in input and ones returned from reducer are fulfilled, or rejects if any of the promises reject. The fulfilled value is the result of the reduction.


Type: Iterable<Promise|any>

Iterated over serially in the reducer function.

reducer(previousValue, currentValue, index)

Type: Function

Expected to return a value. If a Promise is returned, it's awaited before continuing with the next iteration.


Type: unknown

Value to use as previousValue in the first reducer invocation.


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