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.

Install

$ npm install p-reduce

Usage

const pReduce = require('p-reduce');
const humanInfo = require('human-info'); // Not a real module

(async () => {
	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);

	console.log(totalAge);
	//=> 125
})();

API

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.

input

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.

initialValue

Type: unknown

Value to use as previousValue in the first reducer invocation.

Related

• p-each-series - Iterate over promises serially
• p-map-series - Map over promises serially
• p-map - Map over promises concurrently
• More…