https://github.com/sindresorhus/p-reduce

Reduce a list of values using promises into a promise for a value
https://github.com/sindresorhus/p-reduce

Last synced: about 2 months ago
JSON representation

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

• Host: GitHub
• URL: https://github.com/sindresorhus/p-reduce
• Owner: sindresorhus
• License: mit
• Created: 2016-10-21T04:17:21.000Z (over 7 years ago)
• Default Branch: main
• Last Pushed: 2023-02-11T08:31:44.000Z (over 1 year ago)
• Last Synced: 2024-04-13T17:43:02.611Z (3 months ago)
• Language: JavaScript
• Size: 12.7 KB
• Stars: 66
• Watchers: 6
• Forks: 7
• Open Issues: 1
• Metadata Files:
  • Readme: readme.md
  • Funding: .github/funding.yml
  • License: license
  • Security: .github/security.md

Lists

• awesome-promises - p-reduce - Reduce a list of values using promises into a promise for a value (Convenience Utilities / sindresorhus's many Promise utilities ([see notes](https://github.com/sindresorhus/promise-fun)))
• promise-fun - p-reduce

README

        
# 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.

**Please don't use it to [write unreadable code](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-array-reduce.md).**

## Install

```sh

npm install p-reduce

```

## Usage

```js

import pReduce from 'p-reduce';

import humanInfo from 'human-info'; // Not a real package

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`

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](https://github.com/sindresorhus/p-each-series) - Iterate over promises serially

- [p-map-series](https://github.com/sindresorhus/p-map-series) - Map over promises serially

- [p-map](https://github.com/sindresorhus/p-map) - Map over promises concurrently

- [More…](https://github.com/sindresorhus/promise-fun)

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