# p-reduce [](https://travis-ci.org/sindresorhus/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 --save p-reduce ``` ## Usage ```js const pReduce = require('p-reduce'); const humanInfo = require('human-info'); // not a real module const names = [ getUser('sindresorhus').then(info => info.name), 'Addy Osmani', 'Pascal Hartig', 'Stephen Sawchuk' ]; pReduce(names, (total, name) => { return humanInfo(name).then(info => total + info.age); }, 0).then(totalAge => { 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: `any` 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) ## License MIT © [Sindre Sorhus](https://sindresorhus.com)