# 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 p-reduce
```
## Usage
```js
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](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)
Generated by dwww version 1.15 on Thu Jun 27 23:31:11 CEST 2024.