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72 lines
1.7 KiB
72 lines
1.7 KiB
# p-reduce [![Build Status](https://travis-ci.org/sindresorhus/p-reduce.svg?branch=master)](https://travis-ci.org/sindresorhus/p-reduce) |
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> Reduce a list of values using promises into a promise for a value |
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Useful when you need to calculate some accumulated value based on async resources. |
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## Install |
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``` |
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$ npm install --save p-reduce |
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``` |
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## Usage |
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```js |
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const pReduce = require('p-reduce'); |
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const humanInfo = require('human-info'); // not a real module |
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const names = [ |
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getUser('sindresorhus').then(info => info.name), |
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'Addy Osmani', |
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'Pascal Hartig', |
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'Stephen Sawchuk' |
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]; |
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pReduce(names, (total, name) => { |
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return humanInfo(name).then(info => total + info.age); |
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}, 0).then(totalAge => { |
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console.log(totalAge); |
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//=> 125 |
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}); |
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``` |
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## API |
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### pReduce(input, reducer, [initialValue]) |
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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. |
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#### input |
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Type: `Iterable<Promise|any>` |
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Iterated over serially in the `reducer` function. |
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#### reducer(previousValue, currentValue, index) |
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Type: `Function` |
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Expected to return a value. If a `Promise` is returned, it's awaited before continuing with the next iteration. |
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#### initialValue |
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Type: `any` |
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Value to use as `previousValue` in the first `reducer` invocation. |
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## Related |
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- [p-each-series](https://github.com/sindresorhus/p-each-series) - Iterate over promises serially |
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- [p-map-series](https://github.com/sindresorhus/p-map-series) - Map over promises serially |
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- [p-map](https://github.com/sindresorhus/p-map) - Map over promises concurrently |
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- [More…](https://github.com/sindresorhus/promise-fun) |
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## License |
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MIT © [Sindre Sorhus](https://sindresorhus.com)
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