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73 lines
1.7 KiB
Markdown
73 lines
1.7 KiB
Markdown
# p-reduce [](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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