# throat
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Throttle the parallelism of an asynchronous, promise returning, function / functions. This has special utility when you set the concurrency to `1`. That way you get a mutually exclusive lock.
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[](https://travis-ci.org/ForbesLindesay/throat)
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[](https://coveralls.io/r/ForbesLindesay/throat?branch=master)
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[](https://david-dm.org/ForbesLindesay/throat)
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[](https://www.npmjs.com/package/throat)
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[](https://greenkeeper.io/)
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[](https://saucelabs.com/u/throat)
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## Installation
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npm install throat
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## API
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### throat(concurrency)
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This returns a function that acts a bit like a lock (exactly as a lock if concurrency is 1).
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Example, only 2 of the following functions will execute at any one time:
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```js
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const throat = require('throat')(2);
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// alternatively provide your own promise implementation
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const throat = require('throat')(require('promise'))(2);
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const promise = Promise.resolve();
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const resA = throat(() => /* async stuff... */ promise);
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const resB = throat(() => /* async stuff... */ promise);
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const resC = throat(() => /* async stuff... */ promise);
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const resD = throat(() => /* async stuff... */ promise);
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const resE = throat(() => /* async stuff... */ promise);
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```
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### throat(concurrency, worker)
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This returns a function that is an exact copy of `worker` except that it will only execute up to `concurrency` times in parallel before further requests are queued:
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```js
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const throat = require('throat');
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// alternatively provide your own promise implementation
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const throat = require('throat')(require('promise'));
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const input = ['fileA.txt', 'fileB.txt', 'fileC.txt', 'fileD.txt'];
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const data = Promise.all(input.map(throat(2, fileName => readFile(fileName))));
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```
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Only 2 files will be read at a time, sometimes limiting parallelism in this way can improve scalability.
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## License
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MIT
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