98 lines
3.1 KiB
Markdown
98 lines
3.1 KiB
Markdown
promise-limit [](https://www.npmjs.com/package/promise-limit) [](https://www.npmjs.com/package/promise-limit) [](https://travis-ci.org/featurist/promise-limit)
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===
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Limit outstanding calls to promise returning functions, or a semaphore for promises. You might want to do this to reduce load on external services, or reduce memory usage when processing large batches of jobs.
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```sh
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npm install promise-limit
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```
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```js
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var promiseLimit = require('promise-limit')
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var limit = promiseLimit(2)
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var jobs = ['a', 'b', 'c', 'd', 'e']
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Promise.all(jobs.map((name) => {
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return limit(() => job(name))
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})).then(results => {
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console.log()
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console.log('results:', results)
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})
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function job (name) {
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var text = `job ${name}`
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console.log('started', text)
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return new Promise(function (resolve) {
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setTimeout(() => {
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console.log(' ', text, 'finished')
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resolve(text)
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}, 100)
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})
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}
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```
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will output:
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```
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started job a
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started job b
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job a finished
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job b finished
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started job c
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started job d
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job c finished
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job d finished
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started job e
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job e finished
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results: [ 'job a', 'job b', 'job c', 'job d', 'job e' ]
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```
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API
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---
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```js
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var promiseLimit = require('promise-limit')
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promiseLimit(concurrency?: Number) -> limit
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```
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Returns a function that can be used to wrap promise returning functions, limiting them to `concurrency` outstanding calls.
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- `concurrency` the concurrency, i.e. 1 will limit calls to one at a time, effectively in sequence or serial. 2 will allow two at a time, etc. 0 or `undefined` specify no limit, and all calls will be run in parallel.
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limit
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=====
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```js
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limit(fn: () -> Promise<T>) -> Promise<T>
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```
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A function that limits calls to `fn`, based on `concurrency` above. Returns a promise that resolves or rejects the same value or error as `fn`. All functions are executed in the same order in which they were passed to `limit`. `fn` must return a promise.
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* `fn` a function that is called with no arguments and returns a promise. You can pass arguments to your function by putting it inside another function, i.e. `() -> myfunc(a, b, c)`.
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limit.map
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=========
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```js
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limit.map(items: [T], mapper: (T) -> Promise<R>) -> Promise<[R]>
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```
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Maps an array of items using `mapper`, but limiting the number of concurrent calls to `mapper` with the `concurrency` of `limit`. If at least one call to `mapper` returns a rejected promise, the result of `map` is a the same rejected promise, and no further calls to `mapper` are made.
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limit.queue
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===========
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```js
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limit.queue: Number
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```
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Returns the queue length, the number of jobs that are waiting to be started. You could use this to throttle incoming jobs, so the queue doesn't overwhealm the available memory - for e.g. `pause()` a stream.
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## We're Hiring!
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Featurist provides full stack, feature driven development teams. Want to join us? Check out [our career opportunities](https://www.featurist.co.uk/careers/).
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