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# run-queue
A promise based, dynamic priority queue runner, with concurrency limiting.
```js
const RunQueue = require('run-queue')
const queue = new RunQueue({
maxConcurrency: 1
})
queue.add(1, example, [-1])
for (let ii = 0; ii < 5; ++ii) {
queue.add(0, example, [ii])
}
const finished = []
queue.run().then(
console.log(finished)
})
function example (num, next) {
setTimeout(() => {
finished.push(num)
next()
}, 5 - Math.abs(num))
}
```
would output
```
[ 0, 1, 2, 3, 4, -1 ]
```
If you bump concurrency to `2`, then you get:
```
[ 1, 0, 3, 2, 4, -1 ]
```
The concurrency means that they don't finish in order, because some take
longer than others. Each priority level must finish entirely before the
next priority level is run. See
[PRIORITIES](https://github.com/iarna/run-queue#priorities) below. This is
even true if concurrency is set high enough that all of the regular queue
can execute at once, for instance, with `maxConcurrency: 10`:
```
[ 4, 3, 2, 1, 0, -1 ]
```
## API
### const queue = new RunQueue(options)
Create a new queue. Options may contain:
* maxConcurrency - (Default: `1`) The maximum number of jobs to execute at once.
* Promise - (Default: global.Promise) The promise implementation to use.
### queue.add (prio, fn, args)
Add a new job to the end of the queue at priority `prio` that will run `fn`
with `args`. If `fn` is async then it should return a Promise.
### queue.run ()
Start running the job queue. Returns a Promise that resolves when either
all the jobs are complete or a job ends in error (throws or returns a
rejected promise). If a job ended in error then this Promise will be rejected
with that error and no further queue running will be done.
## PRIORITIES
Priorities are any integer value >= 0.
Lowest is executed first.
Priorities essentially represent distinct job queues. All jobs in a queue
must complete before the next highest priority job queue is executed.
This means that if you have two queues, `0` and `1` then ALL jobs in `0`
must complete before ANY execute in `1`. If you add new `0` level jobs
while `1` level jobs are running then it will switch back processing the `0`
queue and won't execute any more `1` jobs till all of the new `0` jobs
complete.