Promise utilities designed for looping.
Limit the concurrency of promises. This can be used to control
how many requests are made to a server, for example. Note:
exceptions will be swallowed in order to prevent an UnhandledPromiseRejection
from being thrown in the case where the promise rejects before the limit is
reached. Therefore, you must handle exceptions on a per promise basis.
Wrapping rateLimit method calls in a try/catch will not work.
// Limit concurrency to at most 3
const limiter = rateLimit(3)
for (const url of urls) {
// Will wait for one promise to finish if limit is reached
await limiter.add(fetch(url))
}
// Wait for unresolved promises to resolve
await limiter.finish()Batch calls via a local queue. This can be used to batch values before writing to a database, for example.
Calls fn when either batchSize, batchBytes, or timeout is reached.
batchSize defaults to 500 and therefore will always be in effect if
no options are provided. You can pass Infinity to disregard batchSize.
If timeout is passed, the timer will be started when the first item is
enqueued and reset when flush is called explicitly or implicitly.
Use maxItemsPerSec and/or maxBytesPerSec to limit throughput.
Call queue.getStats() to get the items/sec and bytes/sec rates.
Call queue.flush() to flush explicitly.
The last result of calling fn can be obtained by referencing lastResult
on the returned object.
const writeToDatabase = async (records) => {...}
const queue = batchQueue(writeToDatabase)
for (const record of records) {
// Will call `fn` when a threshold is met
await queue.enqueue(record)
}
// Call `fn` with remaining queued items
await queue.flush()Types
export type QueueResult<A, B> = {
/** Call `fn` with the items in the queue. */
flush(): Promise<void>
/** Add an item to the queue. When a queue condition is met `flush` will be called. */
enqueue(item: A): Promise<void>
/** The last result returned from calling `fn`. */
lastResult?: Awaited<B>
/** Get the current throughput rates. */
getStats(): QueueStats
}
export interface QueueOptions {
/** Wait for the batch to reach this number of elements before flushing the queue. */
batchSize?: number
/** Wait for the batch to reach this size in bytes before flushing the queue. */
batchBytes?: number
/** Wait this long in ms before flushing the queue. */
timeout?: number
/** Maximum throughput allowed (item/sec). */
maxItemsPerSec?: number
/** Maximum throughput allowed (bytes/sec). */
maxBytesPerSec?: number
}Example
const writeToDatabase = async (records) => {...}
const batchSize = 250
const queue = batchQueue(writeToDatabase, { batchSize })
for (const record of records) {
await queue.enqueue(record)
}
await queue.flush()Limit throughput by sleeping until the rate (units/sec)
is less than or equal to maxUnitsPerSec. Units is intentionally
abstract since it could represent records/sec or bytes/sec, for
example.
Example
// Limit to at most 1000 items/sec
const limiter = throughputLimiter(1000)
for (const batch of batches) {
// Will wait until the rate is <= `maxUnitsPerSec`
await limiter.throttle(batch.length)
console.log('Items/sec %d', limiter.getCurrentRate())
}Types
export interface ThroughputLimiterOptions {
/** The maximum number of start invocations to hold in memory. */
windowLength?: number
/** Number of ms to sleep before checking the rate again. Defaults to 100. */
sleepTime?: number
}Pause a loop by awaiting maybeBlock. When pause is called maybeBlock will
return a promise that is resolved when resume is called. Otherwise,
maybeBlock will return immediately. If timeout is passed, resume will
be called after timeout if it is not manually called first.
const shouldProcess = pausable()
onSomeCondition(shouldProcess.pause)
onSomeOtherCondition(shouldProcess.resume)
for (const record of records) {
await shouldProcess.maybeBlock()
await processRecord(record)
}Defer resolving a promise until done is called.
const delay = (milliseconds: number) => {
const deferred = defer()
setTimeout(deferred.done, milliseconds, '🦄')
return deferred.promise
}Sleep for time ms before resolving the Promise.
// Sleep for one second
await sleep(1000)Call heartbeatFn every interval until promise resolves or rejects.
interval defaults to 1000.
Returns the value of the resolved promise.
const heartbeatFn = () => {
// Emit heartbeat
}
const result = await pacemaker(heartbeatFn, someProm)Wait until the predicate returns truthy or the timeout expires. Returns a promise.
Types
export interface WaitOptions {
/** Wait this long in ms before rejecting. Defaults to 5000 ms. */
timeout?: number
/** Check the predicate with this frequency. Defaults to 50 ms. */
checkFrequency?: number
}Example
let isTruthy = false
setTimeout(() => {
isTruthy = true
}, 250)
await waitUntil(() => isTruthy)