Vanilla Queuing Guide

Queuing stores operations in an ordered buffer and processes them individually. It is the primary Pacer strategy for work that should not be discarded when calls arrive faster than they can run.

Queues are lossless only while they have capacity. A finite maxSize, explicit clearing, expiration, or a processing error can still remove or reject work.

How queuing works

text

Queuing (process one item every 2 ticks)
Timeline: [1 second per tick]
Calls:        ⬇️  ⬇️  ⬇️     ⬇️  ⬇️     ⬇️  ⬇️  ⬇️
Queue:       [ABC]   [BC]    [BCDE]    [DE]    [E]    []
Executed:     ✅     ✅       ✅        ✅      ✅     ✅
             [======================================================]
             ^ Accepted items remain queued until processed

             [Items arrive]    [Process steadily]      [Empty]

Queuing (process one item every 2 ticks)
Timeline: [1 second per tick]
Calls:        ⬇️  ⬇️  ⬇️     ⬇️  ⬇️     ⬇️  ⬇️  ⬇️
Queue:       [ABC]   [BC]    [BCDE]    [DE]    [E]    []
Executed:     ✅     ✅       ✅        ✅      ✅     ✅
             [======================================================]
             ^ Accepted items remain queued until processed

             [Items arrive]    [Process steadily]      [Empty]

The queue can process automatically with a delay between items, or remain stopped for manual processing.

When to use queuing

Choose queuing when:

Every accepted operation should run individually.
Processing order matters.
Incoming work may temporarily exceed processing capacity.
A maximum buffer size should reject excess work explicitly.
FIFO, LIFO, or priority ordering is required.

Choose another utility when:

Only the final call matters. Use debouncing.
Intermediate calls may be discarded while work runs steadily. Use throttling.
Calls should be rejected after a time-window quota. Use rate limiting.
Items should run together. Use batching.
Tasks return Promises or should run concurrently. Use async queuing.

Using queuing in TanStack Pacer

TanStack Pacer provides two core APIs:

queue returns a function that adds an item to an automatically running queue.
Queuer exposes ordering, lifecycle, capacity, expiration, callbacks, and state.

Convenience function

import { queue } from '@tanstack/pacer'

const processItem = queue<number>(
  (item) => {
    console.log('Processing:', item)
  },
  { wait: 1000 },
)

processItem(1) // true, accepted and processed immediately.
processItem(2) // true, accepted and queued.
processItem(3) // true, accepted and queued.

import { queue } from '@tanstack/pacer'

const processItem = queue<number>(
  (item) => {
    console.log('Processing:', item)
  },
  { wait: 1000 },
)

processItem(1) // true, accepted and processed immediately.
processItem(2) // true, accepted and queued.
processItem(3) // true, accepted and queued.

The returned function reports whether the item was accepted. It does not expose queue lifecycle methods.

Class API

import { Queuer } from '@tanstack/pacer'

const queuer = new Queuer<number>(
  (item) => {
    console.log('Processing:', item)
  },
  {
    wait: 1000,
    maxSize: 10,
  },
)

queuer.addItem(1)
queuer.addItem(2)

console.log(queuer.peekAllItems())

import { Queuer } from '@tanstack/pacer'

const queuer = new Queuer<number>(
  (item) => {
    console.log('Processing:', item)
  },
  {
    wait: 1000,
    maxSize: 10,
  },
)

queuer.addItem(1)
queuer.addItem(2)

console.log(queuer.peekAllItems())

Pass initialItems when work is already available at creation time. The queue applies its normal insertion and capacity rules, and automatic processing can begin immediately unless started: false is set.

Results and errors

The synchronous queuer does not retain the wrapped function's return value or catch errors. It removes an item before invoking the function. If the function throws, that item is no longer queued and the error propagates from the current execution.

Use async queuing for Promise results, configurable error handling, retries, abort support, and concurrency.

Ordering items

Automatic processing uses addItemsTo to choose where new items enter and getItemsFrom to choose where items leave.

FIFO

FIFO processes the oldest item first. This is the default.

const queuer = new Queuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'front',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 1, 2, 3.

const queuer = new Queuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'front',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 1, 2, 3.

LIFO

LIFO processes the newest item first.

const queuer = new Queuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'back',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 3, 2, 1.

const queuer = new Queuer(processItem, {
  addItemsTo: 'back',
  getItemsFrom: 'back',
  started: false,
})

queuer.addItem(1)
queuer.addItem(2)
queuer.addItem(3)
queuer.start() // Processes 3, 2, 1.

Priority

Provide getPriority to process higher numeric priorities first. Priority ordering takes precedence over front and back retrieval.

type Task = { name: string; priority: number }

const queuer = new Queuer<Task>(processTask, {
  getPriority: (task) => task.priority,
  started: false,
})

queuer.addItem({ name: 'low', priority: 1 })
queuer.addItem({ name: 'high', priority: 3 })
queuer.addItem({ name: 'medium', priority: 2 })
queuer.start() // Processes high, medium, low.

type Task = { name: string; priority: number }

const queuer = new Queuer<Task>(processTask, {
  getPriority: (task) => task.priority,
  started: false,
})

queuer.addItem({ name: 'low', priority: 1 })
queuer.addItem({ name: 'high', priority: 3 })
queuer.addItem({ name: 'medium', priority: 2 })
queuer.start() // Processes high, medium, low.

Automatic and manual processing

Queues start automatically by default. The first accepted item processes immediately, then wait controls the delay before later items.

const queuer = new Queuer(processItem, {
  wait: 1000,
})

const queuer = new Queuer(processItem, {
  wait: 1000,
})

Set started: false to collect items before processing:

const queuer = new Queuer(processItem, { started: false })

queuer.addItem(1)
queuer.addItem(2)
queuer.start()
queuer.stop()

const queuer = new Queuer(processItem, { started: false })

queuer.addItem(1)
queuer.addItem(2)
queuer.start()
queuer.stop()

stop() cancels the scheduled tick and retains queued items. start() resumes automatic processing.

For manual control:

execute() removes and processes the next item immediately.
getNextItem() removes and returns the next item without processing it.
peekNextItem() returns the next item without removing it.
peekAllItems() returns a copy of the current queue.

Capacity and rejection

Set maxSize to bound the number of waiting items. An item added to a full queue is rejected, addItem() returns false, and onReject runs.

const queuer = new Queuer(processItem, {
  maxSize: 2,
  started: false,
  onReject: (item, queuer) => {
    console.log('Rejected:', item)
    console.log('Total rejections:', queuer.store.state.rejectionCount)
  },
})

queuer.addItem(1) // true
queuer.addItem(2) // true
queuer.addItem(3) // false

const queuer = new Queuer(processItem, {
  maxSize: 2,
  started: false,
  onReject: (item, queuer) => {
    console.log('Rejected:', item)
    console.log('Total rejections:', queuer.store.state.rejectionCount)
  },
})

queuer.addItem(1) // true
queuer.addItem(2) // true
queuer.addItem(3) // false

The active synchronous execution is not part of size; size counts items still waiting in the queue.

Expiring stale items

Use expirationDuration to remove items that have waited too long:

const queuer = new Queuer(processItem, {
  expirationDuration: 5000,
  onExpire: (item) => {
    console.log('Expired:', item)
  },
})

const queuer = new Queuer(processItem, {
  expirationDuration: 5000,
  onExpire: (item) => {
    console.log('Expired:', item)
  },
})

Use getIsExpired for custom logic:

const queuer = new Queuer(processItem, {
  getIsExpired: (item, addedAt) => Date.now() - addedAt > item.maxAge,
})

const queuer = new Queuer(processItem, {
  getIsExpired: (item, addedAt) => Date.now() - addedAt > item.maxAge,
})

Expiration is checked while the automatic processing loop runs. A stopped queue evaluates stale items when processing resumes.

Flushing, clearing, and resetting

Flush

flush() processes waiting items immediately without the configured delay. Pass a count to process only part of the queue.

queuer.flush() // Process all waiting items.
queuer.flush(2) // Process at most two waiting items.

queuer.flush() // Process all waiting items.
queuer.flush(2) // Process at most two waiting items.

flushAsBatch() removes all waiting items and passes them to a separate batch function:

queuer.flushAsBatch((items) => {
  saveItems(items)
})

queuer.flushAsBatch((items) => {
  saveItems(items)
})

Clear

clear() removes all waiting items without processing them. It does not change whether the queue is running.

queuer.clear()

queuer.clear()

Reset

reset() restores state to the default running, empty queue. It does not clear an already scheduled timeout. Call stop() before reset() when scheduled work must be canceled.

queuer.stop()
queuer.reset()

queuer.stop()
queuer.reset()

Configuring and observing the queue

Use setOptions() to update future behavior. Changing started through setOptions() does not call start() or stop().

queuer.setOptions({ wait: 250, maxSize: 20 })
queuer.start()

queuer.setOptions({ wait: 250, maxSize: 20 })
queuer.start()

The wait option may be a function that receives the queuer instance:

const queuer = new Queuer(processItem, {
  wait: (queuer) => (queuer.store.state.size > 20 ? 50 : 250),
})

const queuer = new Queuer(processItem, {
  wait: (queuer) => (queuer.store.state.size > 20 ? 50 : 250),
})

Use callbacks for queue events:

onItemsChange: An item was added or removed.
onExecute: An item was processed.
onReject: An item was rejected.
onExpire: An item expired.

To share a type-checked configuration across instances, define it with queuerOptions().

State

initialState can restore selected queue state that your app has persisted. If it includes items, they take precedence over initialItems; initialState.isRunning likewise takes precedence over started. Restore only durable fields. Pending timers are not restored.

Commonly useful state includes: