Queues in JavaScript

Written by Swapnil RajaPublished 28 Aug 2026

JavaScript has no queue. The obvious implementation with shift() is quadratic, and it is the most common accidental performance bug in this whole subject.

First in, first out

Add at the back, remove from the front. A checkout line. Where a stack reverses order, a queue preserves it - which is why breadth-first search uses one.

The trap

The obvious queue is O(n) per removal

javascript

const queue = []
queue.push(1)      // O(1) — fine
queue.push(2)
queue.shift()      // O(n) — every remaining element slides forward

Processing a million items this way is roughly half a trillion element moves. It will appear to hang.

PreviousStacks in JavaScript NextLinked Lists Explained

An O(1) queue

javascript

class Queue {
  #items = []
  #head = 0

  enqueue(value) {
    this.#items.push(value)
  }

  dequeue() {
    if (this.#head >= this.#items.length) return undefined

    const value = this.#items[this.#head]
    // Release the reference so it can be garbage collected.
    this.#items[this.#head] = undefined
    this.#head++

    // Compact once the dead prefix outgrows the live part, so the
    // array cannot grow without bound in a long-running queue.
    if (this.#head > 32 && this.#head * 2 > this.#items.length) {
      this.#items = this.#items.slice(this.#head)
      this.#head = 0
    }

    return value
  }

  get size() { return this.#items.length - this.#head }
  get isEmpty() { return this.size === 0 }
}

Level order, using a queue

javascript

function levelOrder(root) {
  if (!root) return []

  const out = []
  const queue = [root]
  let head = 0

  while (head < queue.length) {
    const levelSize = queue.length - head
    const level = []

    for (let i = 0; i < levelSize; i++) {
      const node = queue[head++]
      level.push(node.value)
      if (node.left) queue.push(node.left)
      if (node.right) queue.push(node.right)
    }

    out.push(level)
  }

  return out
}

Queues in JavaScript

First in, first out

The trap

The fix: never move the data

Where you will actually use it

Deques, briefly