Queues

A queue is FIFO (first-in, first-out): you enqueue at the back and dequeue from the front. A deque (double-ended queue) allows adds and removes at both ends. The front-dequeue is exactly the operation a plain array does poorly — array.shift is O(n) because every element re-indexes.

Reach for a queue for breadth-first traversal, scheduling, and any 'process in arrival order' task. For an O(1) queue in JavaScript, use a head index into an array (or two stacks) rather than shift.