Queues: First In, First Out

The plain-language idea

Picture the checkout line at a grocery store. The first person to join the line is the first person to get served. Everyone else waits their turn, joining at the back and moving forward as people ahead of them leave.

That's a queue: FIFO, first in, first out. It's the mirror image of the stack you just met. A stack serves the most recent arrival first; a queue serves the earliest arrival first.

Queues show up anywhere "fairness by order of arrival" matters. A printer handling documents in the order they were sent. Customer support tickets answered in the order they came in. A to-do list of background tasks a program needs to work through, one at a time, in the order they were added.

Adding to a queue is usually called enqueue ("join the line"). Removing from the front is called dequeue ("get served and leave"). New people always join at the back; only the front of the line ever gets served.

See it in a real thing: a checkout line

// A plain array can act as a queue: push adds to the back of the line,
// shift removes from the front — the person who's been waiting longest.
const checkoutLine = [];

checkoutLine.push("Priya");  // Priya joins the back of the line
checkoutLine.push("Sam");    // Sam joins behind Priya
checkoutLine.push("Devon");  // Devon joins behind Sam

// The cashier calls the next person — always from the FRONT.
const nextCustomer = checkoutLine.shift();
console.log(nextCustomer); // "Priya" — first in line, first served

console.log(checkoutLine); // ["Sam", "Devon"] — Priya is gone, order preserved

What people usually miss

Watch out

• shift removes from the front, push adds to the back — different ends. This trips people up coming straight from stacks, where both operations touch the same end. A queue deliberately uses two different ends.
• shift on a real array is slower than pop, technically. For learning the concept, don't worry about this — it's a performance detail for later, not something that changes how a queue behaves.
• "First in" means order of arrival, not size or importance. A queue doesn't know or care whether a task is urgent — it only tracks who showed up first. Priority queues (a different structure) exist for when urgency matters.

The genealogy: the stack's mirror image

You just learned that a stack is an array used with rules at one end. A queue is the same array, with one small rule change: add at the back, remove from the front instead of the same end. Same starting material, one flipped rule, a completely different order of service.