Deque (Double-Ended Queue)

A deque (pronounced deck, short for double-ended queue) is a linear collection that lets you add and remove items at both ends — the front and the back — each in constant time. That makes it a superset of two simpler structures: restrict it to one end and it behaves like a stack, add at one end and remove from the other and it behaves like a queue. Your deque() factory returns an object with eight methods that push, pop, and peek at each end, plus size and isEmpty. See double-ended queue for background.

Signature

function deque(): {
  pushFront(value: unknown): void;   // add value at the front
  pushBack(value: unknown): void;    // add value at the back
  popFront(): unknown | undefined;   // remove + return the front, or undefined if empty
  popBack(): unknown | undefined;    // remove + return the back, or undefined if empty
  peekFront(): unknown | undefined;  // read the front without removing, or undefined
  peekBack(): unknown | undefined;   // read the back without removing, or undefined
  size(): number;                    // how many items are in the deque
  isEmpty(): boolean;                // true when the deque holds nothing
};

Examples

const d = deque();
d.pushBack('a');
d.pushBack('b');
d.pushBack('c');
d.popFront(); // 'a' — oldest out first, like a queue
d.popFront(); // 'b'

const d = deque();
d.pushFront(1);
d.pushFront(2);
d.pushFront(3); // front-to-back: [3, 2, 1]
d.peekFront();  // 3
d.popFront();   // 3 — newest out first, like a stack

const d = deque();
d.pushBack(1);
d.pushFront(0);
d.pushBack(2); // front-to-back: [0, 1, 2]
d.peekFront(); // 0
d.peekBack();  // 2
d.popBack();   // 2
d.size();      // 2

Notes

• O(1) at both ends — every push, pop, and peek must run in constant time no matter how many items are stored. A doubly linked list gives you this; a plain array does not, because shifting the front is O(n).
• Empty reads return undefined — popFront, popBack, peekFront, and peekBack on an empty deque must not throw. Returning undefined is the spec.
• Any value is valid — including undefined, null, 0, and false. Don't filter or coerce; use size() or isEmpty() to tell an empty deque from a stored undefined.
• It generalizes a stack and a queue — operate on one end only and it is a stack (last-in, first-out); add at one end and remove from the other and it is a queue (first-in, first-out).
• Each instance is independent — calling deque() twice gives you two separate deques; one's pushes don't appear in the other.