A deque you can push and pop from both ends

Some queues are entered from both ends: a carousel you can scroll either way, a chat that prepends older messages while appending new ones, a sliding window that grows at the front and shrinks at the back.

Implement a Deque class (double-ended queue). It is constructed with no arguments and starts empty. pushFront(value) and pushBack(value) add a value at that end and each return the new size. popFront() and popBack() remove and return the value at that end, or return undefined when the deque is empty. peekFront() and peekBack() return the value at that end without removing it, or undefined when empty. size() returns how many values are held. toArray() returns the values as a plain array ordered front to back, and [] when empty.

const d = new Deque();
d.pushBack("b");
d.pushBack("c");
d.pushFront("a");
d.toArray();   // ["a", "b", "c"]
d.peekFront(); // "a"
d.peekBack();  // "c"

const d = new Deque();
[1, 2, 3, 4].forEach((v) => d.pushBack(v));
d.popFront();  // 1
d.popBack();   // 4
d.toArray();   // [2, 3]