Implementation of Queue data structure using two stacks in JavaScript. Actually, it uses two arrays here(because there is no stack in JavaScript), but you should have a look at your language docs: perhaps it provides embedded stack data structure. (For example: Java provides class "Stack", C# similarly, etc.)

queue-using-two-stacks.js

class Queue {
	constructor() {
		this.inbox = [];
		this.outbox = [];
	}

	enqueue(item) {
		this.inbox.push(item);
	}

	dequeue() {
		while(this.inbox.length) {
			this.outbox.push(this.inbox.pop());
		}

		const returnable = this.outbox.pop();

		while(this.outbox.length) {
			this.inbox.push(this.outbox.pop());
		}

		return returnable;
	}
}

const queue = new Queue();
queue.enqueue(1);
queue.enqueue(3);
queue.enqueue(7);
queue.dequeue();
queue.dequeue();
queue.dequeue();

How can we approach for O(N) for enqueue and O(1) for dequeue.
And what to do for front() in Queue, to get the peek element using peek() of Stack.