Queue

Prompt

Implement a Queue class with the following methods:

• enqueue(item) — adds an item to the back of the queue
• dequeue() — removes and returns the item at the front (return null if empty)
• peek() — returns the front item without removing it (return null if empty)
• isEmpty() — returns true if the queue has no items
• size() — returns the number of items

Playground

Solution

Explanation

A queue is a First-In, First-Out (FIFO) data structure. Think of it like a line at a coffee shop: the first person in line gets served first. New people join at the back, and people leave from the front.

That's the key difference from a stack. A stack is like a stack of plates (last in, first out). A queue is like a line of people (first in, first out).

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

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

  dequeue() {
    if (this.isEmpty()) return null;
    return this.items.shift();
  }

  peek() {
    if (this.isEmpty()) return null;
    return this.items[0];
  }

  isEmpty() {
    return this.items.length === 0;
  }

  size() {
    return this.items.length;
  }
}

The implementation uses an array where:

• enqueue(item) uses Array.push() to add to the end (the "back" of the line).
• dequeue() uses Array.shift() to remove from the beginning (the "front" of the line). This is the main difference from a stack, which removes from the end with pop().
• peek() reads this.items[0] — the front of the queue — without removing it.
• isEmpty() and size() work the same as in a stack.

Just like with the stack, we return null from dequeue() and peek() when the queue is empty, instead of letting JavaScript return undefined.