Introduction

• The queue data structure is a sequential collection of elements that follows the principle of First In First Out (FIFO).

• The first element inserted into the queue is first element to be removed.

• A queue of people. People enter the queue at one end and leave the queue from the other end.

Operations

• enqueue(element) : add an element at the end

• dequeue() : remove an element from the front

• peek() : get the value of the element at the front of the queue without removing it

• isEmpty() : check if is empty

• size() : get the number of elements

• print() : visualise the elements

Code

class Queue {
  constructor() {
    this.items = {};
    this.rear = 0;
    this.front = 0;
  }

  enqueue(item) {
    this.items[this.rear] = item;
    this.rear++;
  }

  dequeue() {
    const item = this.items[this.front];
    delete this.items[this.front];
    this.front++;
    return item;
  }

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

  isEmpty() {
    return this.rear - this.front === 0;
  }

  size() {
    return this.rear - this.front;
  }

  print() {
    console.log(this.items);
  }
}

const queue = new Queue();
console.log(queue.isEmpty()); // true

queue.enqueue(10);
queue.enqueue(20);
queue.enqueue(30);
queue.enqueue(40);
queue.print(); // { '0': 10, '1': 20, '2': 30, '3': 40 }

queue.dequeue();
queue.print(); // { '1': 20, '2': 30, '3': 40 }

console.log(queue.size()); // 3
console.log(queue.peek()); // 20

Usage

• Printers

• CPU task scheduling

• Callback queue in JavaScript runtime