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JavaScript Data Structures: Queue: Enqueue

Intro

Last time, we learned what a Queue is and set it up.

Today, we learn how to enqueue / add a new node to the end of the Queue.


Starter Code ▶️

We start with the code from the last part.

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Queue {
  constructor() {
    this.length = 0;
    this.start = null;
    this.end = null;
  }
}

Thoughts 💭

First, we should think about the constraints and possibilities:

If the Queue is empty:

  • create a new node
  • set the new node as start and end
  • increase the queue’s length by 1
  • return the new node

All remaining cases:

  • create a new node
  • set the new node as the end’s next node
  • set the new node as the new end
  • increase the queue’s length by 1
  • return the new node

Example

// current queue:
A (start) ==> B (end)

// desired queue:
A (start) ==> B       ==> C (end)

Steps:

// current queue:
A (start) ==> B (end)

// set the new node as the end's next node
A (start) ==> B (end) ==> C

// set the new node as the new end
A (start) ==> B       ==> C (end)

// desired queue:
A (start) ==> B       ==> C (end)


Implementation 📝

class Node {
  constructor(value) {
    this.value = value;
    this.next = null;
  }
}

class Queue {
  constructor() {
    this.length = 0;
    this.start = null;
    this.end = null;
  }

  enqueue(value) {
    // create a new node
    const newNode = new Node(value);

    if (!this.length) {
      // set the new node as start and end
      this.start = newNode;
      this.end = newNode;
    } else {
      // set the new node as the end's next node
      this.end.next = newNode;
      // set the new node as the new end
      this.end = newNode;
    }

    // increase the queue's length by 1
    this.length += 1;
    // return the new node
    return newNode;
  }
}

Result

Let’s have a look how to use the enqueue method and its results.

const newQueue = new Queue();

// empty queue
console.log(newQueue);
// Queue { length: 0, start: null, end: null }

console.log(newQueue.enqueue("new A"));
// Node { value: 'new A', next: null }

// queue with 1 node
console.log(newQueue);
// Queue {
//   length: 1,
//   start: Node { value: 'new A', next: null },
//   end: Node { value: 'new A', next: null }
// }

console.log(newQueue.enqueue("new B"));
// Node { value: 'new B', next: null }

// queue with 2 nodes
console.log(newQueue);
// Queue {
//   length: 2,
//   start: Node { value: 'new A', next: Node { value: 'new B', next: null } },
//   end: Node { value: 'new B', next: null }
// }


Next Part ➡️

We will implement our next method to dequeue the first node.

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Questions ❔

  • Do you see some similarities to the Singly Linked List or Doubly Linked List?