Enqueue Operation in DSA Javascript - Execution Trace

Concept Flow - Enqueue Operation

Create new node with value

↓

Is queue empty?

Yes→Set head and tail to new node

|No↓

Set tail.next to new node

↓

Update tail to new node

↓

Done

This flow shows adding a new node to the end of the queue. If empty, head and tail point to it. Otherwise, tail's next points to new node, then tail updates.

Execution Sample

DSA Javascript

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

class Queue {
  constructor() {
    this.head = null;
    this.tail = null;
    this.size = 0;
  }

  enqueue(value) {
    const newNode = new Node(value);
    if (!this.tail) {
      this.head = newNode;
      this.tail = newNode;
    } else {
      this.tail.next = newNode;
      this.tail = newNode;
    }
    this.size++;
  }
}

This code adds a new value to the end of the queue, updating pointers accordingly.

Execution Table

Step	Operation	Nodes in List	Pointer Changes	Visual State
1	Enqueue 10 on empty queue	10	head -> 10, tail -> 10	10 -> null
2	Enqueue 20	10 -> 20	tail.next -> 20, tail -> 20	10 -> 20 -> null
3	Enqueue 30	10 -> 20 -> 30	tail.next -> 30, tail -> 30	10 -> 20 -> 30 -> null
4	Exit	10 -> 20 -> 30	No change	10 -> 20 -> 30 -> null

💡 No more enqueue operations; queue has 3 nodes.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
head	null	Node(10)	Node(10)	Node(10)	Node(10)
tail	null	Node(10)	Node(20)	Node(30)	Node(30)
size	0	1	2	3	3

Key Moments - 3 Insights

Why do we set both head and tail to the new node when the queue is empty?

When the queue is empty (step 1), there is no node, so both head and tail must point to the new single node to represent the queue correctly, as shown in execution_table row 1.

Why do we update tail.next before moving tail to the new node?

Updating tail.next links the old last node to the new node, maintaining the chain. Then tail moves to the new node to mark the new end, as seen in steps 2 and 3 in execution_table.

What happens if we forget to update tail after enqueue?

If tail is not updated, the queue's end pointer stays at the old node, breaking the chain for future enqueues. The visual state would not include the new node at the end, unlike steps 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what does tail point to?

A Node with value 10

B Node with value 30

C Node with value 20

D null

💡 Hint

Check the 'Pointer Changes' column at step 2 in execution_table.

At which step does the queue first have more than one node?

A Step 1

B Step 2

C Step 3

D Exit step

💡 Hint

Look at the 'Nodes in List' column in execution_table to see when nodes count increases.

If we enqueue a new node without updating tail, what will happen to the visual state?

A New node will not appear at the end

B Queue will be empty

C Head will point to new node

D Tail will point to old node

💡 Hint

Refer to key_moments about tail update importance and execution_table steps 2 and 3.

Concept Snapshot

Enqueue Operation in Queue:
- Create new node with value
- If queue empty: head and tail point to new node
- Else: tail.next points to new node
- Update tail to new node
- Visual: nodes linked from head to tail
- Maintains FIFO order

Full Transcript

The enqueue operation adds a new node to the end of a queue. First, a new node is created with the given value. If the queue is empty, both head and tail pointers are set to this new node. Otherwise, the current tail's next pointer is set to the new node, and then the tail pointer is updated to the new node. This keeps the queue linked correctly from head to tail, preserving the order of elements. The execution table shows step-by-step how nodes are added and pointers updated, with the visual state of the queue after each enqueue. Key moments clarify why both head and tail are set on empty, why tail.next is updated before tail, and what happens if tail is not updated. The visual quiz tests understanding of pointer positions and queue state changes during enqueue.

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand the queue behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the end of the queue.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to add an element 5 to a queue implemented as an array named queue in JavaScript?

easy

A. queue.enqueue(5);

B. queue.add(5);

C. queue.push(5);

D. queue.insert(5);

Solution

1. Step 1: Identify JavaScript array method for adding at end

   JavaScript arrays use push() to add elements at the end.

2. Step 2: Match method to enqueue operation

   Enqueue adds to the end, so queue.push(5); is correct syntax.

3. Final Answer:

   queue.push(5); -> Option C

4. Quick Check:

   Array push adds at end [OK]

Hint: Use push() to enqueue in array-based queue [OK]

Common Mistakes:

• Using non-existent methods like add() or insert()
• Confusing enqueue with dequeue methods
• Trying to use enqueue() directly on array

3. What will be the output of the following JavaScript code?

const queue = [];
queue.push(10);
queue.push(20);
queue.push(30);
console.log(queue);

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   First push adds 10: queue = [10]. Second push adds 20: queue = [10, 20]. Third push adds 30: queue = [10, 20, 30].

2. Step 2: Print the queue array

   Logging queue prints the array as it is: [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   push adds at end, so order is [10, 20, 30] [OK]

Hint: push() adds at end, so order stays as inserted [OK]

Common Mistakes:

• Assuming push adds at front
• Confusing output order
• Expecting error due to empty initialization

4. Identify the error in the following enqueue implementation for a queue using an array:

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

medium

A. enqueue method should remove element, not add

B. Using unshift adds element at the front, not the end

C. Array should be initialized as null, not []

D. Missing return statement in enqueue method

Solution

1. Step 1: Understand unshift behavior

   unshift adds element at the start (front) of the array, not the end.

2. Step 2: Compare with enqueue operation

   Enqueue must add element at the end, so unshift is incorrect here.

3. Final Answer:

   Using unshift adds element at the front, not the end -> Option B

4. Quick Check:

   Enqueue adds at end, unshift adds at front [OK]

Hint: Use push() for enqueue, not unshift() [OK]

Common Mistakes:

• Confusing unshift with push
• Thinking unshift adds at end
• Ignoring method behavior differences

5. You have a queue implemented with an array. You want to enqueue multiple elements [7, 8, 9] at once while preserving their order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.unshift(7, 8, 9);

B. queue.concat([7, 8, 9]);

C. queue.push([7, 8, 9]);

D. [7, 8, 9].forEach(e => queue.push(e));

Solution

1. Step 1: Analyze each option for enqueue behavior

   queue.push([7, 8, 9]); adds the entire array as one element, not individual elements. queue.concat([7, 8, 9]); creates a new array but does not modify queue. [7, 8, 9].forEach(e => queue.push(e)); pushes each element individually, preserving order. queue.unshift(7, 8, 9); adds elements at front, reversing order.

2. Step 2: Choose the method that enqueues elements individually at the end

   Using forEach with push adds each element in order at the end of queue.

3. Final Answer:

   [7, 8, 9].forEach(e => queue.push(e)); -> Option D

4. Quick Check:

   forEach + push enqueues elements in order [OK]

Hint: Use forEach with push to enqueue multiple elements in order [OK]

Common Mistakes:

• Using push with array adds nested array
• Using unshift reverses order
• Forgetting to assign concat result back to queue