Enqueue Using Linked List in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new node with the given value.

DSA Javascript

class Node {
  constructor(value) {
    this.value = [1];
    this.next = null;
  }
}

Drag options to blanks, or click blank then click option'

Avalue

Bnext

Cnull

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning 'next' instead of 'value'.

Using 'null' instead of the passed value.

The constructor should assign the passed value to this.value to store the node's data.

2fill in blank

medium

Complete the code to check if the queue is empty before enqueueing.

DSA Javascript

enqueue(value) {
  const newNode = new Node(value);
  if (this.front === [1]) {
    this.front = newNode;
    this.rear = newNode;
  } else {
    this.rear.next = newNode;
    this.rear = newNode;
  }
}

Drag options to blanks, or click blank then click option'

Afalse

Bnull

C0

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for undefined instead of null.

Checking for 0 or false which are unrelated.

The queue is empty if this.front is null, so we check for null to set front and rear.

3fill in blank

hard

Fix the error in updating the rear pointer after enqueueing a new node.

DSA Javascript

enqueue(value) {
  const newNode = new Node(value);
  if (this.front === null) {
    this.front = newNode;
    this.rear = newNode;
  } else {
    this.rear.next = newNode;
    this.rear = [1];
  }
}

Drag options to blanks, or click blank then click option'

Athis.rear.next.next

Bthis.front

Cvalue

DnewNode

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning rear to front or value instead of newNode.

Using next.next which is undefined.

After linking the new node, rear should point to the newNode to keep track of the last node.

4fill in blank

hard

Fill both blanks to complete the enqueue method that adds a new node to the queue.

DSA Javascript

enqueue(value) {
  const newNode = new Node(value);
  if (this.front === [1]) {
    this.front = newNode;
    this.rear = newNode;
  } else {
    this.rear.[2] = newNode;
    this.rear = newNode;
  }
}

Drag options to blanks, or click blank then click option'

Anull

Bundefined

Cnext

Dprev

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined instead of null for empty check.

Using prev instead of next to link nodes.

Check if front is null to detect empty queue, and link newNode by setting rear.next to newNode.

5fill in blank

hard

Fill all three blanks to complete the enqueue method with proper node creation and linking.

DSA Javascript

enqueue([1]) {
  const newNode = new [2]([3]);
  if (this.front === null) {
    this.front = newNode;
    this.rear = newNode;
  } else {
    this.rear.next = newNode;
    this.rear = newNode;
  }
}

Drag options to blanks, or click blank then click option'

Avalue

BNode

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'data' instead of 'value' inconsistently.

Not matching parameter and constructor argument names.

The method takes 'value', creates a new Node with 'value', and links it properly.

Practice

1. What does the enqueue operation do in a queue implemented using a linked list?

easy

A. Deletes the entire linked list

B. Removes an element from the front (head) of the linked list

C. Adds a new element at the end (tail) of the linked list

D. Adds a new element at the front (head) of the linked list

Solution

1. Step 1: Understand enqueue operation

   Enqueue means adding an element to the queue.

2. Step 2: Identify position in linked list

   In a linked list queue, new elements are added at the tail (end).

3. Final Answer:

   Adds a new element at the end (tail) of the linked list -> Option C

4. Quick Check:

   Enqueue = add at tail [OK]

Hint: Enqueue always adds at the tail in linked list queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue operation
• Adding new element at the head instead of tail
• Thinking enqueue removes elements

2. Which of the following is the correct way to create a new node for enqueue in JavaScript linked list?

easy

A. const node = { value: val, next: null };

B. const node = new Node(val); node.next = val;

C. const node = { val: value, next: undefined };

D. const node = Node(val, null);

Solution

1. Step 1: Understand node structure

   A node should have a value and a pointer to next (null initially).

2. Step 2: Check syntax correctness

   const node = { value: val, next: null }; correctly creates an object with value and next set to null.

3. Final Answer:

   const node = { value: val, next: null }; -> Option A

4. Quick Check:

   Node = {{value, next:null}} [OK]

Hint: Node must have value and next=null for enqueue [OK]

Common Mistakes:

• Using wrong property names like 'val' instead of 'value'
• Setting next to undefined instead of null
• Calling Node as a function without new keyword

3. What will be the state of the queue after these operations?

enqueue(10); enqueue(20); enqueue(30);

Assuming an empty queue initially, what is the linked list representation?

medium

A. 10 -> 20 -> 30 -> null

B. 30 -> 20 -> 10 -> null

C. null -> 10 -> 20 -> 30

D. 20 -> 10 -> 30 -> null

Solution

1. Step 1: Enqueue 10

   Queue is empty, so 10 becomes head and tail: 10 -> null

2. Step 2: Enqueue 20 and 30

   20 added at tail: 10 -> 20 -> null, then 30 added: 10 -> 20 -> 30 -> null

3. Final Answer:

   10 -> 20 -> 30 -> null -> Option A

4. Quick Check:

   Order preserved, enqueue adds at tail [OK]

Hint: Enqueue adds at tail, order stays same [OK]

Common Mistakes:

• Reversing order of elements
• Confusing head and tail positions
• Assuming enqueue adds at head

4. Consider this enqueue function snippet:

function enqueue(val) {
  const node = { value: val, next: null };
  if (!this.tail) {
    this.head = node;
  }
  this.tail.next = node;
  this.tail = node;
}

What error will occur when enqueue is called on an empty queue?

medium

A. ReferenceError: this is not defined

B. SyntaxError: Unexpected token

C. No error, works fine

D. TypeError: Cannot read property 'next' of null

Solution

1. Step 1: Analyze empty queue condition

   When queue is empty, this.tail is null or undefined.

2. Step 2: Identify error line

   Code tries this.tail.next = node; but this.tail is null, causing TypeError.

3. Final Answer:

   TypeError: Cannot read property 'next' of null -> Option D

4. Quick Check:

   Accessing next on null causes TypeError [OK]

Hint: Check if tail exists before accessing next [OK]

Common Mistakes:

• Ignoring null check for tail
• Assuming no error on empty queue
• Confusing this.head and this.tail usage

5. You want to implement enqueue in a linked list queue that keeps track of both head and tail. Which of these code snippets correctly enqueues a new value val?

hard

A. const node = { value: val, next: null };
this.tail.next = node;
this.tail = node;

B. const node = { value: val, next: null };
if (!this.head) {
this.head = node;
this.tail = node;
} else {
this.tail.next = node;
this.tail = node;
}

C. const node = { value: val, next: this.head };
this.head = node;

D. const node = { value: val, next: null };
this.head.next = node;
this.head = node;

Solution

1. Step 1: Handle empty queue case

   If head is null, both head and tail should point to new node.

2. Step 2: Handle non-empty queue

   Set tail.next to new node and update tail to new node.

3. Final Answer:

   const node = { value: val, next: null }; if (!this.head) { this.head = node; this.tail = node; } else { this.tail.next = node; this.tail = node; } -> Option B

4. Quick Check:

   Check empty and non-empty cases for enqueue [OK]

Hint: Set head and tail on empty, else add at tail.next [OK]

Common Mistakes:

• Not updating tail on empty queue
• Adding new node at head instead of tail
• Forgetting to update tail pointer