Enqueue Operation in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after enqueue operations?

Consider a queue implemented using an array. What will be the state of the queue after the following enqueue operations?

DSA Javascript

class Queue {
  constructor() {
    this.items = [];
  }
  enqueue(element) {
    this.items.push(element);
  }
  print() {
    return this.items.join(' -> ') + ' -> null';
  }
}

const q = new Queue();
q.enqueue(10);
q.enqueue(20);
q.enqueue(30);
console.log(q.print());

A null

B 30 -> 20 -> 10 -> null

C 10 -> 20 -> 30 -> null

D 10 -> 20 -> null

💡 Hint

Remember, enqueue adds elements to the end of the queue.

The enqueue operation adds elements to the back of the queue. So after adding 10, 20, and 30, the queue will be 10 -> 20 -> 30 -> null.

❓ Predict Output

intermediate

What is the output after enqueueing objects?

Given the following queue implementation, what will be printed after enqueueing three objects?

DSA Javascript

class Queue {
  constructor() {
    this.items = [];
  }
  enqueue(element) {
    this.items.push(element);
  }
  print() {
    return this.items.map(e => e.value).join(' -> ') + ' -> null';
  }
}

const q = new Queue();
q.enqueue({value: 'a'});
q.enqueue({value: 'b'});
q.enqueue({value: 'c'});
console.log(q.print());

A a -> b -> c -> null

B c -> b -> a -> null

C a -> c -> b -> null

D null

💡 Hint

Enqueue adds elements to the end, preserving order.

The queue stores objects in the order they are enqueued. The print method extracts the 'value' property, so the output is 'a -> b -> c -> null'.

❓ Predict Output

advanced

What is the output after enqueue with size limit?

A queue has a maximum size of 3. What will be the output after trying to enqueue four elements?

DSA Javascript

class Queue {
  constructor(maxSize) {
    this.items = [];
    this.maxSize = maxSize;
  }
  enqueue(element) {
    if (this.items.length < this.maxSize) {
      this.items.push(element);
    }
  }
  print() {
    return this.items.join(' -> ') + ' -> null';
  }
}

const q = new Queue(3);
q.enqueue(1);
q.enqueue(2);
q.enqueue(3);
q.enqueue(4);
console.log(q.print());

A null

B 1 -> 2 -> 3 -> 4 -> null

C 4 -> 3 -> 2 -> 1 -> null

D 1 -> 2 -> 3 -> null

💡 Hint

The queue does not accept elements beyond its max size.

The queue only accepts the first three elements because of the maxSize check. The fourth enqueue is ignored.

❓ Predict Output

advanced

What error does this enqueue code raise?

What error will be raised when running this enqueue method?

DSA Javascript

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

const q = new Queue();
q.enqueue(5);

A SyntaxError: Unexpected token

B TypeError: this.items.add is not a function

C ReferenceError: element is not defined

D No error, queue contains 5

💡 Hint

Check the method used to add elements to an array.

Arrays in JavaScript do not have an 'add' method. The correct method is 'push'. Using 'add' causes a TypeError.

🧠 Conceptual

expert

Which option correctly describes enqueue operation in a linked list based queue?

In a queue implemented using a linked list with head and tail pointers, which statement about the enqueue operation is correct?

A Enqueue adds a new node at the tail and updates the tail pointer.

B Enqueue removes the node at the tail and updates the tail pointer.

C Enqueue removes the node at the head and updates the head pointer.

D Enqueue adds a new node at the head and updates the head pointer.

💡 Hint

Think about where new elements enter a queue.

In a linked list queue, enqueue adds a new node at the tail end and moves the tail pointer to this new node.

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