Peek Front Element of Queue in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the front element of the queue without removing it.

DSA Javascript

function peek(queue) {
  return queue[1];
}

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

A[0]

Bshift()

Cpop()

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() removes the element, not just peeks.

Using pop() returns the last element, not the front.

The front element of a queue implemented as an array is at index 0. Using queue[0] returns it without removing.

2fill in blank

medium

Complete the code to check if the queue is empty before peeking the front element.

DSA Javascript

function peek(queue) {
  if (queue[1] === 0) {
    return null;
  }
  return queue[0];
}

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

Acount

Blength

Csize

Dempty

Attempts:

3 left

💡 Hint

Common Mistakes

Using size or count which are not valid properties for arrays.

Using empty which is not a property.

Arrays have a length property to check how many elements they contain. If length is 0, the queue is empty.

3fill in blank

hard

Fix the error in the peek function that tries to return the front element but causes an error when the queue is empty.

DSA Javascript

function peek(queue) {
  if (queue.length === 0) {
    return null;
  }
  return queue[1];
}

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

Alength

Bpop()

Cshift()

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() or shift() which remove elements.

Returning length which is a number, not the element.

Accessing queue[0] safely returns the front element. Using pop() or shift() removes elements, and length is not an element.

4fill in blank

hard

Fill both blanks to create a function that returns the front element or null if the queue is empty.

DSA Javascript

function peek(queue) {
  if (queue[1] === 0) {
    return [2];
  }
  return queue[0];
}

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

Alength

Bundefined

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning undefined instead of null.

Checking a wrong property like size or count.

Check if queue.length is 0 to detect empty queue, then return null to indicate no front element.

5fill in blank

hard

Fill all three blanks to implement peek with a default parameter and safe return.

DSA Javascript

function peek(queue = [1]) {
  if (queue[2] === 0) {
    return [3];
  }
  return queue[0];
}

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

A[]

Blength

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined as default parameter which causes errors.

Returning undefined instead of null.

Default the queue to an empty array []. Check queue.length for emptiness. Return null if empty.

Practice

1. What does the peek operation do in a queue?

easy

A. Returns the front element without removing it

B. Removes the front element from the queue

C. Adds a new element to the back of the queue

D. Checks if the queue is empty

Solution

1. Step 1: Understand the peek operation

   The peek operation looks at the front element of the queue but does not remove it.

2. Step 2: Compare with other queue operations

   Removing is done by dequeue, adding by enqueue, and checking empty is a separate check.

3. Final Answer:

   Returns the front element without removing it -> Option A

4. Quick Check:

   Peek = front element without removal [OK]

Hint: Peek shows front without changing queue [OK]

Common Mistakes:

• Confusing peek with dequeue
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the front element of a queue named myQueue in JavaScript if it is implemented as an array?

easy

A. myQueue.peek()

B. myQueue[0]

C. myQueue.front()

D. myQueue.getFront()

Solution

1. Step 1: Understand array-based queue front access

   In JavaScript, arrays use index 0 for the first element, so myQueue[0] accesses the front.

2. Step 2: Check other options for validity

   Methods like peek(), front(), or getFront() are not standard array methods.

3. Final Answer:

   myQueue[0] -> Option B

4. Quick Check:

   Array front element = index 0 [OK]

Hint: Use index 0 to peek front in array queue [OK]

Common Mistakes:

• Using non-existent methods on arrays
• Using wrong index like myQueue[1]
• Confusing peek with dequeue method

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

const queue = [];
queue.push(10);
queue.push(20);
queue.push(30);
console.log(queue[0]);

medium

A. 10

B. 20

C. 30

D. undefined

Solution

1. Step 1: Understand queue insertion using push

   Elements 10, 20, 30 are added to the end of the array, so queue = [10, 20, 30].

2. Step 2: Access front element using index 0

   queue[0] returns the first element, which is 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Front element after pushes = 10 [OK]

Hint: Front element is first pushed value at index 0 [OK]

Common Mistakes:

• Thinking last pushed element is front
• Using queue.pop() instead of queue[0]
• Confusing index 0 with index 1

4. Identify the error in the following code that tries to peek the front element of a queue implemented as an array:

const queue = [5, 15, 25];
function peek() {
  return queue.pop();
}
console.log(peek());

medium

A. queue is not defined inside peek()

B. pop() returns undefined for non-empty arrays

C. peek() should use queue.push() instead

D. Using pop() removes the last element instead of peeking the front

Solution

1. Step 1: Understand pop() behavior on arrays

   pop() removes and returns the last element, not the front.

2. Step 2: Correct way to peek front element

   To peek front, access queue[0] without removing it.

3. Final Answer:

   Using pop() removes the last element instead of peeking the front -> Option D

4. Quick Check:

   pop() removes last, not front [OK]

Hint: pop() removes last; peek front uses index 0 [OK]

Common Mistakes:

• Using pop() to peek front
• Confusing pop() with shift()
• Trying to push in peek function

5. You have a queue implemented as an array: let q = [100, 200, 300]. You want to peek the front element safely, but the queue might be empty sometimes. Which code snippet correctly peeks the front element without causing an error?

hard

A. console.log(q.pop());

B. console.log(q[0]);

C. console.log(q.length > 0 ? q[0] : 'Queue is empty');

D. console.log(q.shift());

Solution

1. Step 1: Check for empty queue before peeking

   Accessing q[0] when empty returns undefined but may be unclear; better to check length.

2. Step 2: Use conditional to safely peek front

   Using q.length > 0 ? q[0] : 'Queue is empty' returns front if exists, else message.

3. Final Answer:

   console.log(q.length > 0 ? q[0] : 'Queue is empty'); -> Option C

4. Quick Check:

   Safe peek uses length check before access [OK]

Hint: Check length before peek to avoid errors [OK]

Common Mistakes:

• Peeking without checking empty queue
• Using pop() or shift() which remove elements
• Assuming q[0] always exists