Peek Front Element of Queue in DSA Javascript - Execution Trace

Concept Flow - Peek Front Element of Queue

Start

↓

Check if queue is empty?

Yes→Return 'Queue is empty'

No↓

Access front element

↓

Return front element

↓

End

The flow checks if the queue has elements; if yes, it returns the front element without removing it.

Execution Sample

DSA Javascript

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

This code shows how to peek the front element of a queue represented as an array.

Execution Table

Step	Operation	Queue State	Pointer Changes	Visual State
1	Start peek operation	[10, 20, 30]	front pointer undefined	10 -> 20 -> 30 -> null
2	Check if queue is empty	[10, 20, 30]	front pointer undefined	10 -> 20 -> 30 -> null
3	Queue not empty, access front element	[10, 20, 30]	front pointer at index 0	10 -> 20 -> 30 -> null
4	Return front element value	[10, 20, 30]	front pointer at index 0	10 -> 20 -> 30 -> null
5	End peek operation	[10, 20, 30]	front pointer at index 0	10 -> 20 -> 30 -> null

💡 Peek ends after returning the front element without modifying the queue.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
queue	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]	[10, 20, 30]
front pointer	undefined	0 (points to 10)	0 (points to 10)	0 (points to 10)
front element value	undefined	10	10	10

Key Moments - 3 Insights

Why do we check if the queue is empty before peeking?

Because if the queue is empty, there is no front element to return. The execution_table row 2 shows this check to avoid errors.

Does peeking remove the front element from the queue?

No, peeking only reads the front element without removing it. The queue state stays the same throughout, as seen in all execution_table rows.

What does the 'front pointer' represent in this operation?

It points to the first element in the queue (index 0). This is shown in execution_table row 3 and variable_tracker after step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the queue state at step 4?

A [20, 30]

B [10, 20, 30]

C []

D [30]

💡 Hint

Check the 'Queue State' column at step 4 in the execution_table.

At which step does the front pointer get assigned to the first element?

A Step 3

B Step 1

C Step 2

D Step 5

💡 Hint

Look at the 'Pointer Changes' column in the execution_table to find when front pointer is set.

If the queue was empty, what would happen at step 2?

A Return the front element value

B Assign front pointer to index 0

C Return 'Queue is empty' message

D Remove an element from queue

💡 Hint

Refer to the concept_flow diagram and execution_table step 2 for the empty queue check.

Concept Snapshot

Peek Front Element of Queue:
- Check if queue is empty
- If not empty, access element at front (index 0)
- Return this element without removing it
- Queue remains unchanged
- Useful to see next item to process

Full Transcript

Peeking the front element of a queue means looking at the first item without removing it. The process starts by checking if the queue is empty to avoid errors. If the queue has elements, we access the element at the front, which is at index 0 in an array representation. This element is returned as the peek result. The queue itself does not change during this operation. The execution steps show the queue state remains the same, and a pointer to the front element is used to access it. This operation is useful when you want to see the next item to process without modifying the queue.

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