Peek Front Element of Queue in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'peek' operation do in a queue?

It shows the front element of the queue without removing it, so you can see who is next without changing the queue.

beginner

Why is peek useful in a queue?

Peek lets you check the next item to be processed without changing the order or removing it, like looking at the first person in line without letting them go.

beginner

In a queue implemented with an array, how do you peek the front element?

You look at the element at index 0, because the front of the queue is always the first element in the array.

beginner

What happens if you peek an empty queue?

Usually, you get a special value like null or undefined to show the queue is empty and there is no front element.

beginner

Show the JavaScript code to peek the front element of a queue stored as an array named 'queue'.

const front = queue.length > 0 ? queue[0] : null;
console.log(front);

What does the peek operation return in a queue?

A Adds a new element at the front

B The last element and removes it

C Removes all elements

D The front element without removing it

Peek returns the front element without removing it, so you can see who is next.

If a queue is empty, what should peek return?

A null or undefined

B The first element

C An error always

D The last element

Peek should return null or undefined to show the queue is empty and no front element exists.

In a queue implemented as an array, which index holds the front element?

A length - 1

B 0

C 1

D length / 2

The front element is at index 0 in an array-based queue.

What is the difference between peek and dequeue in a queue?

A Dequeue removes the front element, peek does not

B Both add elements

C Both remove the front element

D Peek removes the front element, dequeue does not

Dequeue removes the front element, peek only shows it without removing.

Which of these is a correct JavaScript code to peek the front element of a queue array named 'queue'?

A const front = queue[queue.length - 1];

B const front = queue.pop();

C const front = queue[0];

D const front = queue.push();

queue[0] accesses the front element without removing it.

Explain how to peek the front element of a queue and why it is useful.

Think about looking at the first person in line without letting them go.

Write a simple JavaScript code snippet to peek the front element of a queue stored as an array.

Use a conditional (ternary) operator to handle empty 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