Peek Front Element of Queue in DSA Typescript - Build from Scratch

Peek Front Element of Queue

📖 Scenario: You are managing a line of customers waiting for service at a store. You want to check who is at the front of the line without removing them.

🎯 Goal: Build a simple queue using an array and write code to peek at the front element of the queue.

📋 What You'll Learn

Create a queue as an array with exact customer names

Create a variable to hold the front element

Write code to peek (access) the front element without removing it

Print the front element

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting for service or tasks waiting to be processed.

💼 Career

Understanding queues and how to peek at elements is important for programming tasks like scheduling, buffering, and managing resources efficiently.

1

Create the queue array

Create an array called queue with these exact strings in order: 'Alice', 'Bob', 'Charlie', 'Diana'.

DSA Typescript

# Your code here

Hint

Use square brackets [] to create an array and separate names with commas.

2

Create a variable for the front element

Create a variable called frontElement and set it to null initially.

DSA Typescript

const queue: string[] = ['Alice', 'Bob', 'Charlie', 'Diana'];
# Your code here

Hint

Use let to create a variable that can change later.

3

Peek the front element of the queue

Set frontElement to the first element of the queue array without removing it.

DSA Typescript

const queue: string[] = ['Alice', 'Bob', 'Charlie', 'Diana'];
let frontElement: string | null = null;
# Your code here

Hint

Use queue[0] to access the first item in the array.

4

Print the front element

Print the value of frontElement using console.log.

DSA Typescript

const queue: string[] = ['Alice', 'Bob', 'Charlie', 'Diana'];
let frontElement: string | null = null;
frontElement = queue[0];
# Your code here

Hint

Use console.log(frontElement); to show the front element in the console.

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 an 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 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 removal [OK]

Common Mistakes:

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

2. Which of the following is the correct TypeScript syntax to peek the front element of a queue implemented as an array named queue?

easy

A. const front = queue.pop();

B. const front = queue.shift();

C. const front = queue.push();

D. const front = queue[0];

Solution

1. Step 1: Understand array methods for queue

   In a queue using an array, the front element is at index 0.

2. Step 2: Identify correct syntax for peek

   Accessing queue[0] returns the front without removing it. pop() removes from end, shift() removes from front, push() adds to end.

3. Final Answer:

   const front = queue[0]; -> Option D

4. Quick Check:

   Peek = queue[0] [OK]

Hint: Peek front is queue[0], not pop or shift [OK]

Common Mistakes:

• Using pop() which removes last element
• Using shift() which removes front element
• Using push() which adds element

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

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

medium

A. undefined
[10, 20, 30]

B. 10
[20, 30]

C. 10
[10, 20, 30]

D. 30
[10, 20, 30]

Solution

1. Step 1: Peek front element from queue array

   Accessing queue[0] returns 10, the first element.

2. Step 2: Check if queue is modified

   Since we only accessed queue[0], the array remains unchanged as [10, 20, 30].

3. Final Answer:

   10 [10, 20, 30] -> Option C

4. Quick Check:

   Peek returns 10, queue unchanged [OK]

Hint: Peek does not remove, queue stays same [OK]

Common Mistakes:

• Assuming peek removes element
• Confusing front with last element
• Expecting undefined for valid index

4. Identify the error in this TypeScript code that tries to peek the front element of a queue:

class Queue {
  items: number[] = [];
  peek() {
    return this.items.pop();
  }
}

medium

A. pop() removes last element, should use items[0]

B. pop() is correct for peek operation

C. items array should be private

D. peek method should return void

Solution

1. Step 1: Understand pop() behavior

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

2. Step 2: Correct way to peek front

   To peek front, return this.items[0] without removing it.

3. Final Answer:

   pop() removes last element, should use items[0] -> Option A

4. Quick Check:

   pop() ≠ peek front [OK]

Hint: pop() removes last, peek front is items[0] [OK]

Common Mistakes:

• Using pop() for peek
• Not returning front element correctly
• Ignoring array indexing

5. You have a queue implemented as a linked list with nodes having value and next. How do you peek the front element safely when the queue might be empty?

hard

A. Return head.value directly without checks

B. Check if head is null; if not, return head.value, else return null

C. Traverse to the last node and return its value

D. Remove the head node and return its value

Solution

1. Step 1: Understand linked list queue front

   The front element is at the head node of the linked list.

2. Step 2: Handle empty queue case

   If head is null, queue is empty, so return null or undefined safely.

3. Step 3: Return front value safely

   If head exists, return head.value without removing the node.

4. Final Answer:

   Check if head is null; if not, return head.value, else return null -> Option B

5. Quick Check:

   Peek front safely = check head then return value [OK]

Hint: Check head null before peek to avoid errors [OK]

Common Mistakes:

• Not checking for empty queue
• Returning value without null check
• Removing node instead of peeking