Peek Top Element of Stack in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The 'peek' operation shows the top element of the stack without removing it. It lets you see what is on top without changing the stack.

beginner

In a stack, which end do we add or remove elements from?

We add or remove elements from the top of the stack only. This is called Last In, First Out (LIFO).

intermediate

What will happen if you peek an empty stack?

Peeking an empty stack usually returns null, undefined, or an error because there is no top element to show.

intermediate

Why is peek useful in stack operations?

Peek is useful because it lets you check the top element without changing the stack. This helps in decision making without losing data.

beginner

Show a simple JavaScript code snippet to peek the top element of a stack represented by an array.

const stack = [1, 2, 3];
const top = stack[stack.length - 1];
console.log(top); // Output: 3

What does the peek operation return in a stack?

A The top element without removing it

B The bottom element

C Removes and returns the top element

D Removes and returns the bottom element

Peek returns the top element without removing it from the stack.

If a stack is empty, what should peek return?

A The first element

B An error or null/undefined

C The last element

D The whole stack

Peeking an empty stack usually returns null, undefined, or an error because there is no top element.

Which data structure principle does stack follow?

A None of the above

B First In, First Out (FIFO)

C Random Access

D Last In, First Out (LIFO)

Stack follows Last In, First Out (LIFO) principle.

In JavaScript, how do you access the top element of a stack stored as an array named 'stack'?

A stack.pop()

B stack[0]

C stack[stack.length - 1]

D stack.push()

The top element is at index stack.length - 1.

What is the difference between peek and pop in a stack?

A Pop removes the top element, peek only shows it

B Both only show the top element

C Both remove the top element

D Peek removes the top element, pop only shows it

Pop removes and returns the top element, peek only returns it without removing.

Explain how the peek operation works in a stack and why it is useful.

Think about looking at the top book on a stack without taking it off.

Write a simple JavaScript code snippet to peek the top element of a stack represented by an array.

Use array indexing with length property.

Practice

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

easy

A. Returns the top element without removing it

B. Removes the top element and returns it

C. Adds a new element to the top

D. Checks if the stack is empty

Solution

1. Step 1: Understand stack operations

   A stack stores elements with last-in, first-out order. Peek shows the top element without removing it.

2. Step 2: Differentiate peek from pop

   Pop removes and returns the top element, peek only returns it without removal.

3. Final Answer:

   Returns the top element without removing it -> Option A

4. Quick Check:

   Peek = top element without removal [OK]

Hint: Peek shows top without removing it [OK]

Common Mistakes:

• Confusing peek with pop operation
• Thinking peek adds elements
• Assuming peek checks emptiness

2. Which of the following is the correct syntax to peek the top element of a stack implemented as an array named stack in JavaScript?

easy

A. stack.peek()

B. stack.pop()

C. stack[stack.length - 1]

D. stack.top()

Solution

1. Step 1: Understand JavaScript array indexing

   JavaScript arrays use zero-based indexing. The last element is at index length - 1.

2. Step 2: Identify correct peek syntax

   To peek, access the last element without removing it: stack[stack.length - 1]. stack.pop() removes the element, stack.peek() and stack.top() are not default methods.

3. Final Answer:

   stack[stack.length - 1] -> Option C

4. Quick Check:

   Peek = stack[stack.length - 1] [OK]

Hint: Use stack[stack.length - 1] to peek top [OK]

Common Mistakes:

• Using stack.pop() which removes element
• Using non-existent methods like peek() or top()
• Accessing stack[stack.length] which is undefined

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

const stack = [10, 20, 30];
const top = stack[stack.length - 1];
console.log(top);

medium

A. 10

B. 30

C. 20

D. undefined

Solution

1. Step 1: Identify the stack elements and length

   The stack array is [10, 20, 30]. Its length is 3.

2. Step 2: Calculate the top element index and value

   stack.length - 1 = 3 - 1 = 2. stack[2] is 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   Top element = 30 [OK]

Hint: Top element is last array item: stack[length-1] [OK]

Common Mistakes:

• Using stack[stack.length] which is undefined
• Confusing first element with top element
• Printing stack.pop() instead of peek

4. Find the error in this code snippet that tries to peek the top element of a stack:

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

medium

A. stack.length should be stack.length - 1

B. stack is not defined

C. peek function should remove the element

D. console.log is missing parentheses

Solution

1. Step 1: Analyze the peek function index

   stack.length returns the number of elements, but array indices go from 0 to length - 1.

2. Step 2: Correct the index to access top element

   Use stack.length - 1 to get the last element. Using stack.length returns undefined.

3. Final Answer:

   stack.length should be stack.length - 1 -> Option A

4. Quick Check:

   Index = length - 1 for top element [OK]

Hint: Use length - 1 to access last element [OK]

Common Mistakes:

• Using stack.length as index causing undefined
• Thinking peek removes element
• Missing parentheses in console.log (not in this code)

5. Given a stack implemented as an array, how can you safely peek the top element without causing an error if the stack is empty?

function safePeek(stack) {
  // Your code here
}

hard

A. return stack[stack.length];

B. return stack[0];

C. return stack.pop();

D. return stack.length > 0 ? stack[stack.length - 1] : null;

Solution

1. Step 1: Check if stack is empty

   Before accessing the top element, verify stack length is greater than zero to avoid undefined access.

2. Step 2: Return top element or null safely

   If stack is not empty, return stack[stack.length - 1], else return null to indicate empty stack.

3. Final Answer:

   return stack.length > 0 ? stack[stack.length - 1] : null; -> Option D

4. Quick Check:

   Safe peek uses length check and conditional [OK]

Hint: Check length > 0 before peek to avoid errors [OK]

Common Mistakes:

• Accessing stack[stack.length] causing undefined
• Using pop which removes element
• Returning stack[0] which is bottom, not top