Peek Top Element of Stack in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the top element of the stack without removing it.

DSA Javascript

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

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

Ashift()

Bpop()

Cpush()

Dslice(-1)[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() removes the top element instead of just peeking.

Using push() or shift() does not return the top element.

Using slice(-1)[0] returns the last element of the array (top of the stack) without removing it.

2fill in blank

medium

Complete the code to check if the stack is empty before peeking.

DSA Javascript

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

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

A0

B1

C-1

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Checking length === 1 instead of 0 causes wrong empty check.

Using undefined or -1 as length check is incorrect.

Checking if stack.length === 0 means the stack is empty, so peek should return null.

3fill in blank

hard

Fix the error in the peek function to correctly return the top element or null if empty.

DSA Javascript

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

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

A0

B-1

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Returning undefined instead of null for empty stack.

Accessing stack[stack.length] which is undefined.

Return null when stack is empty. Also, stack[stack.length] is undefined, should use stack[stack.length - 1].

4fill in blank

hard

Fill both blanks to complete the peek function that safely returns the top element or null if empty.

DSA Javascript

function peek(stack) {
  if (stack.length === [1]) {
    return [2];
  }
  return stack[stack.length - 1];
}

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

A0

Bnull

Cundefined

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 instead of 0 for empty check.

Returning undefined instead of null.

Check if length is 0 to detect empty stack and return null in that case.

5fill in blank

hard

Fill all three blanks to implement peek using a ternary operator.

DSA Javascript

const peek = stack => stack.length === [1] ? [2] : stack[stack.length [3] 1];

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

A0

Bnull

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' instead of '-' for index calculation.

Returning undefined instead of null for empty stack.

If stack is empty (length 0), return null; else return last element at index length - 1.

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