Peek Top Element of Stack in DSA Javascript

Mental Model

A stack is like a pile of plates where you only see and use the top plate. Peeking means looking at the top plate without removing it.

Analogy: Imagine a stack of books on a table. You can only see and touch the top book. Peeking is like glancing at the title of the top book without taking it off.

Top -> [ 3 ] -> [ 2 ] -> [ 1 ] -> null
          ↑
        (peek)

Dry Run Walkthrough

Input: stack: push 1, push 2, push 3; peek top element

Goal: To see the top element of the stack without removing it

Step 1: push 1 onto stack

Top -> [ 1 ] -> null

Why: We start with one element; it becomes the top

Step 2: push 2 onto stack

Top -> [ 2 ] -> [ 1 ] -> null

Why: New element goes on top, pushing previous down

Step 3: push 3 onto stack

Top -> [ 3 ] -> [ 2 ] -> [ 1 ] -> null

Why: Again, new element is on top

Step 4: peek top element

Top -> [ 3 ] -> [ 2 ] -> [ 1 ] -> null
          ↑
        (peek)

Why: We look at the top element without removing it

Result:

Top -> [ 3 ] -> [ 2 ] -> [ 1 ] -> null
Peeked element: 3

Annotated Code

DSA Javascript

class Stack {
  constructor() {
    this.items = [];
  }

  push(element) {
    this.items.push(element); // add element on top
  }

  peek() {
    if (this.items.length === 0) {
      return null; // empty stack check
    }
    return this.items[this.items.length - 1]; // return top element
  }

  print() {
    let result = '';
    for (let i = this.items.length - 1; i >= 0; i--) {
      result += `[ ${this.items[i]} ] -> `;
    }
    result += 'null';
    return result;
  }
}

// Driver code
const stack = new Stack();
stack.push(1);
stack.push(2);
stack.push(3);
console.log('Stack: ' + stack.print());
console.log('Peeked element: ' + stack.peek());

this.items.push(element); // add element on top

push new element to the top of stack

if (this.items.length === 0) { return null; // empty stack check }

check if stack is empty before peeking

return this.items[this.items.length - 1]; // return top element

return the top element without removing it

Output

Stack: [ 3 ] -> [ 2 ] -> [ 1 ] -> null

Peeked element: 3

Complexity Analysis

Time: O(1) because peek just returns the last element without looping

Space: O(n) because stack stores n elements in an array

vs Alternative: Compared to popping which removes the element, peek is faster since it does not modify the stack

Edge Cases

empty stack

peek returns null indicating no elements

DSA Javascript

if (this.items.length === 0) {
      return null; // empty stack check
    }

When to Use This Pattern

When you need to see the last added item without removing it, use the peek operation on a stack.

Common Mistakes

Mistake: Removing the top element instead of just viewing it during peek
Fix: Return the last element without calling pop or modifying the stack

Summary

Peek shows the top element of the stack without removing it.

Use peek when you want to check the last added item but keep the stack unchanged.

Remember peek just looks at the top, it does not change the stack.

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