Pop Operation on Stack in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove the top element from the stack.

DSA Javascript

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

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

Apush

Bshift

Cpop

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop() will add an element instead of removing.

Using shift() or unshift() removes or adds elements at the start, not the end.

The pop() method removes the last element added to the stack, which is the top element.

2fill in blank

medium

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

DSA Javascript

function safePop(stack) {
  if (stack.[1] === 0) return null;
  return stack.pop();
}

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

Acount

Bsize

Clength()

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using size or count which are not valid properties for arrays in JavaScript.

Using length() as a function instead of a property.

The length property gives the number of elements in the array (stack).

3fill in blank

hard

Fix the error in the pop function to avoid popping from an empty stack.

DSA Javascript

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

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

Aundefined

Bnull

Cfalse

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Returning undefined can be confusing as it may mean variable not set.

Returning 0 or false might be mistaken for valid stack elements.

Returning null clearly indicates no element was popped because the stack was empty.

4fill in blank

hard

Fill both blanks to pop the top element and store it in a variable.

DSA Javascript

function popAndStore(stack) {
  const topElement = stack.[1]();
  return [2];
}

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

Apop

Bpush

CtopElement

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() instead of pop() will add an element instead of removing.

Returning stack instead of the popped element.

pop() removes the top element, which is stored in topElement and then returned.

5fill in blank

hard

Fill all three blanks to pop an element only if the stack is not empty, else return null.

DSA Javascript

function conditionalPop(stack) {
  if (stack.[1] === 0) {
    return [2];
  } else {
    return stack.[3]();
  }
}

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

Alength

Bnull

Cpop

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using shift() instead of pop() changes stack behavior.

Returning undefined or 0 instead of null for empty stack.

Check stack length, return null if empty, else pop the top element.

Practice

1. What does the pop() operation do in a stack?

easy

A. Removes and returns the top element of the stack

B. Adds a new element to the bottom of the stack

C. Returns the bottom element without removing it

D. Clears all elements from the stack

Solution

1. Step 1: Understand stack behavior

   A stack follows Last In First Out (LIFO) principle, so the last added element is on top.

2. Step 2: Define pop operation

   The pop operation removes and returns the top element of the stack, following LIFO.

3. Final Answer:

   Removes and returns the top element of the stack -> Option A

4. Quick Check:

   pop() = remove top element [OK]

Hint: Pop always removes the last added item [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek (which only returns)
• Assuming pop removes bottom element

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

easy

A. pop(stack);

B. stack.popElement();

C. stack.pop();

D. stack.remove();

Solution

1. Step 1: Recall JavaScript array pop method

   JavaScript arrays have a built-in method pop() to remove the last element.

2. Step 2: Match correct syntax

   The correct syntax to pop from an array named stack is stack.pop();

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Use array.pop() to remove last element [OK]

Hint: Use array.pop() to remove top stack element [OK]

Common Mistakes:

• Using non-existent methods like popElement()
• Calling pop as a standalone function
• Using remove() which is not valid for arrays

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

const stack = [10, 20, 30];
const popped = stack.pop();
console.log(popped);
console.log(stack);

medium

A. 20
[10, 20, 30]

B. undefined
[10, 20]

C. 30
[10, 20, 30]

D. 30
[10, 20]

Solution

1. Step 1: Understand pop() effect on array

   pop() removes the last element (30) and returns it, so popped is 30.

2. Step 2: Check stack after pop

   After pop, stack becomes [10, 20] because 30 is removed.

3. Final Answer:

   30 [10, 20] -> Option D

4. Quick Check:

   pop() returns last element and removes it [OK]

Hint: pop() returns last item and removes it from array [OK]

Common Mistakes:

• Expecting pop() to return second last element
• Assuming stack remains unchanged after pop
• Confusing pop() return value with stack content

4. Identify the error in the following code snippet that tries to pop an element from a stack:

let stack = [1, 2, 3];
let popped = stack.pop;
console.log(popped);

medium

A. pop is not a function

B. pop method is not called, missing parentheses

C. stack is not defined

D. pop returns undefined always

Solution

1. Step 1: Check how pop is used

   The code assigns stack.pop without parentheses, so it assigns the function itself, not the result.

2. Step 2: Correct usage of pop

   To pop and get the value, pop must be called with parentheses: stack.pop().

3. Final Answer:

   pop method is not called, missing parentheses -> Option B

4. Quick Check:

   Use stack.pop() to call pop method [OK]

Hint: Always use parentheses to call pop() method [OK]

Common Mistakes:

• Forgetting parentheses when calling pop
• Thinking pop is a property, not a function
• Assuming pop returns undefined without call

5. Given a stack implemented as an array stack = [5, 10, 15], what will be the state of the stack after performing two consecutive pop() operations?

hard

A. [5]

B. [10]

C. [5, 10]

D. []

Solution

1. Step 1: Perform first pop()

   First pop removes 15, stack becomes [5, 10].

2. Step 2: Perform second pop()

   Second pop removes 10, stack becomes [5].

3. Final Answer:

   [5] -> Option A

4. Quick Check:

   Two pops remove last two elements [OK]

Hint: Each pop removes one top element [OK]

Common Mistakes:

• Removing elements from the bottom instead of top
• Assuming pop does not change stack
• Confusing number of pops with elements removed