Pop Operation on Stack in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after popping from the stack?

Consider the following TypeScript code that uses a stack implemented as an array. What will be printed after the pop operation?

DSA Typescript

const stack: number[] = [10, 20, 30, 40];
const popped = stack.pop();
console.log(stack);

A [10, 20, 30]

B [10, 20, 30, 40]

C [20, 30, 40]

D []

💡 Hint

Remember that pop() removes the last element from the array.

The pop() method removes the last element (40) from the stack, so the remaining stack is [10, 20, 30].

❓ Predict Output

intermediate

What is the value of popped element?

Given the stack and pop operation below, what is the value stored in popped?

DSA Typescript

const stack: string[] = ['a', 'b', 'c'];
const popped = stack.pop();
console.log(popped);

A undefined

B 'a'

C 'c'

D 'b'

💡 Hint

Pop returns the last element removed from the stack.

The last element 'c' is removed and returned by pop().

❓ Predict Output

advanced

What happens when popping from an empty stack?

What will be the output of the following code?

DSA Typescript

const stack: number[] = [];
const popped = stack.pop();
console.log(popped);

A null

B 0

C Throws an error

D undefined

💡 Hint

Check what pop() returns when the array is empty.

When popping from an empty array, pop() returns undefined without error.

🧠 Conceptual

advanced

How does pop operation affect stack size?

If a stack has 5 elements and you perform 2 pop operations, how many elements remain?

A 3

B 2

C 5

D 0

💡 Hint

Each pop removes one element from the stack.

Starting with 5 elements, popping twice removes 2 elements, leaving 3.

🔧 Debug

expert

Identify the error in pop operation code

What error will this code produce when run?

DSA Typescript

class Stack {
  private items: number[] = [];
  pop(): number | undefined {
    return this.items.pop();
  }
}
const stack = new Stack();
console.log(stack.pop());

A SyntaxError: Unexpected token

B Type 'number | undefined' is not assignable to type 'number'.

C Runtime error: Cannot read property 'pop' of undefined

D No error, prints 0

💡 Hint

Consider what pop() returns when the array is empty and the method's return type.

The pop() method can return undefined if the array is empty, but the method signature expects a number only, causing a type error.

Practice

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

easy

A. Clears all elements from the stack

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

C. Returns the bottom element without removing it

D. Removes and returns the top element of 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 this top element from the stack.

3. Final Answer:

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

4. Quick Check:

   pop() removes top element = A [OK]

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

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

2. Which of the following is the correct TypeScript syntax to pop an element from a stack array named stack?

easy

A. pop(stack);

B. stack.remove();

C. stack.pop();

D. stack.delete();

Solution

1. Step 1: Recall array pop method in TypeScript

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

2. Step 2: Match correct syntax

   The correct syntax to pop from stack is stack.pop();. Other options are invalid methods.

3. Final Answer:

   stack.pop(); -> Option C

4. Quick Check:

   Array pop method = stack.pop() [OK]

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

Common Mistakes:

• Using non-existent methods like remove() or delete()
• Calling pop as a function with stack as argument
• Confusing pop with other array methods

3. What will be the output after executing the following TypeScript code?

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

medium

A. 30
[10, 20]

B. 20
[10, 20, 30]

C. 30
[10, 20, 30]

D. undefined
[10, 20]

Solution

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

   pop() removes and returns the last element. Here, 30 is removed and returned.

2. Step 2: Check console outputs

   First console.log prints popped value 30. Second prints updated stack [10, 20].

3. Final Answer:

   30 [10, 20] -> Option A

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
• Thinking pop() does not modify the array
• Confusing pop() return value with stack after pop

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

const stack: number[] = [];
const popped = stack.pop();
console.log(popped);

medium

A. pop() returns undefined when stack is empty, no error

B. Runtime error because pop() on empty array is invalid

C. Syntax error due to missing type annotation

D. pop() removes first element instead of last

Solution

1. Step 1: Check pop() behavior on empty array

   pop() returns undefined if the array is empty; it does not throw an error.

2. Step 2: Verify code correctness

   Code runs without error and prints undefined.

3. Final Answer:

   pop() returns undefined when stack is empty, no error -> Option A

4. Quick Check:

   pop() on empty array = undefined [OK]

Hint: pop() returns undefined if stack is empty, no crash [OK]

Common Mistakes:

• Expecting runtime error on empty pop()
• Confusing pop() with shift() behavior
• Thinking pop() removes first element

5. Given a stack implemented as an array stack = [5, 10, 15, 20], write a TypeScript function popTwo that pops two elements and returns them as an array. What will be the returned value and the stack after calling popTwo(stack)?

hard

A. [15, 20], stack = [5, 10]

B. [20, 15], stack = [5, 10]

C. [20, 15], stack = [10, 5]

D. [10, 5], stack = [15, 20]

Solution

1. Step 1: Understand pop() order and stack state

   pop() removes last element first. First pop returns 20, second pop returns 15.

2. Step 2: Determine returned array and stack after two pops

   Returned array is [20, 15]. Stack after pops is [5, 10].

3. Final Answer:

   [20, 15], stack = [5, 10] -> Option B

4. Quick Check:

   Two pops remove last two elements in order [20, 15] [OK]

Hint: Pop twice returns last two elements in reverse order [OK]

Common Mistakes:

• Returning popped elements in wrong order
• Assuming stack order changes after pop
• Mixing up stack contents after pops