Stack Concept and LIFO Principle in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the content of the stack from top to bottom after this operation sequence?

Consider a stack initially empty. We perform these operations in order:
push(5), push(10), pop(), push(7), pop(), push(3), push(8), pop()
What is the content of the stack from top to bottom after these operations?

DSA Javascript

const stack = [];
stack.push(5);
stack.push(10);
stack.pop();
stack.push(7);
stack.pop();
stack.push(3);
stack.push(8);
stack.pop();
console.log(stack);

A [3, 5]

B [8, 3, 5]

C [5, 7, 3]

D [5, 3]

💡 Hint

Remember that pop removes the top element and push adds on top.

After the operations, the stack has 3 on top and 5 below it, so the array is [5, 3] but since top is last element, console.log shows [5,3].

🧠 Conceptual

intermediate

Which statement best describes the LIFO principle in stacks?

Choose the option that correctly explains the Last In First Out (LIFO) principle used in stacks.

A The last element added is the last one to be removed.

B Elements are removed in the reverse order they were added.

C Elements are removed randomly regardless of order.

D The first element added is the first one to be removed.

💡 Hint

Think about how a stack of plates works.

LIFO means the last element added is the first to be removed, so elements come out in reverse order of insertion.

🔧 Debug

advanced

What error does this stack code produce?

Look at this JavaScript stack code snippet and identify the error it causes when run.

DSA Javascript

const stack = [];
stack.push(1);
stack.push(2);
stack.pop();
stack.pop();
stack.pop(); // Extra pop
console.log(stack);

A undefined (no error, empty stack after pops)

B RangeError: Stack underflow

C TypeError: Cannot pop from empty stack

D ReferenceError: stack is not defined

💡 Hint

What happens if you pop from an empty array in JavaScript?

In JavaScript, popping from an empty array returns undefined and does not throw an error.

🚀 Application

advanced

How many items remain in the stack after this sequence?

A stack starts empty. We perform:
push(4), push(9), push(2), pop(), push(7), pop(), pop(), push(5)
How many items are in the stack now?

DSA Javascript

const stack = [];
stack.push(4);
stack.push(9);
stack.push(2);
stack.pop();
stack.push(7);
stack.pop();
stack.pop();
stack.push(5);
console.log(stack.length);

A 2

B 3

C 1

D 4

💡 Hint

Count pushes and pops carefully.

After all operations, the stack has two items: 4 and 5.

📝 Syntax

expert

Which option correctly implements a stack push method in JavaScript?

Given a Stack class with an array named 'items', which push method is correct?

DSA Javascript

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

  push(element) {
    // Which option is correct here?
  }
}

A this.items.append(element);

B this.items.add(element);

C this.items.push(element);

D this.items.insert(element);

💡 Hint

Check JavaScript array methods for adding elements at the end.

JavaScript arrays use push() to add elements at the end. add(), insert(), append() are not valid methods.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Elements are removed based on their value

B. First In, First Out - the first element added is the first to be removed

C. Elements are removed randomly

D. Last In, First Out - the last element added is the first to be removed

Solution

1. Step 1: Understand the LIFO principle

   LIFO means the last element you put in is the first one you take out.

2. Step 2: Apply to stack behavior

   Stacks follow LIFO, so the newest item is removed first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option D

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack like a stack of plates, last added removed first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking elements are removed randomly
• Assuming removal depends on element value

2. Which of the following is the correct way to push an element '5' onto a stack named stack in JavaScript?

easy

A. stack.insert(5);

B. stack.add(5);

C. stack.push(5);

D. stack.enqueue(5);

Solution

1. Step 1: Recall JavaScript stack operations

   JavaScript arrays use push() to add elements at the end (top of stack).

2. Step 2: Match method to stack push

   push() adds element to stack top, so stack.push(5); is correct.

3. Final Answer:

   stack.push(5); -> Option C

4. Quick Check:

   Push method adds element to stack [OK]

Hint: Use push() to add elements to stack in JavaScript [OK]

Common Mistakes:

• Using add() or insert() which are not array methods
• Confusing enqueue() with queue operation
• Trying to use non-existent methods

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

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

medium

A. 20
[10]

B. 10
[20]

C. 10
[10, 20]

D. 20
[20]

Solution

1. Step 1: Trace push operations

   stack.push(10) adds 10, stack.push(20) adds 20. Stack is now [10, 20].

2. Step 2: Trace pop operation

   stack.pop() removes last element 20 and returns it. Stack becomes [10].

3. Final Answer:

   20 [10] -> Option A

4. Quick Check:

   Pop returns last pushed element 20 [OK]

Hint: Pop returns last pushed element, stack shrinks by one [OK]

Common Mistakes:

• Thinking pop returns first element
• Confusing push and pop order
• Expecting stack unchanged after pop

4. Identify the error in the following code snippet that tries to implement a stack using an array:

const stack = [];
stack.push(1);
stack.push(2);
const top = stack.pop;
console.log(top);

medium

A. pop is used without parentheses, so it returns the function, not the popped value

B. push method is incorrect, should be pushElement

C. stack should be initialized as an object, not an array

D. console.log cannot print stack elements

Solution

1. Step 1: Check pop usage

   stack.pop is a function reference; missing () means no call, so top is function, not value.

2. Step 2: Understand effect on output

   console.log(top) prints function code, not popped element. Correct usage is stack.pop().

3. Final Answer:

   pop is used without parentheses, so it returns the function, not the popped value -> Option A

4. Quick Check:

   Call pop() with () to get value [OK]

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

Common Mistakes:

• Forgetting parentheses on pop()
• Thinking push method is wrong
• Confusing array with object for stack

5. You have a stack initially empty. You perform these operations in order:

push(1), push(2), pop(), push(3), pop(), pop()

What is the final output of each pop operation in sequence?

hard

A. [2, 3, 3]

B. [2, 3, 1]

C. [2, 3, undefined]

D. [1, 3, 2]

Solution

1. Step 1: Perform push(1) and push(2)

   Stack becomes [1, 2].

2. Step 2: pop() removes last pushed element

   pop() returns 2, stack is now [1].

3. Step 3: push(3)

   Stack becomes [1, 3].

4. Step 4: pop() returns 3, stack is [1]

5. Step 5: pop() returns 1, stack is empty

6. Final Answer:

   [2, 3, 1] -> Option B

7. Quick Check:

   Pop returns last pushed elements in reverse order [OK]

Hint: Track stack after each operation to find pop outputs [OK]

Common Mistakes:

• Mixing order of popped elements
• Assuming pop returns undefined when stack not empty
• Confusing push and pop effects