Why Stack Exists and What Problems It Solves in DSA Javascript - See It Work

Why Stack Exists and What Problems It Solves

📖 Scenario: Imagine you are organizing a stack of plates in your kitchen. You can only add or remove the top plate. This is how a stack data structure works in programming.Stacks help solve problems where the last thing added is the first thing you need to remove, like undo actions in apps or checking if brackets in code are balanced.

🎯 Goal: You will create a simple stack using a JavaScript array, add items to it, remove items, and see how the stack behaves.

📋 What You'll Learn

Create a stack as an empty array called stack

Add three items to the stack in order: 'plate1', 'plate2', 'plate3'

Remove the top item from the stack and store it in a variable called removedItem

Print the current state of the stack and the removed item

💡 Why This Matters

🌍 Real World

Stacks are used in web browsers to manage the history of pages you visit, allowing you to go back to the previous page.

💼 Career

Understanding stacks helps in debugging, writing compilers, and solving problems that require reversing or undoing actions.

1

Create an empty stack

Create an empty array called stack to represent the stack.

DSA Javascript

// Your code here

Hint

Use [] to create an empty array and assign it to stack.

2

Add three plates to the stack

Add 'plate1', 'plate2', and 'plate3' to the stack using the push method in that order.

DSA Javascript

const stack = [];
// Your code here

Hint

Use stack.push(item) to add items to the top of the stack.

3

Remove the top plate from the stack

Remove the top item from the stack using the pop method and store it in a variable called removedItem.

DSA Javascript

const stack = [];
stack.push('plate1');
stack.push('plate2');
stack.push('plate3');
// Your code here

Hint

Use stack.pop() to remove the top item from the stack and assign it to removedItem.

4

Print the stack and removed item

Print the current stack and the removedItem using console.log.

DSA Javascript

const stack = [];
stack.push('plate1');
stack.push('plate2');
stack.push('plate3');
const removedItem = stack.pop();
// Your code here

Hint

Use console.log(stack) and console.log(removedItem) to show the stack and the removed plate.

Practice

1. Why do we use a stack data structure in programming?

easy

A. To store data randomly

B. To store data in a first-in, first-out (FIFO) order

C. To manage data in a last-in, first-out (LIFO) order

D. To sort data automatically

Solution

1. Step 1: Understand stack behavior

   A stack stores data so the last item added is the first one removed, called LIFO.

2. Step 2: Compare with other data structures

   Unlike queues (FIFO), stacks focus on reversing order or tracking recent items.

3. Final Answer:

   To manage data in a last-in, first-out (LIFO) order -> Option C

4. Quick Check:

   Stack = LIFO order [OK]

Hint: Stack means last added is first removed [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking stack sorts data automatically
• Assuming stack stores data randomly

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

easy

A. myStack.add(5);

B. myStack.push(5);

C. myStack.insert(5);

D. myStack.enqueue(5);

Solution

1. Step 1: Recall JavaScript stack method

   JavaScript arrays use push() to add elements at the end, acting as stack push.

2. Step 2: Check other options

   add(), insert(), and enqueue() are not standard array methods for stack push.

3. Final Answer:

   myStack.push(5); -> Option B

4. Quick Check:

   Push method adds element to stack [OK]

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

Common Mistakes:

• Using enqueue which is for queues
• Using add or insert which are not JS array methods
• Confusing push with pop

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. 20
[20, 10]

D. 10
[10]

Solution

1. Step 1: Trace push operations

   First, 10 is pushed, stack = [10]. Then 20 is pushed, stack = [10, 20].

2. Step 2: Trace pop operation

   Pop removes last element 20, so popped = 20, stack = [10].

3. Final Answer:

   20 [10] -> Option A

4. Quick Check:

   Pop returns last pushed element [OK]

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

Common Mistakes:

• Thinking pop removes first element
• Confusing push and pop order
• Expecting stack to be empty after pop

4. Identify the error in the following code that tries to implement a stack push operation:

let stack = [];
function pushToStack(element) {
  stack.add(element);
}

medium

A. Function name should be pushStack instead of pushToStack

B. Stack should be declared with const instead of let

C. Missing return statement in function

D. Using add() method which does not exist on arrays

Solution

1. Step 1: Check array methods for adding elements

   JavaScript arrays use push(), not add(), to add elements.

2. Step 2: Verify other options

   Using let is fine; return is not needed for push; function name is valid.

3. Final Answer:

   Using add() method which does not exist on arrays -> Option D

4. Quick Check:

   Use push() to add elements to array [OK]

Hint: Arrays use push(), not add() [OK]

Common Mistakes:

• Using add() instead of push()
• Thinking return is needed for push
• Confusing variable declaration keywords

5. You want to reverse a string using a stack in JavaScript. Which approach correctly uses a stack to reverse the string "hello"?

hard

A. Push each character onto stack, then pop all to build reversed string

B. Push characters in reverse order, then pop to get original string

C. Use stack to sort characters alphabetically

D. Push only vowels onto stack, then pop to reverse string

Solution

1. Step 1: Understand stack reversal logic

   Pushing characters in original order and popping reverses their order, reversing the string.

2. Step 2: Evaluate other options

   Push characters in reverse order, then pop to get original string reverses twice, giving original string; A sorts, not reverses; D only reverses vowels, not full string.

3. Final Answer:

   Push each character onto stack, then pop all to build reversed string -> Option A

4. Quick Check:

   Stack reverses order by LIFO [OK]

Hint: Push original order, pop to reverse [OK]

Common Mistakes:

• Pushing characters in reverse order first
• Using stack to sort instead of reverse
• Only pushing vowels instead of all characters