Why Stack Exists and What Problems It Solves in DSA Javascript - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack.

DSA Javascript

stack.[1](5);

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

Apush

Bpop

Cpeek

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push removes an element instead of adding.

Using peek only views the top element without adding.

Using shift removes the first element, not related to stack.

The push method adds an element to the top of the stack.

2fill in blank

medium

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

DSA Javascript

const top = stack.[1]();

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

Apush

Bpop

Cpeek

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push adds an element instead of removing.

Using peek only views the top element without removing.

Using unshift adds or removes from the start, not the top.

The pop method removes and returns the top element of the stack.

3fill in blank

hard

Fix the error in the code to check the top element without removing it.

DSA Javascript

const top = stack.[1]();

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

Ashift

Bpush

Cpeek

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop removes the element instead of just viewing.

Using push adds a new element instead of viewing.

Using shift removes the first element, not the top.

The peek method returns the top element without removing it.

4fill in blank

hard

Fill both blanks to create a stack and add an element.

DSA Javascript

const stack = new [1]();
stack.[2](10);

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

AStack

Bpush

Cpop

DQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using Queue instead of Stack changes the data structure.

Using pop instead of push removes an element instead of adding.

We create a new Stack and use push to add an element.

5fill in blank

hard

Fill all three blanks to remove the top element and check the new top.

DSA Javascript

stack.[1]();
const newTop = stack.[2]();
console.log(newTop === [3]);

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

Apush

Bpeek

C5

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop adds an element instead of removing.

Using pop instead of peek to view new top removes again.

Comparing to wrong value causes false result.

We remove the top with pop, then check the new top with peek, and compare it to 5.

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