Push Operation on Stack in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after pushing elements on a stack?

Consider the following JavaScript code that pushes elements onto a stack implemented as an array. What will be the printed stack after all push operations?

DSA Javascript

const stack = [];
stack.push(5);
stack.push(10);
stack.push(15);
console.log(stack.join(' -> ') + ' -> null');

A 5 -> 10 -> 15 -> null

B 10 -> 15 -> 5 -> null

C null -> 5 -> 10 -> 15

D 15 -> 10 -> 5 -> null

💡 Hint

Remember that push adds elements to the end of the array, so the first pushed element is at the start of the array.

In JavaScript, arrays used as stacks add elements at the end with push. So the stack array after pushing 5, 10, then 15 is [5, 10, 15]. Printing with join shows elements in order pushed.

❓ Predict Output

intermediate

What is the stack content after pushing and popping?

Given the following code, what will be the printed stack after the operations?

DSA Javascript

const stack = [];
stack.push(1);
stack.push(2);
stack.pop();
stack.push(3);
console.log(stack.join(' -> ') + ' -> null');

A 2 -> 3 -> null

B 1 -> 2 -> 3 -> null

C 1 -> 3 -> null

D 3 -> 1 -> null

💡 Hint

Pop removes the last pushed element before the next push.

Push 1 and 2 makes stack [1,2]. Pop removes 2, stack is [1]. Push 3 adds 3, stack is [1,3].

🧠 Conceptual

advanced

How does push operation affect stack size?

If a stack currently has 4 elements, what will be its size after pushing 3 more elements?

A 7

B 4

C 3

D 1

💡 Hint

Push adds one element each time.

Each push adds one element, so 4 + 3 = 7 elements after pushing 3 more.

❓ Predict Output

advanced

What error occurs when pushing on a non-array stack?

What error will this code produce?

DSA Javascript

const stack = null;
stack.push(1);

A No error, stack becomes [1]

B ReferenceError: stack is not defined

C SyntaxError: Unexpected token

D TypeError: Cannot read property 'push' of null

💡 Hint

You cannot call methods on null.

stack is null, so calling push on it causes a TypeError.

🚀 Application

expert

Trace the stack content after multiple push and pop operations

Given the following code, what is the final printed stack?

DSA Javascript

const stack = [];
stack.push('a');
stack.push('b');
stack.push('c');
stack.pop();
stack.push('d');
stack.pop();
stack.push('e');
console.log(stack.join(' -> ') + ' -> null');

A a -> b -> c -> e -> null

B a -> b -> e -> null

C a -> b -> d -> e -> null

D a -> e -> null

💡 Hint

Track each push and pop step by step.

Push a,b,c: stack=[a,b,c]. Pop removes c: [a,b]. Push d: [a,b,d]. Pop removes d: [a,b]. Push e: [a,b,e].

Practice

1. What does the push operation do in a stack data structure?

easy

A. Checks if the stack is empty

B. Removes the top element from the stack

C. Returns the bottom element of the stack

D. Adds an element to the top of the stack

Solution

1. Step 1: Understand stack operations

   A stack follows Last In First Out (LIFO). The push operation adds an element on top.

2. Step 2: Identify the role of push

   It inserts a new element at the top, increasing stack size by one.

3. Final Answer:

   Adds an element to the top of the stack -> Option D

4. Quick Check:

   Push = Add top element [OK]

Hint: Push means add on top of stack [OK]

Common Mistakes:

• Confusing push with pop operation
• Thinking push removes elements
• Mixing stack with queue operations

2. Which of the following is the correct syntax to push an element 5 into a stack array named stack in JavaScript?

easy

A. stack.add(5);

B. stack.push(5);

C. push.stack(5);

D. stack.insert(5);

Solution

1. Step 1: Recall JavaScript array push syntax

   JavaScript arrays use push() method to add elements at the end.

2. Step 2: Match correct method call

   stack.push(5); correctly calls push on the array named stack.

3. Final Answer:

   stack.push(5); -> Option B

4. Quick Check:

   Array push method = stack.push(value) [OK]

Hint: Use arrayName.push(value) to add element [OK]

Common Mistakes:

• Using non-existent methods like add or insert
• Reversing method and object like push.stack()
• Forgetting parentheses after push

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

const stack = [];
stack.push(10);
stack.push(20);
stack.push(30);
console.log(stack);

medium

A. [10, 20]

B. [30, 20, 10]

C. [10, 20, 30]

D. Error: push is not a function

Solution

1. Step 1: Trace push operations on empty stack

   Initially stack is empty []. After push(10), stack = [10]. After push(20), stack = [10, 20]. After push(30), stack = [10, 20, 30].

2. Step 2: Understand console.log output

   Logging stack prints the array as is: [10, 20, 30].

3. Final Answer:

   [10, 20, 30] -> Option C

4. Quick Check:

   Push adds at end, so stack = [10,20,30] [OK]

Hint: Push adds at end, array grows left to right [OK]

Common Mistakes:

• Assuming push adds at start (top as front)
• Confusing stack order with queue
• Expecting reversed array output

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

let stack = 5;
stack.push(10);

medium

A. stack is not an array, so push method is undefined

B. push method requires two arguments

C. stack.push should be stack.add

D. No error, code runs fine

Solution

1. Step 1: Check variable type of stack

   stack is assigned number 5, not an array. Numbers do not have push method.

2. Step 2: Understand push method applicability

   push is an array method. Calling push on a number causes error: push is not a function.

3. Final Answer:

   stack is not an array, so push method is undefined -> Option A

4. Quick Check:

   push only works on arrays [OK]

Hint: Only arrays have push method [OK]

Common Mistakes:

• Assuming push works on numbers
• Thinking push needs multiple arguments
• Confusing push with add method

5. Given a stack implemented as an array, how would you push multiple elements [1, 2, 3] onto it so that 3 is on top after all pushes?
Choose the correct code snippet.

hard

A. stack.push(1); stack.push(2); stack.push(3);

B. stack.push(3); stack.push(2); stack.push(1);

C. stack.push([1, 2, 3]);

D. stack = [1, 2, 3];

Solution

1. Step 1: Understand push order for stack

   Push adds elements on top. To have 3 on top, push 1 first, then 2, then 3.

2. Step 2: Analyze options

   Sequential pushes of 1 then 2 then 3 add elements individually with 3 on top. Pushing [1,2,3] adds a single array element with [1,2,3] on top. Reverse pushes put 1 on top. Assignment replaces the stack entirely.

3. Final Answer:

   stack.push(1); stack.push(2); stack.push(3); -> Option A

4. Quick Check:

   Push in order to have last element on top [OK]

Hint: Push elements in order, last pushed is top [OK]

Common Mistakes:

• Pushing array [1,2,3] as single element
• Pushing in reverse order
• Replacing stack instead of pushing