Push Operation on Stack in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the push operation do in a stack?

It adds a new element to the top of the stack, increasing its size by one.

beginner

In a stack, where is the new element placed when you perform a push?

The new element is placed on the top of the stack, above all existing elements.

intermediate

What happens if you push an element onto a full stack (fixed size)?

It causes a stack overflow error because there is no space to add the new element.

beginner

Show the stack after pushing elements 5, then 10, then 15 in order.

Stack top -> 15 -> 10 -> 5 -> null (bottom)

intermediate

Why is the push operation considered a constant time operation (O(1))?

Because it only adds one element at the top without needing to move other elements.

What does the push operation do in a stack?

A Adds an element to the top

B Removes the top element

C Checks if the stack is empty

D Returns the bottom element

Push adds a new element on top of the stack.

If you push 3 elements onto an empty stack, how many elements will it have?

A 2

B 3

C 1

D 0

Each push adds one element, so 3 pushes mean 3 elements.

What is the time complexity of the push operation on a stack?

A O(n)

B O(n^2)

C O(log n)

D O(1)

Push is a constant time operation because it only adds one element on top.

Where is the new element placed when you push it onto a stack?

A At the bottom

B In the middle

C At the top

D Random position

Push always adds the element at the top of the stack.

What error occurs if you push onto a full fixed-size stack?

A Stack overflow

B Stack underflow

C Null pointer exception

D Index out of bounds

Pushing onto a full stack causes stack overflow.

Explain the push operation on a stack and what happens to the stack after pushing an element.

Think about adding a new plate on top of a stack of plates.

Describe what happens if you try to push an element onto a stack that is already full.

Imagine trying to add a book to a full shelf with no room.

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