Stack Implementation Using Array in DSA Javascript - Build from Scratch

Stack Implementation Using Array

📖 Scenario: Imagine you are managing a stack of books. You can only add or remove the top book. This is like a stack data structure in programming.

🎯 Goal: You will build a simple stack using an array in JavaScript. You will add items, remove the top item, and then show the stack.

📋 What You'll Learn

Create an empty array called stack to hold the stack items.

Create a variable called maxSize and set it to 5 to limit the stack size.

Write code to add three items: 'Book1', 'Book2', and 'Book3' to the stack using push only if the stack size is less than maxSize.

Write code to remove the top item from the stack using pop.

Print the final state of the stack.

💡 Why This Matters

🌍 Real World

Stacks are used in undo features, browser history, and managing tasks where last added is first removed.

💼 Career

Understanding stack implementation helps in coding interviews and building efficient software that manages data in order.

1

Create an empty stack array

Create an empty array called stack to hold the stack items.

DSA Javascript

# Your code here

Hint

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

2

Set the maximum stack size

Create a variable called maxSize and set it to 5 to limit the stack size.

DSA Javascript

const stack = [];
# Your code here

Hint

Use const maxSize = 5; to set the limit.

3

Add three books to the stack

Use if statements to check if stack.length is less than maxSize. If yes, add 'Book1', 'Book2', and 'Book3' to the stack using stack.push().

DSA Javascript

const stack = [];
const maxSize = 5;
# Your code here

Hint

Use stack.push('Book1') inside an if that checks stack.length < maxSize.

4

Remove the top book and print the stack

Remove the top item from the stack using stack.pop(). Then print the final stack using console.log(stack).

DSA Javascript

const stack = [];
const maxSize = 5;
if (stack.length < maxSize) {
    stack.push('Book1');
}
if (stack.length < maxSize) {
    stack.push('Book2');
}
if (stack.length < maxSize) {
    stack.push('Book3');
}
# Your code here

Hint

Use stack.pop() to remove the last item and console.log(stack) to show the stack.

Practice

1. What is the main characteristic of a stack data structure implemented using an array?

easy

A. Sorted Order

B. Last In, First Out (LIFO)

C. Random Access

D. First In, First Out (FIFO)

Solution

1. Step 1: Understand stack behavior

   A stack adds and removes elements from the same end, so the last element added is the first to be removed.

2. Step 2: Identify the correct term

   This behavior is called Last In, First Out (LIFO).

3. Final Answer:

   Last In, First Out (LIFO) -> Option B

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack always removes the last added item first [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking stack allows random access
• Assuming stack keeps elements sorted

2. Which of the following is the correct way to add an element to a stack implemented as an array in JavaScript?

easy

A. stack.push(element);

B. stack.pop(element);

C. stack.shift(element);

D. stack.unshift(element);

Solution

1. Step 1: Identify method to add element at the end

   In JavaScript arrays, push() adds an element to the end of the array.

2. Step 2: Confirm stack push operation

   Stack push means adding element on top, which corresponds to array's end, so push() is correct.

3. Final Answer:

   stack.push(element); -> Option A

4. Quick Check:

   Use push() to add element to stack [OK]

Hint: Use push() to add on top of stack [OK]

Common Mistakes:

• Using pop() which removes element
• Using shift() or unshift() which affect start of array

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

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

medium

A. [10, 20, 30]

B. [30]

C. [20, 30]

D. [10, 30]

Solution

1. Step 1: Trace push and pop operations

   Initially stack = [] push(10) -> [10] push(20) -> [10, 20] pop() removes last element 20 -> [10] push(30) -> [10, 30]

2. Step 2: Check final stack state

   After all operations, stack contains [10, 30].

3. Final Answer:

   [10, 30] -> Option D

4. Quick Check:

   Push and pop update stack correctly [OK]

Hint: Pop removes last pushed, push adds at end [OK]

Common Mistakes:

• Assuming pop removes first element
• Confusing order of elements after pop

4. Identify the error in the following stack implementation code and select the correct fix:

class Stack {
  constructor() {
    this.items = [];
  }
  push(element) {
    this.items.unshift(element);
  }
  pop() {
    return this.items.pop();
  }
}

medium

A. Use push() instead of unshift() in push method

B. Use shift() instead of pop() in pop method

C. No error, code is correct

D. Use pop() instead of push() in push method

Solution

1. Step 1: Analyze push method

   push uses unshift which adds element at start of array.

2. Step 2: Analyze pop method

   pop removes element from end of array. This causes mismatch because push adds at start but pop removes from end.

3. Step 3: Fix mismatch

   To maintain stack behavior, push should add at end using push(), so pop removes last added element correctly.

4. Final Answer:

   Use push() instead of unshift() in push method -> Option A

5. Quick Check:

   push() and pop() must match ends [OK]

Hint: Push and pop must operate on same array end [OK]

Common Mistakes:

• Mixing unshift with pop causes wrong stack order
• Thinking shift() and pop() are interchangeable
• Ignoring array end consistency

5. You want to implement a stack using an array that also tracks the maximum element at any time efficiently. Which approach is best?

hard

A. Scan entire array to find max after each pop

B. Sort the array after each push to find max

C. Use two stacks: one for elements, one for max values

D. Store max element only in a variable updated on push

Solution

1. Step 1: Understand max tracking challenge

   Finding max after each operation must be efficient, not scanning entire array.

2. Step 2: Evaluate options

   Sorting after each push is costly. Scanning array after pop is slow. Storing max in variable fails if max is popped.

3. Step 3: Use two stacks approach

   One stack stores elements, second stack stores current max at each push. This allows O(1) max retrieval.

4. Final Answer:

   Use two stacks: one for elements, one for max values -> Option C

5. Quick Check:

   Two stacks track max efficiently [OK]

Hint: Use auxiliary stack to track max values [OK]

Common Mistakes:

• Sorting array each time is inefficient
• Single variable max fails when max is popped
• Scanning array after pop is slow