Queue Implementation Using Array in DSA Javascript - Build from Scratch

Queue Implementation Using Array

📖 Scenario: You are helping a small bakery manage the queue of customers waiting to buy fresh bread. The bakery wants to keep track of customers in the order they arrive and serve them one by one.

🎯 Goal: Build a simple queue using an array in JavaScript. You will add customers to the queue, remove them when served, and finally display the current queue.

📋 What You'll Learn

Create an array called queue with initial customers

Create a variable called newCustomer to add to the queue

Add the newCustomer to the end of the queue

Remove the first customer from the queue to simulate serving

Print the final state of the queue

💡 Why This Matters

🌍 Real World

Queues are used in many places like customer service lines, printer job management, and task scheduling where order matters.

💼 Career

Understanding queues helps in software development roles that involve managing tasks, requests, or data streams in order.

1

Create the initial queue array

Create an array called queue with these exact customers in order: 'Alice', 'Bob', 'Charlie'

DSA Javascript

// Create an array called queue with 'Alice', 'Bob', 'Charlie'
// Your code here

Hint

Use square brackets [] to create an array and separate names with commas.

2

Add a new customer to the queue

Create a variable called newCustomer and set it to 'David'

DSA Javascript

const queue = ['Alice', 'Bob', 'Charlie'];
// Create a variable newCustomer and set it to 'David'
// Your code here

Hint

Use const newCustomer = 'David'; to create the variable.

3

Add the new customer to the queue and serve the first customer

Add newCustomer to the end of queue using push(). Then remove the first customer from queue using shift()

DSA Javascript

const queue = ['Alice', 'Bob', 'Charlie'];
const newCustomer = 'David';
// Add newCustomer to queue using push()
// Remove the first customer from queue using shift()
// Your code here

Hint

Use queue.push(newCustomer); to add and queue.shift(); to remove the first customer.

4

Print the final queue

Print the queue array to show the current customers waiting

DSA Javascript

const queue = ['Alice', 'Bob', 'Charlie'];
const newCustomer = 'David';
queue.push(newCustomer);
queue.shift();
// Print the queue array
// Your code here

Hint

Use console.log(queue); to print the array.

Practice

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

easy

A. It stores elements in sorted order

B. It follows Last-In-First-Out (LIFO) order

C. It follows First-In-First-Out (FIFO) order

D. It allows random access to elements

Solution

1. Step 1: Understand queue behavior

   A queue adds elements at the end and removes from the front, so the first element added is the first removed.

2. Step 2: Compare with options

   FIFO means First-In-First-Out, which matches queue behavior. LIFO is for stacks, random access is for arrays, and sorted order is unrelated.

3. Final Answer:

   It follows First-In-First-Out (FIFO) order -> Option C

4. Quick Check:

   Queue = FIFO [OK]

Hint: Queue always removes the oldest element first [OK]

Common Mistakes:

• Confusing queue with stack (LIFO)
• Thinking queue allows random access
• Assuming queue sorts elements automatically

2. Which of the following is the correct syntax to add an element `x` to the end of a queue implemented as an array named queue in JavaScript?

easy

A. queue.push(x);

B. queue.pop(x);

C. queue.shift(x);

D. queue.unshift(x);

Solution

1. Step 1: Identify method to add at end

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

2. Step 2: Check other methods

   pop() removes from end, shift() removes from front, unshift() adds at front.

3. Final Answer:

   queue.push(x); -> Option A

4. Quick Check:

   push adds at end [OK]

Hint: Use push() to add at end of array [OK]

Common Mistakes:

• Using pop() which removes instead of adds
• Using shift() or unshift() incorrectly
• Confusing push() with unshift()

3. What will be the output of the following JavaScript code implementing a queue using an array?

const queue = [];
queue.push(10);
queue.push(20);
const removed = queue.shift();
queue.push(30);
console.log(queue);

medium

A. [10, 20, 30]

B. [20, 30]

C. [30, 20]

D. [10, 30]

Solution

1. Step 1: Trace queue operations

   Initially empty array. After push(10), queue = [10]. After push(20), queue = [10, 20].

2. Step 2: Remove first element with shift()

   shift() removes 10, so queue = [20]. Then push(30), queue = [20, 30].

3. Final Answer:

   [20, 30] -> Option B

4. Quick Check:

   After shift and push, queue = [20, 30] [OK]

Hint: shift() removes front, push() adds end [OK]

Common Mistakes:

• Thinking shift() removes from end
• Confusing order of elements after operations
• Assuming push adds at front

4. Identify the error in the following queue implementation using an array and suggest the fix:

const queue = [];
queue.push(1);
queue.push(2);
const removed = queue.pop();
console.log(queue);

medium

A. pop() removes from end; use shift() to remove from front

B. push() should be replaced with unshift()

C. pop() should be replaced with push()

D. No error; code works correctly

Solution

1. Step 1: Understand queue removal operation

   Queue removes elements from the front, but pop() removes from the end.

2. Step 2: Correct removal method

   Use shift() to remove the first element to maintain FIFO order.

3. Final Answer:

   pop() removes from end; use shift() to remove from front -> Option A

4. Quick Check:

   Queue removal = shift() [OK]

Hint: Use shift() to remove front element in queue [OK]

Common Mistakes:

• Using pop() which removes last element
• Confusing push() and unshift() roles
• Thinking pop() removes front element

5. Given a queue implemented as an array, how can you efficiently check if the queue is empty before removing an element to avoid errors?

medium

A. Check if queue.length > 1 before calling shift()

B. Check if queue[0] === undefined before calling pop()

C. Call shift() directly without checking

D. Check if queue.length === 0 before calling shift()

Solution

1. Step 1: Understand empty queue condition

   An empty queue has length 0, so no elements to remove.

2. Step 2: Safe removal check

   Check queue.length === 0 before calling shift() to avoid removing from empty array.

3. Final Answer:

   Check if queue.length === 0 before calling shift() -> Option D

4. Quick Check:

   Empty queue check = length === 0 [OK]

Hint: Always check length === 0 before removing [OK]

Common Mistakes:

• Checking wrong index for emptiness
• Calling shift() without checking length
• Checking length > 1 instead of 0