Enqueue Operation in DSA Javascript - Build from Scratch

Implementing Enqueue Operation in a Queue

📖 Scenario: You are managing a line of customers waiting to buy tickets at a counter. Each new customer joins the end of the line. This line is called a queue.

🎯 Goal: Build a simple queue using an array and add new customers to the end using the enqueue operation.

📋 What You'll Learn

Create an array called queue with initial customers

Create a variable called newCustomer to hold the name of the customer to add

Write code to add newCustomer to the end of queue using the enqueue operation

Print the final state of queue after adding the new customer

💡 Why This Matters

🌍 Real World

Queues are used in real life to manage lines, like customers waiting for service or tasks waiting to be processed.

💼 Career

Understanding queues and enqueue operations is important for software developers working on scheduling, task management, and real-time systems.

1

Create the initial queue

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

DSA Javascript

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

Hint

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

2

Add a new customer name

Create a variable called newCustomer and set it to the string '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

Enqueue the new customer

Use the push method on queue to add newCustomer to the end of the array

DSA Javascript

const queue = ['Alice', 'Bob', 'Charlie'];
const newCustomer = 'David';
// Add newCustomer to the end of queue using push
// Your code here

Hint

Use queue.push(newCustomer); to add the new customer at the end.

4

Print the final queue

Print the queue array to show the final order of customers after adding newCustomer

DSA Javascript

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

Hint

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

Practice

1. What does the enqueue operation do in a queue data structure?

easy

A. Adds an element to the end of the queue

B. Removes an element from the front of the queue

C. Checks if the queue is empty

D. Returns the size of the queue

Solution

1. Step 1: Understand the queue behavior

   A queue works like a line where new people join at the end.

2. Step 2: Define enqueue operation

   Enqueue means adding a new element at the end of the queue.

3. Final Answer:

   Adds an element to the end of the queue -> Option A

4. Quick Check:

   Enqueue = Add at end [OK]

Hint: Enqueue always adds at the back of the queue [OK]

Common Mistakes:

• Confusing enqueue with dequeue
• Thinking enqueue removes elements
• Mixing enqueue with checking size

2. Which of the following is the correct syntax to add an element 5 to a queue implemented as an array named queue in JavaScript?

easy

A. queue.enqueue(5);

B. queue.add(5);

C. queue.push(5);

D. queue.insert(5);

Solution

1. Step 1: Identify JavaScript array method for adding at end

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

2. Step 2: Match method to enqueue operation

   Enqueue adds to the end, so queue.push(5); is correct syntax.

3. Final Answer:

   queue.push(5); -> Option C

4. Quick Check:

   Array push adds at end [OK]

Hint: Use push() to enqueue in array-based queue [OK]

Common Mistakes:

• Using non-existent methods like add() or insert()
• Confusing enqueue with dequeue methods
• Trying to use enqueue() directly on array

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

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

medium

A. [10, 20, 30]

B. [30, 20, 10]

C. [10, 30, 20]

D. Error

Solution

1. Step 1: Trace each enqueue operation

   First push adds 10: queue = [10]. Second push adds 20: queue = [10, 20]. Third push adds 30: queue = [10, 20, 30].

2. Step 2: Print the queue array

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

3. Final Answer:

   [10, 20, 30] -> Option A

4. Quick Check:

   push adds at end, so order is [10, 20, 30] [OK]

Hint: push() adds at end, so order stays as inserted [OK]

Common Mistakes:

• Assuming push adds at front
• Confusing output order
• Expecting error due to empty initialization

4. Identify the error in the following enqueue implementation for a queue using an array:

class Queue {
  constructor() {
    this.items = [];
  }
  enqueue(element) {
    this.items.unshift(element);
  }
}

medium

A. enqueue method should remove element, not add

B. Using unshift adds element at the front, not the end

C. Array should be initialized as null, not []

D. Missing return statement in enqueue method

Solution

1. Step 1: Understand unshift behavior

   unshift adds element at the start (front) of the array, not the end.

2. Step 2: Compare with enqueue operation

   Enqueue must add element at the end, so unshift is incorrect here.

3. Final Answer:

   Using unshift adds element at the front, not the end -> Option B

4. Quick Check:

   Enqueue adds at end, unshift adds at front [OK]

Hint: Use push() for enqueue, not unshift() [OK]

Common Mistakes:

• Confusing unshift with push
• Thinking unshift adds at end
• Ignoring method behavior differences

5. You have a queue implemented with an array. You want to enqueue multiple elements [7, 8, 9] at once while preserving their order. Which code snippet correctly enqueues all elements in order?

hard

A. queue.unshift(7, 8, 9);

B. queue.concat([7, 8, 9]);

C. queue.push([7, 8, 9]);

D. [7, 8, 9].forEach(e => queue.push(e));

Solution

1. Step 1: Analyze each option for enqueue behavior

   queue.push([7, 8, 9]); adds the entire array as one element, not individual elements. queue.concat([7, 8, 9]); creates a new array but does not modify queue. [7, 8, 9].forEach(e => queue.push(e)); pushes each element individually, preserving order. queue.unshift(7, 8, 9); adds elements at front, reversing order.

2. Step 2: Choose the method that enqueues elements individually at the end

   Using forEach with push adds each element in order at the end of queue.

3. Final Answer:

   [7, 8, 9].forEach(e => queue.push(e)); -> Option D

4. Quick Check:

   forEach + push enqueues elements in order [OK]

Hint: Use forEach with push to enqueue multiple elements in order [OK]

Common Mistakes:

• Using push with array adds nested array
• Using unshift reverses order
• Forgetting to assign concat result back to queue