Enqueue Operation in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the enqueue operation do in a queue?

It adds an element to the end of the queue, following the First-In-First-Out (FIFO) rule.

beginner

In a queue, where is the new element added during an enqueue operation?

The new element is added at the rear (or tail) of the queue.

beginner

Why is enqueue operation important in real-life scenarios like waiting lines?

Because it models how new people join the end of a line, ensuring fairness by serving people in the order they arrive.

intermediate

What happens if you try to enqueue an element into a full queue (fixed size)?

The operation fails or returns an error because there is no space to add a new element.

beginner

Show the result of enqueueing 5 into the queue: 1 -> 2 -> 3 -> null

After enqueueing 5, the queue becomes: 1 -> 2 -> 3 -> 5 -> null

Where does the enqueue operation add an element in a queue?

A It removes an element

B At the front (start) of the queue

C At the rear (end) of the queue

D In the middle of the queue

Enqueue always adds an element at the rear (end) of the queue.

What is the main rule a queue follows?

A Last-In-First-Out (LIFO)

B First-In-First-Out (FIFO)

C Random order

D Priority order

A queue follows FIFO, meaning the first element added is the first to be removed.

If a queue is empty, what happens when you enqueue an element?

A The element is added as the first and only element

B The operation fails

C The element is ignored

D The queue deletes itself

When a queue is empty, enqueue adds the element as the first item.

What is a common problem when enqueueing in a fixed-size queue that is full?

A Overflow

B Sorting error

C Underflow

D No problem

Overflow happens when trying to enqueue into a full fixed-size queue.

After enqueueing 4 into queue: 7 -> 8 -> null, what is the new queue?

A 4 -> 7 -> 8 -> null

B 8 -> 7 -> 4 -> null

C 7 -> 4 -> 8 -> null

D 7 -> 8 -> 4 -> null

Enqueue adds 4 at the rear, so the queue becomes 7 -> 8 -> 4 -> null.

Explain the enqueue operation in a queue and how it affects the order of elements.

Think about how people join a line.

Describe what happens when you enqueue an element into an empty queue.

Start from no elements.

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