Queue Concept and FIFO Principle in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this queue operation sequence?

Consider a queue where we enqueue numbers 1, 2, 3 in order, then dequeue twice, then enqueue 4. What is the state of the queue?

DSA Javascript

const queue = [];
queue.push(1);
queue.push(2);
queue.push(3);
queue.shift();
queue.shift();
queue.push(4);
console.log(queue.join(' -> ') + ' -> null');

A 3 -> 4 -> null

B 1 -> 2 -> 3 -> 4 -> null

C 2 -> 3 -> 4 -> null

D 4 -> null

💡 Hint

Remember, shift() removes the first element (front) of the queue.

After enqueueing 1,2,3 the queue is 1 -> 2 -> 3 -> null.
Dequeuing twice removes 1 and 2.
Queue becomes 3 -> null.
Enqueue 4 adds it to the end.
Final queue: 3 -> 4 -> null.

🧠 Conceptual

intermediate

Which statement best describes the FIFO principle in queues?

Choose the statement that correctly explains the FIFO (First In First Out) principle in queues.

A Elements are removed randomly regardless of insertion order.

B The last element added is the first one to be removed.

C The first element added is the first one to be removed.

D Elements are removed based on their value, not order.

💡 Hint

Think about a line of people waiting for service.

FIFO means the first element that enters the queue is the first to leave, like people lining up and being served in order.

🔧 Debug

advanced

What error occurs in this queue implementation?

This code tries to dequeue from an empty queue. What error will it cause?

DSA Javascript

const queue = [];
queue.shift();
console.log(queue.join(' -> ') + ' -> null');

A RangeError: Maximum call stack size exceeded

B TypeError: Cannot read property 'join' of undefined

C ReferenceError: queue is not defined

D undefined is returned, no error

💡 Hint

What does shift() return when the array is empty?

Calling shift() on an empty array returns undefined but does not throw an error.
The queue remains empty, so join works fine and prints ' -> null'.

❓ Predict Output

advanced

What is the output after these queue operations?

A queue starts empty. We enqueue 5, then enqueue 10, dequeue once, enqueue 15, dequeue once, then enqueue 20. What is the queue state?

DSA Javascript

const queue = [];
queue.push(5);
queue.push(10);
queue.shift();
queue.push(15);
queue.shift();
queue.push(20);
console.log(queue.join(' -> ') + ' -> null');

A 10 -> 15 -> 20 -> null

B 15 -> 20 -> null

C 5 -> 10 -> 15 -> 20 -> null

D 20 -> null

💡 Hint

Track each enqueue and dequeue step carefully.

Enqueue 5,10: queue is 5 -> 10
Dequeue once removes 5: queue is 10
Enqueue 15: queue is 10 -> 15
Dequeue once removes 10: queue is 15
Enqueue 20: queue is 15 -> 20

🚀 Application

expert

How many items remain in the queue after this sequence?

Starting with an empty queue, perform these operations: enqueue 1, enqueue 2, dequeue, enqueue 3, dequeue, dequeue, enqueue 4. How many items are in the queue now?

DSA Javascript

const queue = [];
queue.push(1);
queue.push(2);
queue.shift();
queue.push(3);
queue.shift();
queue.shift();
queue.push(4);
console.log(queue.length);

A 1

B 2

C 3

D 0

💡 Hint

Count carefully how many items are removed and added.

Operations:
enqueue 1,2 → queue: 1,2
dequeue removes 1 → queue: 2
enqueue 3 → queue: 2,3
dequeue removes 2 → queue: 3
dequeue removes 3 → queue empty
enqueue 4 → queue: 4
Length is 1.

Practice

1. What does FIFO stand for in the context of a queue?

easy

A. First In, First Out

B. First In, Final Out

C. Fast Input, Fast Output

D. First Input, First Output

Solution

1. Step 1: Understand FIFO meaning

   FIFO means the first element added is the first one to be removed.

2. Step 2: Match the acronym

   First In, First Out exactly matches FIFO.

3. Final Answer:

   First In, First Out -> Option A

4. Quick Check:

   FIFO = First In, First Out [OK]

Hint: Remember queue removes in order added, first comes out first [OK]

Common Mistakes:

• Confusing FIFO with LIFO (stack)
• Thinking FIFO means last in first out
• Mixing up acronym words

2. Which of the following is the correct way to add an element 5 to a queue named q in JavaScript using an array?

easy

A. q.unshift(5)

B. q.pop(5)

C. q.shift(5)

D. q.push(5)

Solution

1. Step 1: Understand queue insertion

   In a queue implemented with an array, new elements are added at the end.

2. Step 2: Identify correct method

   Array method push adds element at the end, so q.push(5) is correct.

3. Final Answer:

   q.push(5) -> Option D

4. Quick Check:

   Use push to add at queue end [OK]

Hint: Use push to add, shift to remove in JS queue [OK]

Common Mistakes:

• Using pop to add (pop removes last)
• Using shift to add (shift removes first)
• Using unshift adds at front, not end

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

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

medium

A. [30, 20]

B. [10, 20, 30]

C. [20, 30]

D. [10, 30]

Solution

1. Step 1: Add elements to queue

   Elements 10, 20, 30 are added in order using push, so queue = [10, 20, 30].

2. Step 2: Remove first element

   Using shift removes the first element (10), so queue becomes [20, 30].

3. Final Answer:

   [20, 30] -> Option C

4. Quick Check:

   Shift removes first, queue left with [20, 30] [OK]

Hint: Shift removes first element, push adds at end [OK]

Common Mistakes:

• Thinking pop removes first element
• Confusing order of elements after shift
• Expecting original array unchanged

4. The following code is intended to remove the first element from a queue implemented as an array. What is the error?

const queue = [1, 2, 3];
queue.pop();
console.log(queue);

medium

A. pop() removes the last element, not the first

B. pop() is not a valid array method

C. pop() adds an element instead of removing

D. pop() removes all elements

Solution

1. Step 1: Understand pop() behavior

   pop() removes the last element from an array, not the first.

2. Step 2: Identify correct method for first element removal

   To remove the first element in a queue, shift() should be used instead of pop().

3. Final Answer:

   pop() removes the last element, not the first -> Option A

4. Quick Check:

   pop removes last, shift removes first [OK]

Hint: Use shift() to remove first, pop() removes last [OK]

Common Mistakes:

• Using pop() to remove first element
• Confusing pop() with shift()
• Expecting pop() to remove all elements

5. You have a queue implemented as an array: let q = [];. You perform these operations:

q.push(1);
q.push(2);
q.shift();
q.push(3);
q.shift();
q.push(4);
console.log(q);

What is the final state of the queue?

hard

A. [2, 3, 4]

B. [3, 4]

C. [1, 2, 3, 4]

D. [4]

Solution

1. Step 1: Trace each operation on queue

   Start with empty q = [].
   push(1) -> [1]
   push(2) -> [1, 2]
   shift() removes 1 -> [2]
   push(3) -> [2, 3]
   shift() removes 2 -> [3]
   push(4) -> [3, 4]

2. Step 2: Confirm final queue state

   After all operations, queue contains [3, 4].

3. Final Answer:

   [3, 4] -> Option B

4. Quick Check:

   FIFO order maintained, final queue is [3, 4] [OK]

Hint: Track push adds end, shift removes front step-by-step [OK]

Common Mistakes:

• Not removing elements with shift correctly
• Assuming push adds at front
• Mixing order of elements after shifts