Queue Implementation Using Array in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a queue in data structures?

A queue is a collection where elements are added at the back and removed from the front, following the First-In-First-Out (FIFO) order.

beginner

How do you add an element to a queue implemented using an array?

You add (enqueue) the element at the end (back) of the array using the array's push method.

beginner

How do you remove an element from a queue implemented using an array?

You remove (dequeue) the element from the front of the array using the array's shift method.

beginner

What happens if you try to dequeue from an empty queue?

Since the queue is empty, there is no element to remove, so the dequeue operation returns undefined to indicate the queue is empty.

intermediate

Why might using an array for a queue be inefficient for many dequeue operations?

Because removing from the front (shift) causes all other elements to move one position forward, which takes extra time for large arrays.

In a queue implemented with an array, which method adds an element to the queue?

A pop()

B shift()

C push()

D unshift()

push() adds an element to the end of the array, which is the back of the queue.

Which method removes the front element from a queue implemented using an array?

A shift()

B pop()

C push()

D unshift()

shift() removes the first element from the array, which is the front of the queue.

What is the order in which elements come out of a queue?

A Sorted order

B Last-In-First-Out

C Random order

D First-In-First-Out

Queues follow First-In-First-Out (FIFO) order.

What will happen if you call dequeue on an empty queue implemented with an array?

A Returns the last element

B Returns null or undefined

C Throws an error

D Adds a new element

Dequeue returns undefined to indicate the queue is empty.

Why might using an array's shift method repeatedly be inefficient for a queue?

A Because shift moves all elements forward after removal

B Because shift removes elements from the end

C Because shift sorts the array

D Because shift adds elements to the front

shift removes the first element and shifts all other elements forward, which can be slow for large arrays.

Explain how to implement a queue using an array and how enqueue and dequeue operations work.

Think about how arrays add and remove elements at different ends.

Describe one disadvantage of using an array to implement a queue and why it happens.

Consider what happens inside the array when removing the first element.

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