Coding Interview Preparation

JavaScript Coding Interview Questions

Easy

1. Write a function that returns the number of vowels in a string.
2. Write a function that capitalizes the first letter of every word in a sentence.
3. Find the longest word in a sentence.
4. Check whether a string contains only unique characters.
5. Convert an array of strings into a single string separated by commas.
6. Find the sum of all numbers in an array using reduce().
7. Find the difference between two arrays.
8. Group an array of objects by a specific property.
9. Remove falsy values from an array.
10. Find the most frequent element in an array.
11. Flatten a nested array by one level.
12. Write a function that returns a random integer within a given range.
13. Count how many times a specific character appears in a string.
14. Sort an array of objects by a specific property.
15. Write a function that checks whether two objects have the same keys and values.
16. Convert a JavaScript object into a query string.
17. Write a function that safely accesses a deeply nested object property.

Medium

18. Implement your own version of Array.prototype.map().
19. Implement your own version of Array.prototype.filter().
20. Implement your own version of Array.prototype.reduce().
21. Implement a debounce function.
22. Implement a throttle function.
23. Create a function that retries a failed Promise a specified number of times.
24. Write a function that runs multiple Promises and returns their results in the original order.
25. Implement a simple Promise from scratch.
26. Create a function that executes a callback only once.
27. Implement memoization for a function.
28. Write a function to deep-clone a JavaScript object.
29. Implement a custom event emitter with on(), off(), and emit() methods.
30. Create a function that converts a callback-based function into a Promise-based function.
31. Implement an LRU cache using JavaScript data structures.
32. Write a function that limits the number of concurrent asynchronous operations.
33. Implement a function that detects circular references in an object.
34. Build a simple task queue that executes asynchronous tasks one by one.

Hard

35. Implement a simplified version of Promise.all().
36. Implement a simplified version of Promise.race().
37. Implement a simplified version of Promise.allSettled().
38. Build a simple pub/sub system using JavaScript.
39. Implement a virtual DOM diffing function.
40. Build a simple autocomplete system that searches and ranks results as the user types.

JavaScript Interview Questions

These are some of the most important JavaScript interview questions every developer should know, from closures and hoisting to promises, async/await, CORS, debouncing, throttling, and more.

🔥 SQL Interview Case Studies (Advanced Business Scenarios)

🧠 Case Study 1: Find Repeat Customers
📊 Orders Table
order_id customer_id
1 101
2 102
3 101

❓ Business Question
Find customers who placed more than 1 order.

✅ Solution
SELECT customer_id,
COUNT(*) AS total_orders
FROM orders
GROUP BY customer_id
HAVING COUNT(*) > 1;

🧠 Case Study 2: Highest Paid Employee Per Department
❓ Business Question
Find highest paid employee in every department.

✅ Solution
WITH RankedEmployees AS (
SELECT *,
ROW_NUMBER() OVER(
PARTITION BY department
ORDER BY salary DESC
) rn
FROM employees
)
SELECT *
FROM RankedEmployees
WHERE rn = 1;

🧠 Case Study 3: Find Inactive Customers
❓ Business Question
Customers who haven't ordered in the last 6 months.

✅ Solution
SELECT c.customer_id,
c.name
FROM customers c
LEFT JOIN orders o
ON c.customer_id = o.customer_id
GROUP BY c.customer_id, c.name
HAVING MAX(o.order_date) <
DATE_SUB(CURDATE(), INTERVAL 6 MONTH);

🧠 Case Study 4: Product Generating Highest Revenue
📊 Sales Table
product_id amount

✅ Solution
SELECT product_id,
SUM(amount) AS revenue
FROM sales
GROUP BY product_id
ORDER BY revenue DESC
LIMIT 1;

🧠 Case Study 5: Employee Salary Above Department Average

✅ Solution
WITH DeptAvg AS (
SELECT department,
AVG(salary) avg_salary
FROM employees
GROUP BY department
)

SELECT e.*
FROM employees e
JOIN DeptAvg d
ON e.department = d.department
WHERE e.salary > d.avg_salary;

🧠 Case Study 6: Running Total Sales
❓ Business Question
Calculate cumulative sales.

✅ Solution
SELECT order_date,
amount,
SUM(amount) OVER(
ORDER BY order_date
) AS running_total
FROM sales;

🧠 Case Study 7: Top 3 Products Per Category

✅ Solution
WITH RankedProducts AS (
SELECT *,
ROW_NUMBER() OVER(
PARTITION BY category
ORDER BY sales DESC
) rn
FROM products
)
SELECT *
FROM RankedProducts
WHERE rn <= 3;

🎯 Practice Tasks
1️⃣ Find customer with highest number of orders
2️⃣ Find lowest salary employee per department
3️⃣ Find products never sold
4️⃣ Find departments with average salary > company average
5️⃣ Find month with lowest sales

⚡️ Mini Challenge 🔥
Banking Scenario
Tables:
Accounts
account_id customer_name

Transactions
transaction_id account_id amount transaction_date

Question
👉 Find the top 3 customers with highest total transaction amount in the last 1 year.

🔥 Interview Tip
When solving case studies:
1️⃣ Understand business question
2️⃣ Identify tables
3️⃣ Identify JOINs
4️⃣ Identify Aggregations
5️⃣ Decide whether Window Function is needed