Technical interviews can be nerve-wracking experiences, especially when it comes to backend interview coding challenges. Coding challenges can be tough and they test your coding skills, ability to solve problems, and the fundamentals of backend concepts. However, with practice and preparation, backend interview coding challenges can be a piece of cake.
With enough practice, you can approach any interview with confidence and significantly increase your chances of landing the job. In this blog, we will delve into some backend interview coding challenges for JavaScript, SQL, MongoDB, and more.
JavaScript Backend Interview Coding Challenges
These backend interview coding challenges are designed to test your understanding of asynchronous operations and fundamental JavaScript concepts.
Problem 1: Asynchronous Code Execution
Challenge: Write a function that fetches data from an API and processes it asynchronously. Ensure that the function handles errors and provides a fallback mechanism.
Solution:
- Use ‘async/await’ for cleaner asynchronous code
- Implement error handling using ‘try/catch’
- Provide a fallback mechanism if the API fails.
Problem 2: Asynchronous Data Fetching
Challenge: Write a function that fetches data from two APIs asynchronously and combines their results. The function should handle errors from both APIs.
Solution:
async function fetchData() {
      try {
const [data1, data2] = await Promise.all ([
      fetch(‘https://api.example.com/data1’).then(res => res.json()),
      fetch(‘https://api.example.com/data2’).then(res => res.json())
       ]);
        return { data1, data2 };
        } catch (error) {;
        console.error(‘Error fetching data:’, error);
        )
}
Problem 3: Event Loop Understanding
Challenge: Explain how JavaScript’s event loop works and demonstrate it with a code example that involves setTimeout and promises.
Solution:
- Discuss the single-threaded nature of JavaScript and how the event loop handles asynchronous operations.
- Provide a code example showing how tasks are queued and executed.
SQL Backend Interview Coding Challenges
These backend interview coding challenges are for testing your fundamentals of SQL queries.
Problem 4: Complex Joins
Challenge: Write a SQL query that retrieves data from multiple tables using INNER JOIN, LEFT JOIN, and RIGHT JOIN.
Solution:
- Use multiple joins to combine data from related tables
- Optimize the query for performance using indexing and filtering conditions
Problem 5: Aggregation and Grouping
Challenge: Write a query that groups data by a specific column and calculates the average, sum, and count of another column.
Solution:
- Use GROUP BY and aggregate functions like AVG(), SUM(), and COUNT() to derive insights from the data.
Also read: Top SQL Interview Questions for Developers
Problem 6: Employee Hierarchy
Challenge:Â Write a SQL query to find all employees directly reporting to a given manager.
Solution:
SELECT employee_name
FROM employees
WHERE manager_id = (SELECT id FROM employees WHERE employee_name = ‘John Doe’)
Problem 7: Data Aggregation
Challenge: Write a SQL query to calculate the total sales, average sales, and number of sales for each product category.
Solution:
SELECT category, SUM(sales) AS total_sales, AVG(sales) AS
average_sales, COUNT(*) AS number_of_sales
FROM products
GROUP BY category;
MongoDB Backend Interview Coding Challenges
These backend interview coding challenges assess your skills in NoSQL data modeling, query optimization, and handling unstructured data in MongoDB.
Problem 8: Data Modeling
Challenge: Design a MongoDB schema for a blog application that supports posts, comments, and user profiles.
Solution:
- Use embedded documents for comments within posts
- Reference user profiles in posts and comments for better scalability
Problem 9: Query Optimization
Challenge: Write a MongoDB query to fetch posts that contain a specific keyword in the title, sorted by the number of comments.
Solution:
- Use a text index to search for the keyword
- Sort the results using ‘$sort’ based on the count of comments
Problem 10: User Activity Log
Challenge: Design a MongoDB query to find all users who logged in more than five times in the last week.
Solution (Javascript):
db.logs.aggregate([
     { $match: { action: “login”, date: { $gte: new Date(new Date() –
7 * 24 * 60 * 60 * 1000) } } },
     { $group: { _id: “$user_id”, login_count: { $sum: 1 } } },
     { $match: { login_count: { $gt: 5 } } }
]);
Django Backend Interview Coding Challenges
These backend interview coding challenges are intended to evaluate your expertise in Django’s ORM, middleware, and caching mechanisms.
Problem 11: ORM Querying
Challenge: Write a Django ORM query to fetch all users who have posted more than five blog articles.
Solution:
- Use ‘annotate()’ and ‘filter()’ to count related posts and apply the condition
Problem 12: Middleware Implementation
Challenge: Create custom middleware that logs the request method and path for every incoming request.
Solution:
- Implement a Django middleware class with ‘process_request’ method to log details.
Problem 13: Custom User Authentication
Challenge: Implement a custom authentication backend in Django that allows users to log in using either their username or email.
Solution (Python):
from django.contrib.auth.backends import ModelBackend
from django.contrib.auth import get_user_model
class EmailOrUsernameBackend(ModelBackend):
       def authenticate(self, request, username=None, password=None, **kwargs):
       UserModel = get_user_model()
       try:
           user = UserModel.objects.get(Q(username=username) |
       Q(email=username))
           except UserModel.DoesNotExist:
                   return None
           if user.check_password(password):
                    return user
            return None
PHP Backend Interview Coding Challenges
These backend interview coding challenges aim to test your proficiency in secure file handling, error management, and database interaction using PHP.
Problem 14: File Handling
Challenge: Write a PHP script that reads a CSV file and inserts the data into a MySQL database.
Solution:
- Use ‘fgetcsv()’ to parse the CSV file
- Use ‘PDO’ or ‘MySQLi’ to insert data securely into the database.
Problem 15: Secure File Upload
Challenge: Write a PHP script to securely upload a file to the server, checking for file type and size limits.
Solution:
if ($_FILES[‘file’][‘size’] > 500000) {
     die(“File is too large.”);
}
$allowed = [‘jpg’, ‘jpeg’, ‘png’, ‘pdf’];
$ext = strtolower(pathinfo($_FILES[‘file’][‘name’],
PATHINFO_EXTENSION));
if (!in_array($ext, $allowed)) {
   die(“Invalid file type.”);
}
move_uploaded_file($_FILES[‘file’][‘tmp_name’], “uploads/” .
basename($_FILES[‘file’][‘name’]));
Problem 16: Prepared Statements for SQL Injection Prevention
Challenge:
Implement a PHP script that securely inserts user input into a MySQL database using prepared statements.
Solution:
$conn = new mysqli($servername, $username, $password, $dbname);
$stmt = $conn->prepare(“INSERT INTO users (name, email) VALUES (?, ?)”);
$stmt->bind_param(“ss”, $name, $email);
$name = $_POST[‘name’];
$email = $_POST[’email’];
$stmt->execute();
$stmt->close();
$conn->close();
Problem 17: Error Handling
Challenge: Implement error handling in a PHP script that performs a division operation, ensuring division by zero is handled gracefully.
Solution:
- Use ‘try/catch’ blocks to handle ‘DivisionByZeroError’
- Return a custom error message if an error occurs.
API Backend Interview Coding Challenges
These backend interview coding challenges focus on your ability to design secure and efficient RESTful APIs and implement features like JWT authentication and rate limiting.
Problem 18: RESTful API Design
Challenge: Design a RESTful API for a task management system with endpoints for creating, updating, and deleting tasks.
Solution:
- Follow RESTful principles with proper HTTP methods (‘GET’, ‘POST’, ‘PUT’, ‘DELETE’)
- Ensure the API follows the correct resource naming conventions.
Problem 19: Rate Limiting
Challenge: Implement rate limiting for an API to prevent abuse.
Solution: Use middleware to track requests and apply limits based on the client’s IP address.
Node.js Backend Interview Coding Challenges
These backend interview coding challenges assess your ability to handle asynchronous file operations and implement scalable solutions using Node.js and its core modules.
Problem 20: Real-time Data
Challenge: Create a real-time chat application using Node.js and WebSockets.
Solution:
- Use ‘socket.io’ to manage WebSocket connections
- Ensure that messages are broadcast to all connected clients.
Problem 21: File Streaming
Problem: Implement a Node.js script that streams a large file to the client without consuming too much memory.
Solution:
const fs = require(‘fs’); const http = require(‘http’);
http.createServer((req, res) => {
     const stream = fs.createReadStream(‘largefile.txt’);
     stream.pipe(res);
}).listen(3000)
Problem 22: Asynchronous File Processing
Challenge: Write a Node.js script to process a large file asynchronously without blocking the event loop.
Solution:
- Use streams to process the file in chunks
- Handle errors and events to manage the flow
Problem 23: Cluster Module for Load Balancing
Challenge: Use the cluster module in Node.js to create a simple load-balanced server that utilizes all CPU cores.
Solution:
const cluster = require(‘cluster’);
const http = require(‘http’);
const numCPUs = require(‘os’).cpus().length;
if (cluster.isMaster) {
    for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
    }
     cluster.on(‘exit’, (worker, code, signal) => {
     console.log(`Worker ${worker.process.pid} died`);
     }); } else {
             http.createServer((req, res) => {
             res.writeHead(200);
             res.end(‘Hello World’);
             }).listen(8000);
}
Also read: Top Node JS Interview Questions to Practice
20 Backend Interview Coding Challenges Examples For Practice
Use the backend interview coding challenges to test your skills and whether you understand the fundamentals of JavaScript, Python, and SQL.
- Implement a function to reverse a string.
- Write a SQL query to find the nth highest salary.
- Create a MongoDB aggregation pipeline to group data by category.
- Design a RESTful API for a blog platform.
- Implement a binary search algorithm.
- Write a Django view to handle file uploads.
- Implement a priority queue in JavaScript.
- Design a database schema for an e-commerce application.
- Write a Node.js script to fetch and process JSON data from an API.
- Implement pagination in an SQL query.
- Create a PHP script to send an email with attachments.
- Write a function to detect a cycle in a linked list.
- Implement a caching mechanism in Node.js.
- Create a MongoDB query to find documents with missing fields.
- Write a Django model to handle user profiles.
- Implement input validation in a PHP form.
- Write an SQL query to calculate the median value.
- Implement a rate-limited API in Node.js.
- Create a Node.js script to process CSV files asynchronously.
- Write a function to find the longest palindromic substring in a string.
Ace Your Backend Interview Confidently With Interview Kickstart
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FAQs – Backend Interview Coding Challenges
Q1: How should I prepare for backend interview coding challenges?
Focus on understanding fundamental concepts like data structures, algorithms, and database management. Practice coding regularly on platforms like LeetCode, HackerRank, and CodeSignal.
Q2: Are coding challenges the only part of a backend interview?
No, backend interviews also assess your understanding of system design, API integration, and sometimes your knowledge of specific frameworks like Django or Node.js.
Q3: What programming languages are commonly used in backend coding challenges?
Languages like Python, Java, JavaScript (Node.js), PHP, and SQL are frequently used in backend coding challenges.
Q4: How can I manage time effectively during backend coding interviews?
Start by thoroughly understanding the problem before jumping into coding. Allocate time to plan your approach, write clean and efficient code, and test your solution. Practice solving problems within time limits to build speed and accuracy.
Q5: What should I focus on when solving backend interview coding challenges?
Focus on writing clean, maintainable code with proper error handling. Prioritize understanding the problem’s requirements, and think about scalability, performance, and security when crafting your solution.
Related reads:
Back-End Developer Interview Questions
Best Way to Learn Data Structures and Algorithms
Most Popular Back-end Development Languages to Get a Job at FAANG
Backend Developer Roadmap — How to Become a Backend Developer?
