1. API Development: Design and implementation of RESTful APIs using Node.js and Express.js, including creating endpoints for data interactions and handling server-side logic.

2. Database Management: Utilizing MongoDB for database operations such as designing schemas, managing collections, and maintaining data integrity.

3. Integration with Frontend: Collaborating with frontend developers to integrate APIs with React.js interfaces, ensuring seamless data flow between server and client.

4. Testing and Debugging: Implementing testing strategies for APIs to ensure reliability, including unit and integration testing, and debugging any arising issues.

5. Performance Optimization: Monitoring and enhancing the performance of backend services, including analyzing server load, optimizing database queries, and implementing caching.

6. Documentation and Collaboration: Creating comprehensive API documentation and collaborating with team members through code reviews and version control with tools like Git.