PureCode Software interview questions

What is normalization and denormalization? When would you use each? Explain different types of joins in SQL with examples. How do you optimize SQL queries for performance? Discuss the concept of middleware in Express.js. Provide examples of middleware functions. How does Node.js handle concurrency? Explain with respect to event loop and non-blocking I/O. Explain the concept of closures in JavaScript. Provide an example. What are the differences between let, const, and var in JavaScript?