SonarLint has identified 300+ rules in TypeScript projects, with five common issues that are frequently invoked to flag bugs or security hotspots. These issues can have varying consequences on the application, from confusing types to memory leaks and hard-to-spot bugs. The top 5 issues are optional property declarations, creating and dropping objects immediately, unused local variables and functions, non-empty statements that don't change control flow or have a side-effect, and assignments within sub-expressions. SonarLint can help developers catch these issues as they write code, saving time and keeping the code clean and bug-free. By addressing these common issues, developers can ensure their application is more maintainable in development and works better in production.