TypeScript is a free and open-source high-level programming language. It allows specifying the types of data being passed around within the code, and has the ability to report errors when the types don't match. In TypeScript 5.1, there is a new feature that supports linked editing for JSX tag names. This feature, also known as “mirrored cursors”, enables editors to automatically edit multiple locations simultaneously.

In JavaScript, when a function finishes running without using a return statement, it returns the value undefined. However, in earlier versions of TypeScript, functions were required to have at least one return statement unless they were explicitly declared as returning void or any type. This meant that even if you stated that a function returns undefined, you still had to include a return statement.