What does the default function return in the programming language you use?

It depends on the language. In Python, a function without an explicit return returns None. In Java, a function with a return type of void returns nothing. In JavaScript, a function without an explicit return returns undefined. In Go, a function without an explicit return at the end, if a return type is declared, will cause a panic if no value is returned. If no return type is declared, the function can be used as a procedure. In C/C++, a function without a specified return type defaults to int, although this is considered outdated practice and may lead to compiler warnings.