Python

Writing Functions in Python (and Why You Will Want To)

The moment copy-pasting the same lines for the third time stops being okay is usually the moment you need a function.

JavaScript

Functions, Arrow Functions, and What Scope Actually Means

Arrow functions aren't just shorter syntax. The real difference is how they handle this, and it matters.

C++

Functions in C++, and the Value-vs-Reference Question

Every parameter in a C++ function makes a decision: does it get a copy of the data, or the real thing?