Parameters, multiple returns, variadic, closures, and defer.

Variadic Functions and Closures

Variadic — accept "any number of" arguments

A function whose last parameter is ...T accepts zero or more T values. Inside the function, the parameter is a slice []T.

The nums... syntax (suffix on the call site) is how you turn a slice back into individual arguments. fmt.Println itself is variadic — that's why you can pass it any number of values.

Closures — functions that capture variables

A closure is a function that references variables from the enclosing scope. The closure keeps those variables alive even after the outer function returns.

Each call to counter() creates a fresh n. The returned function "captures" that specific n and keeps mutating it.