Functions.

Ninth class. “Income − expense” repeats. Here we learn an ordinary function declaration — a named recipe. Arrow functions come later in Data.

function remaining(income, expense) { return income - expense; }

Parameters are inputs. return sends a result out. Call: remaining(2000, 750) → 1250.

Mini example:

function double(n) { return n * 2; } console.log(double(5));

Common mistake: forgetting return (result is undefined); or calling the function without parentheses.