Functions

A function packages a piece of logic under a name so you can run it again without retyping it. There are two common ways to write one:

function double(n) {
  return n * 2;
}

const triple = (n) => n * 3;

The first is a function declaration. The second is an arrow function stored in a const — a shorter syntax that's especially popular for small helper functions and callbacks. When an arrow function's body is a single expression, you can even skip the return keyword and the curly braces, as shown above (that's called an "implicit return").

Parameters are the named inputs a function expects (n above); arguments are the actual values you pass in when calling it (double(5)). A parameter can have a default value, used only when the caller doesn't supply that argument:

function greet(name = "friend") {
  return "Hello, " + name + "!";
}
greet();          // "Hello, friend!"
greet("Priya");   // "Hello, Priya!"

The return statement sends a value back to wherever the function was called, and immediately ends the function — any code after a return inside the same block never runs. A function with no return statement implicitly returns undefined.

Variables declared inside a function (including its parameters) exist only inside that function — this is called scope. A variable declared inside one function is invisible outside it, which is exactly what lets two different functions safely use a variable with the same name without conflicting.