Declaring a function

A function is a reusable piece of code that you give a name to. You call it by passing arguments, it runs its body and can return a value with return.

Classic declaration

The most common form is the function declaration:

function saluta(nome) {
  return `Ciao, ${nome}!`;
}

saluta('Anna'); // 'Ciao, Anna!'
saluta('Marco'); // 'Ciao, Marco!'

Things to note:

• nome is the parameter (a variable local to the function).
• 'Anna' is the argument (the value passed at call time).
• Without return, the function returns undefined.

Function expression

Functions in JS are values: they can be assigned to a variable.

const quadrato = function (n) {
  return n * n;
};

quadrato(4); // 16

The two forms are almost equivalent, with one important difference:

• function declaration is "hoisted": you can call it even before the line where it appears in the code.
• function expression follows const/let rules: it exists only after the assignment.

No explicit return

function logga(msg) {
  console.log(`[LOG] ${msg}`);
  // no return → the function returns undefined
}

const r = logga('ciao'); // prints "[LOG] ciao"
r; // undefined

Try it

Review exercise