Default and rest parameters

JavaScript lets you make parameters optional (with a default value) and accept a variable number of arguments with the rest operator (...).

Default parameters

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

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

The default applies only when the argument is undefined (including the "not passed" case). For null or 0 the default does not kick in:

function f(x = 10) {
  return x;
}
f(); // 10
f(undefined); // 10
f(null); // null
f(0); // 0

Rest: ...names

To accept a variable number of arguments, collect them into an array with ...:

function somma(...numeri) {
  let totale = 0;
  for (const n of numeri) totale += n;
  return totale;
}

somma(); // 0
somma(5); // 5
somma(1, 2, 3, 4); // 10

Inside the function, numeri is a real array (not the magic arguments object of old functions).

The rest must be the last parameter:

function f(primo, ...altri) {
  /* ok */
}
// function f(...altri, ultimo) {} // SyntaxError

Spread at call time

The other face of ... is the spread operator, which "explodes" an array into separate arguments at call time:

function somma(a, b, c) {
  return a + b + c;
}

const nums = [1, 2, 3];
somma(...nums); // 6, equivalent to somma(1, 2, 3)

Try it

Review exercise