JavaScript Functions

Optional Arguments

The following code is allowed and executes without any problem:

function square(x) { return x * x; }
console.log(square(4, true, "hedgehog"));
// → 16

We defined square with only one parameter. Yet when we call it with three, the language doesn't complain. It ignores the extra arguments and computes the square of the first one.

JavaScript is extremely broad-minded about the number of arguments you pass to a function. If you pass too many, the extra ones are ignored. If you pass too few, the missing parameters get assigned the value undefined.

The downside of this is that it is possible - likely, even - that you'll accidentally pass the wrong number of arguments to functions. And no one will tell you about it.

The upside is that this behavior can be used to allow a function to be called with different numbers of arguments. For example, this minus function tries to imitate the - operator by acting on either one or two arguments:

 function minus(a, b) {
  if (b === undefined) return -a;
  else return a - b;
}

console.log(minus(10));
// → -10
console.log(minus(10, 5));
// → 5

If you write an = operator after a parameter, followed by an expression, the value of that expression will replace the argument when it is not given.

For example, this version of power makes its second argument optional. If you don't provide it or pass the value undefined, it will default to two, and the function will behave like square.

 power(base, exponent = 2) {
  let result = 1;
  for (let count = 0; count < exponent; count++) {
    result *= base;
  }
  return result;
}

console.log(power(4));
// → 16
console.log(power(2, 6));
// → 64