Javascript : optional parameters in function

Let's say I have this :

function concatenate(a, b, c) {
    // Concatenate a, b, and c
}

How do I handle those calls ?

x = concatenate(a)


x = concatenate(a, b)

x = concatenate(a, c)

How can I make my function aware of the parameter I gave to it ?

Answer

Any unfilled argument will be undefined.

concatenate(a, c) is equivalent to concatenate(a, b). You cannot pass the third parameter without passing the second; but you can pass undefined (or null, I suppose) explicitly: concatenate(a, undefined, c).

In the function, you can check for undefined and replace with a default value.

Alternately, you can use an object argument to imitate keyword arguments: concatenate({a: a, c: c}).