Optional Arguments in JavaScript

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, the following 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;



// → -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.

function power(base, exponent = 2) {

let result = 1;

for (let count = 0;

count < exponent; count++) {

result *= base;


return result;



// → 16

console.log(power(2, 6));

// → 64

In the next chapter, we will see a way in which a function body can get at the whole list of arguments it was passed (see “Rest Parameters” on page 74). This is helpful because it makes it possible for a function to accept any num­ber of arguments. For example, console.log does this—it outputs all of the values it is given.

console.log(“C”, “O”, 2);

//→ C O 2

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,

No Starch Press; 3rd edition.

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