JavaScript: comma after opening bracket of parameter in syntax example

Question

The syntax for methods such as Array.prototype.indexOf() look like this:

arr.indexOf(searchElement[, fromIndex = 0])

What does [, ...] mean, and why is the comma inside of the brackets?

Answer 1

The brackets themselves mean "optional", and the = 0 gives the default value should you decide to omit that parameter. The reason why the comma is inside the brackets is because it forms part of the optional bit - if the second parameter is omitted, so is the comma.

In other words, you can use indexOf with only searchElement, in which case fromIndex is assumed to be zero. Or you can specify your own value for fromIndex if you don't want to start searching at element number zero.

So the first two below are equivalent while the third will start searching at a different point in the array:

x = haystack.indexOf (needle);
x = haystack.indexOf (needle, 0);
x = haystack.indexOf (needle, 42);

The Mozilla Developer Network has this to say on the matter (my emphasis):

fromIndex: The index to start the search at.

If the index is greater than or equal to the array's length, -1 is returned, which means the array will not be searched. If the provided index value is a negative number, it is taken as the offset from the end of the array.

Note: if the provided index is negative, the array is still searched from front to back. If the calculated index is less than 0, then the whole array will be searched.

Default: 0 (entire array is searched).