How to explain this Javascript Syntax

Question

I was looking at a few things in Javascript. When I got to the call() function I had a look at this on the mozilla js developer page:

Syntax:

fun.call(thisArg[, arg1[, arg2[, ...]]])

I understand what the function does and how it operates but what on earth is that syntax trying to say? Why is there a comma right after that bracket?

[,

Similarly when I was looking up the reduce() function I understand how it works but the syntax shows this:

arr.reduce(callback[, initialValue])

Shouldn't this be something like arr.reduce(callback, initialvalue) instead?

As a noob, the syntax leads me to believe that callback is an array and it has an empty first element and a second element called initialValue. Is there somewhere that I can read about this syntax so that it makes sense, or could some kind person please provide me with some clarity?

EDIT: Also callback refers to a function yes? So why is it callback[] and not callback()?

score 5 · Answer 1 · answered Oct 30 '16 at 07:46

5

[...] indicates that what is inside is optional. For example, arr.reduce(callback[, initialValue]) can be either arr.reduce(callback) or arr.reduce(callback, initialValue).

The comma is right there meaning that if you will include the second argument, you must put the comma.

answered Oct 30 '16 at 07:46

Bundit J.

141
1
4

Thanks for the response. If you look at the second example I provided there is no [...] however. So how am I supposed to know that one of the arguments is optional? – yoonsi Oct 30 '16 at 07:49
Does it imply that the element after the comma (initialValue) is optional and that I must provide the comma if I use this argument? That would make sense. I do wonder though why callback is callback[] and not callback() – yoonsi Oct 30 '16 at 07:51
2

Just for clarity - it is not a JS syntax, it is just a documentation syntax. – Roman Hocke Oct 30 '16 at 07:51

score 2 · Answer 2 · answered Oct 30 '16 at 07:59

That is a documentation convention and not actual JS syntax.

Square brackets indicate the argument is optional.

The ellipsis indicates that any number of arguments can go there.

fun.call(thisArg[, arg1[, arg2[, ...]]])

means you could say:

 fun.call(foo);
 fun.call(foo, 1);
 fun.call(foo, 1, 2);
 fun.call(foo, 1, 2, 3);
 fun.call(foo, 1, 2, 3, 4);
 fun.call(foo, 1, 2, 3, 4, 5);

etc

arr.reduce(callback[, initialValue])

means you coul dsay:

arr.reduce(foo);

or

arr.reduce(foo, bar);

… but not

arr.reduce(foo, bar, baz);

How to explain this Javascript Syntax

2 Answers2