Passing anonymous vars in While loop syntax - coffeescript

Question

Im just starting with Coffeescript and running the examples presented in "Programming in CoffeeScript book".
In the while loops section i got intrigued why the call to the times function has to declared as stated bellow.
times = (number_of_times, callback) ->
index = 0
while index++ < number_of_times
callback(index)
return null
times 5, (index) ->
console.log index
I was struggling a bit to read the code and when i've tried:
times (5, (index)) ->
console.log index
It returns an error.
Could you provide some help understanding this code please?

Answer 1

A standard function definition is structured like this:
name = (arg, ...) ->
body
so there's not much to say about your times definition. So let us look at your call to times:
times 5, (index) ->
console.log index
This part:
(index) ->
console.log index
is just another function definition but this one is anonymous. We can rewrite your call using a named function to help clarify things:
f = (index) -> console.log index
times 5, f
And we can fill in the optional parentheses to really spell it out:
f = (index) -> console.log(index)
times(5, f)
Once everything has been broken down you should see that the 5 and (index) in:
times 5, (index) ->
console.log index
have nothing to do with each other so grouping them in parentheses:
times (5, (index)) ->
console.log index
doesn't make sense. If you wanted to add parentheses to that times call to clarify the structure (which is quite useful when the callback function is longer) you need to know two things:
No space between the function name and the opening parenthesis around the arguments. If there is a space then CoffeeScript will think you're using the parentheses to group things within the argument list.
The parentheses need to surround the entire argument list and that includes the callback function's body.
Give that, you'd write:
times(5, (index) ->
console.log index
)
or perhaps:
times(5, (index) -> console.log(index))
With console.log was a non-native function you could even:
times(5, console.log)
but that will give you a TypeError in some browsers so don't go that far.