Jerome Argument Names

demo.js

function add (a, b) {
  return a + b
}

var result = add(1, 2);
console.log(result);

Thanks Andrew, I can see that parameters 'a' and 'b' serve as placeholders for any values that are sent down to the function when called. These values are then added up (manipulated) to produce the result we want. - understood

But here, ( (https://gist.github.com/auchomage/80e8c7ce5debfb7b44343462ad802582) ). In file 'location2.js', line 5. What value(s) are being passed if any?

If we want values why not use the return result of 'callback()'

ie callback(){
return some value
}

then we can say

someVar = callback(); // this would assign the return value of calling the function 'callback()', to a a variable (here called 'someVar'), that can be used as we see fit.

But that isn't occurring here and I can't see why. I hope you can see what I mean, thank you.