Two sets of parentheses after function call

Question

I was looking how filters works in Angularjs and I saw that we need to send 2 sets of parentheses.

$filter(\'number\')(number[, fractionSize])

Answer 1

It is the same as this:

var func = $filter('number');
func(number[, fractionSize]);

The $filter() function returns a pointer to another function.