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

$filter('number') returns a function that accepts two arguments, the first being required (a number) and the second one being optional (the fraction size).

It's possible to immediately call the returned function:

$filter('number')('123')

Alternatively, you may keep the returned function for future use:

var numberFilter = $filter('number');

numberFilter('123')