javascript - Two sets of parentheses after function call

It means that the first function ($filter) returns another function and then that returned function is called immediately. For Example:

function add(x){   return function(y){     return x + y;   }; }  var addTwo = add(2);  addTwo(4) === 6; // true add(3)(4) === 7; // true 

$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') 

It is the same as this:

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

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

with ES6 or later versions you can do it that way;

const divideBoth = (x) => (y) => {    return x / y; }; 

one of the reasons that makes this function type useful is when you have a react.js component that needs have callback function instead of doing it inline way(which is ()=>return value) you can do it the way we did previously. But it is not recommended to use in event callbacks because it gets execute in the first render which might cause issues sometimes