Partial function application with arbitrary depth

add-on-steroids.js

function add(x) {
  
  // here's where it all starts, at the edge: add(x)
  var sum = x;
  
  // this is where the funk gets in: we're building a function that adds to
  // the above sum and also provides a method to get to that sum, allowing
  // to call the function arbitrarily often but also get to the sum,
  // eventually. As in add(1)(2)(3)(4)(5).get()
  var result = function(x) {
    sum += x;
    return result;
  };
  result.get = function() {
    return sum;
  }
  
  return result;
}

alert(add(1)(2)(3)(4).get() === 10);