function add(){var a,v,n,f;a=[].slice.call(arguments),v=a[0]||0;if(a.length){n=a.slice(1).reduce(function(s,x){return s+x},v),f=add.bind(0,n);f.valueOf=function(){return n};return f}return v}

function test(a, b) {
  if (a == b) {
    console.log('OK');
  } else {
    console.log(a + ' != ' + b);
  }
}

test(add(1, 2), 3);
test(add(3)(4)(), 7);
test(add(3)(4)(5), 12);

var three = add(3);
var four = add(4);
test(three, 3);
test(four, 4);
test(three(5), 8);
test(three(6), 9);
test(three(four), 7);
test(three(four)(three(four)), 14);