exercise_4.js

var add = (function(a) {
  return function(b) {
    return a + b;
  }
});


var mul = (function(a) {
  return function(b) {
    var total = a * b;
    return total;
  }
});

function applyf(f) {
  return f;
}

addf = applyf(add);
console.log(addf(2)(3)); // 5
console.log(applyf(mul)(2)(3)); // 6