what is currying

Curried Function
// Non-curried
function add(a, b, c) {
  return a + b + c
}

add(1, 2, 3)
//-> 6

// Curried
function addd(a) {
  return function (b) {
    return function (c) {
      return a + b + c
    }
  }
}

addd(1)(2)(3)
//-> 6

a technique that applies a function 
to its arguments one at a time, with 
each application returning a new function 
that accepts the next argument.