addKata

add kata

var addTwo = add(2);
addTwo; // 2
addTwo + 5; // 7
addTwo(3); // 5
addTwo(3)(5); // 10

add.js

var add = function(inc = 0, value = 0) {
  var f = function(n) {
    // In the case of no arguments there are two possibilities:
    //  1. return this so that the object is returned (chaining is enabled ie this works: add(2)()()(3)(4))
    //  2. return a number so that the return type is a number (return type consistency ie this returns a number: add(2)() )
    //
    // I chose option 1 here
    return arguments.length === 0 ? this : add(0, n + value + inc);
  }
  f.valueOf = function() { return Number.prototype.valueOf.apply(value + inc, arguments); }
  f.toString = function() { return Number.prototype.toString.apply(value + inc, arguments); }
  f.prototype = Number;
  return f;
};