デフォルト引数をつける

defaults_for.js

var isDictionary = function(obj) {
  return obj && obj.constructor === Object;
};

var args_resolution = function(keys, vals, given) {
  var dict, args = vals.slice();
  if (isDictionary(given[given.length - 1])) {
    dict = given.pop();
    for (var key in dict) {
      var index = keys.indexOf(key);
      if (index !== -1) {
        args[index] = dict[key];
      }
    }
  }
  for (var i = 0, imax = Math.min(given.length, args.length); i < imax; ++i) {
    args[i] = given[i];
  }
  if (dict) {
    args.push(dict);
  }
  return args;
};

var defaults_for = function(func) {
  var body = "" + func;
  var def = body.replace(/^\s*function\s*\(.+\)\s*{\s*(?:"use strict";)?\s*\/\/\s*{([^\n]+?)}[\S\s]+$/m, "$1");
  if (def !== body) {
    var origin = func, keys = [], vals = [];
    def = def.split(",");
    for (var i = 0, imax = def.length; i < imax; ++i) {
      var items = def[i].trim().split("=");
      keys.push(items[0].trim());
      vals.push(items.length === 1 ? 0 : eval(items[1]));
    };
    func = function() {
      return origin.apply(this, args_resolution(keys, vals, Array.prototype.slice.call(arguments)));
    };
  }
  return func;
};

usage.js

var madd = defaults_for(function(val, mul, add) {
  // { val=0, mul=1, add=0 }
  return val * mul + add;
});

console.log(madd(10));           // 10 (= 10 * 1 +  0)
console.log(madd(10, 2));        // 20 (= 10 * 2 +  0)
console.log(madd(10, {add:20})); // 30 (= 10 * 1 + 20)


var Int = (function() {
  function Int(n) {
    this.value = n|0;
  }
  Int.prototype.madd = defaults_for(function(mul, add) {
    // { mul=1, add=0 }
    return (this.value * mul + add)|0;
  });
  return Int;
})();

var i = new Int(Math.PI);
console.log(i.madd(2));               // 6 (= 3 * 2 + 0)
console.log(i.madd({mul:3, add:-9})); // 0 (= 3 * 3 - 9)