Casual argument parser in JavaScript

hike.js

"use strict";

/****** Library START ******/

var hike = function() {
  var user_fn, i, len, rule_strs, rule_parsed,
      rule = {}, /* arg length -> [arg name] */
      _arguments = Array.prototype.slice.call(arguments);

  user_fn = _arguments.pop();
  if (toString.call(user_fn) !== '[object Function]') {
    throw "Need user function in the last argument";
  }
  rule_strs = _arguments;

  len = rule_strs.length;
  // Parse rules in inverted order to prioritize former rules
  for (i=len-1; i>=0; i-=1) {
    rule_parsed = _hike_parse(rule_strs[i]);
    rule[rule_parsed.length] = rule_parsed;
  }

  return function() {
    var this_rule, i, args = {},
        args_len = arguments.length;

    this_rule = rule[args_len];
    if (this_rule === void 0) {
      throw "Cannot match argument rule. Args length: "+args_len;
    }
    
    for (i=0; i<args_len; i+=1) {
      args[this_rule[i]] = arguments[i];
    }
    user_fn(args);
  };
};

var _hike_parse = function(rule_str) {
  var i, len, args, rule = [];
  args = rule_str.split(',');
  len = args.length;
  for (i=0; i<len; i+=1) {
    // TODO: Support annotation (e.g. type, required, ..)
    rule.push(args[i].trim());
  }
  return rule;
};

/****** Library END ******/

var fade = hike("duration, complete", 
	         "options",
	         "duration, easing, complete",
	     function(args){
	       console.log(args);
	     });

// http://api.jquery.com/fadeIn/

fade(100,function() { console.log("Faded"); });
fade({
	"duration" : 100,
	"easing"   : "swing",
	"completed": function() { console.log("Faded"); }
});
fade(100,"swing",function() { console.log("Faded"); });