function(foo,bar){
   ...
}

Function.prototype.argumentNames = function() {
  var names = this.toString().match(/^[\s\(]*function[^(]*\(([^)]*)\)/)[1]
    .replace(/\/\/.*?[\r\n]|\/\*(?:.|[\r\n])*?\*\//g, '')
    .replace(/\s+/g, '').split(',');
  return names.length == 1 && !names[0] ? [] : names;
};

function valera(x, y, z) {
    defaultArgs(arguments, {y: 5, z: 10});
    ...
}

function valera(x, y, z) {
    defaultArgs(arguments, {y: 5, z: 10});
    ...
}

function valera(x, y, z) {
    y=y || 5, z = z || 10;
    ...
   // или так
  y = typeof y === 'undefined' ? 5 : y ;
  z = typeof z === 'undefined' ? 10 : z ;
}

function defaultArgs(/* arguments */){
	var len = arguments.length - 1;
	
	for(var i = 0; i < len; i ++){
		arguments[0][i] = arguments[i + 1];
	}
}
	
function valera(x, y, z){
	defaultArgs(arguments, 5,6,7);
	/* code */
}

defaultArgs(arguments, {y: 5, z: 10});

foo ( { bar : 'ololo', one : 'two' } );

foo ( 'ololo', 'two' );

(function(a, b, c, d){
  alert([a,b,c,d] + '\n' + arguments.length);
  arguments[0] = 10;  arguments[1] = 13;
  arguments[2] = 11;  arguments[3] = 12;
  arguments.length = 4;
  alert([a,b,c,d] + '\n' + arguments.length);
})(5, 4)

function lambda(fnc, context, defaults){ // другого имени не придумал
	var defaultArgs = [], key, names;
	if(defaults){
		names = fnc.toString().match(/^[\s\(]*function[^(]*\(([^)]*)\)/)[1]
		                      .replace(/\/\/.*?[\r\n]|\/\*(?:.|[\r\n])*?\*\//g, '')
		                      .replace(/\s+/g, '').split(',');
		names = names.length == 1 && !names[0] ? [] : names;

		for(var i = 0, length = names.length; i < length; ++i){
			key = names[i];
			defaultArgs.push(defaults[key]);
		}
	}

	return function(){
			return fnc.apply(context, lambda.merge(arguments, defaultArgs));
		}
}
lambda.merge = function(arr1, arr2){
	var length = Math.max(arr1.length, arr2.length),
		arr = [];
	for(var i = 0; i < length; ++i){
		arr.push(i < arr1.length ? arr1[i] : arr2[i]);
	}
	return arr;
};


alert(lambda(function(a, b){alert(this); return a + b}, 'Hello, world!', {a: 50, b: 77, c: 42})(10));

function merge(arr1, arr2){
	var length = Math.max(arr1.length, arr2.length),
		arr = [];
	for(var i = 0; i < length; ++i){
		arr.push(i < arr1.length ? arr1[i] : arr2[i]);
	}
	return arr;
}

function withDefaults(fn, defaults, cnx){
	if(!defaults){
		defaults = [];
	}
	if(!cnx) {
		cnx = this;
	}
	
	return function(/* args for fn*/){
		return fn.apply(cnx, merge(arguments, defaults));
	}
		
}
	
function mul(a, b){
	alert(a + "*" + b + " = " + a*b);
}
	
withDefaults(mul, [3,5])(5,7);
withDefaults(mul, [3,5])();
withDefaults(mul, [3,5])(6);

fncName(a, b, undefined, undefined, 'red');

fncName({
  speed: a,
  count: b,
  codeName: 'red'
});

Object.prototype.set = function(props){
	for(var any in props) if(props.hasOwnProperty(any)){
		if(typeof props[any] === "object" && !props[any] instanceof Array){
			if(!this[any]){
				this[any] = {};
			}
			this[any].set(props[any]);
		}else{
			this[any] = props[any];
		}
		
	}
	//для удобства
	return this;
}
var a = {}.set({codename: "red", value: 5, style: {color: "red", border: "1px solid red"}, fields: [1,2,3,4,5,6,7,8]});

//смотреть в FF
alert(a.toSource());
a.set({some: true});
alert(a.toSource());

Array.prototype.copy = function(){
	var len = this.length, nuarr = [];
	for(var i = 0; i < len; i ++){
		if(i in this){
			if(typeof this[i] === "object" && !this[i] instanceof Array){
				nuarr[i] = {}.set(this[i]);
			}else if(this[i] instanceof Array){
				nuarr[i] = this[i].copy();
			}else{
				nuarr[i] = this[i];
			}
		}
	}
	
	return nuarr;
}

Object.prototype.set = function(props){
	for(var any in props) if(props.hasOwnProperty(any)){
		if(typeof props[any] === "object" && !props[any] instanceof Array){
			if(!this[any]){
				this[any] = {};
			}
			
			this[any].set(props[any]);
		}else if(props[any] instanceof Array){
			this[any] = props[any].copy();
		}else{
			this[any] = props[any];
		}
		
	}
	//для удобства
	return this;
}

function arr_cycle ( arr, func ) {

// явл. ли массивом
if ( arr.constructor !== Array ) throw new Error( "'arr' must be Array" );

for ( var i = 0; i < arr.length; i++ )
           i in arr &&  func.apply(window, [ arr[i], i, arr ] );
 
}

...
if ( arr = arr || [] ).constructor  ...

function f(a, b){alert(this); return a + b}

function fWrapper(){
    return lambda(f, 'Hello, world!', {a: 50, b: 77, c: 42});
}