extend.js

    function extend(target/*, any number of source objects*/){
        var i = 1,
            len = arguments.length,
            withPrototype = arguments[len-1] === true,
            obj, prop;
        
        target || (target = {});
        for (; i < len; i++){
            obj = arguments[i];
            if (typeof obj === 'object'){
                for (prop in obj){
                    if (withPrototype || obj.hasOwnProperty(prop)){
                        target[prop] = obj[prop];
                    }
                }
            }
        }
        return target;
    }