[Антон Крамолов]

function extend() {
    var target = arguments[0];
    
    for (var i = 1; i < arguments.length; ++i) {
        var options = arguments[i];
        
        for (var prop in options) {          
            if (typeof options[prop] == 'object') {
               target[prop] = extend({}, options[prop]);
            }
            else {
                target[prop] = options[prop];
            }
        }
    }
    
    return target;
}

var a = {x: 'X', y: {yx: 'XX', yy: 'YY'}};
var b = {z: 'Z'};
var c = extend({}, a, b);
a.y.yx = 'TEST';
alert(c.y.yx);