clone 克隆对象

function clone(obj){
    var o;
    switch(typeof obj){
        case 'undefined': break;
        case 'string'   : o = obj + '';break;
        case 'number'   : o = obj - 0;break;
        case 'boolean'  : o = obj;break;
        case 'object'   :
            if(obj === null){
                o = null;
            }else{
                if(obj instanceof Array){
                    o = [];
                    for(var i = 0, len = obj.length; i < len; i++){
                        o.push(clone(obj[i]));
                    }
                }else{
                    o = {};
                    for(var k in obj){
                        o[k] = clone(obj[k]);
                    }
                }
            }
            break;
        default:
            o = obj;break;
    }
    return o;
}