请问xhr.responseType是怎么用的？

case 4:  
            if ( xhr.status == 200 ) {  
                var text = document.getElementById("text");  
                text.innerHTML = "xhr请求成功";  
                console.log(xhr.response);  
                console.log(xhr.responseType);  
                console.log(xhr.responseText);  
                if(xhr.response.valueOf("update")) {  

                } else {  

                }  
            } else {  
                var text = document.getElementById("text");  
                text.innerHTML = "xhr请求失败";  
            }  
            break;  
        default :  
            break;

在执行这段代码是输出是如下：
应用HBuilder已启动...
null at index.js:42
json at index.js:43
{"update":true,"version":"1.0","title":"TestApplication","name":"测试应用","entry":"index.html","description":"测试应用的第一个版本","downloadurl":"http://","serveraddress":"http://","file":[B@440faed3} at index.js:44
Uncaught TypeError: Cannot call method 'valueOf' of null at index.js:45

可以看到代码执行到第二个if判断时报了错，可是我明明在new xhr时已经执行type为json，为什么收到的xhr,response还是null呢？