js字符串转JSONparse函数错误

错误如下

img


json格式没有问题，单独在本地解析没有问题，我觉得可能是异步导致的问题，这是前端代码

var httpRequest = new XMLHttpRequest();
            httpRequest.open('GET',"http://" + UrlName + "/homeposts/" + urlLast,true);
            httpRequest.send();
            httpRequest.onreadystatechange = function() {
                let a = httpRequest.responseText;
                console.log(a);
                let obj = JSON.parse(a); //这里解析json
                console.log(obj);
            }


if (httpRequest.readyState == 4 && httpRequest.status == 200){}

要等到完成请求后再解析，加上判断条件