Javascript 本地调用时，AJAX返回响应为0，状态为4

我正在尝试实现一个网站，该网站通过XMLHttpRequest调用本地php文件以获取json数据。正在从HTML表单调用登录函数。文件/var/www/picnit/api/member.api确实存在，并且具有足够的权限，并且已经使用Chrome的Postman应用程序进行了测试

//URL for member functions
var memberurl='/picnit/api/member.php';

//Request to be sent to the middleware
var request;

function login() {
    //Get and validate user inputs
    var username = $("input#Username").val();
    var password = $("input#Password").val();
    //Validation code omitted

    //Getting here means that the inputs validated

    //Request to be sent to the middleware
    request = new XMLHttpRequest();

    //Create function that handles response
    request.onreadystatechange = function () {
        //Make sure request is ready, we only care about when done
        if(request.readyState === 4) {
            //Good data, proceed to login
            if(request.status === 200) {
                alert('200');
            }

            //Invalid username/password combo
            else if(request.status === 204) {
                alert('204');
            }

            //Our request messed up
            else if(request.status === 400) {
                alert('400');
            }

            //Something else went wrong
            else {
                alert('Unknown error: ' + request.status);
            }
        }
    };

    //Open request
    request.open('POST', memberurl, true);

    //Send request
    request.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    var parms = 'username='+username+'&password='+password;
    request.send(parms);

    return false;
}

这总是返回一个带有未知错误的警报：0，我无法找出原因。我不认为它可以跨域，因为它在同一台服务器上，一个AWS EC2实例。我不完全确定什么是相同的域策略

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我一直在这里搜索其他帖子，还没有找到答案。感谢您在advanced中的帮助。

您是从中打开此页面的吗http://localhost/ 或file:///var/www/?