检测浏览器是否不支持具有完整CORS支持的XMLHttpRequest

Question

How do I check if a browser supports the native JavaScript XMLHttpRequest object with CORS? I am testing in IE 8 which as I understood only supports XDomainRequest , however doing the following in IE 8:

typeof window.XMLHttpRequest
"object"

I would have thought that would be undefined . Basically how can I detect if a browser supports the full XMLHttpRequest with CORS support?

Should I do the inverse?

if(type window.XDomainRequest === 'object') {
    // Assume the browser does not support XMLHttpRequest with CORS
}

Answer 1

I use this, 1 means XMLHttpRequest support cors, 2 means cors is supported via XDomainRequest, 0 means no CORS support

var  corsSupportLevel = null;    
function supportsCorsInternal() {

                if (corsSupportLevel !== null) return corsSupportLevel;

                var xhr = new XMLHttpRequest();
                if (typeof xhr.withCredentials !== 'undefined') {
                    // Supports CORS
                    corsSupportLevel = 1;
                } else if (typeof XDomainRequest !== "undefined") {
                    // IE
                    corsSupportLevel = 2;
                } else {
                    corsSupportLevel = 0;
                }
                return corsSupportLevel;
            }