var s3Error = (new window.DOMParser()).parseFromString(request.response, "text/xml"); var errMsg = s3Error.firstChild.children[1].innerHTML;

These two lines of code are generating an error in IE11. I know that browser sucks, but I have to support it. Not sure if you can build a workaround for the DOMParser, but it says that the children is undefined. This is in all 4 places that it is used in the code.