Load binary file in JavaScript and convert to base64

var url = 'file:///c/temp/file.xlsx';
var xhr = new XMLHttpRequest();
xhr.open("get", url, true);
xhr.onreadystatechange = function () {
    if (xhr.readyState == 4 && xhr.status == 200) {
        var blob = new Blob([xhr.response], {
                type: "octet/stream"
            });
        var reader = new window.FileReader();
        reader.readAsDataURL(blob);
        reader.onloadend = function () {
            base64data = reader.result;
            console.log(base64data);
        }
    }
}
xhr.responseType = "arraybuffer";
xhr.send();