Get binary data with XHR in most browsers including IE8+

xhrBinary.js

/*!
 * xhrBinary.js
 * Author: Jared Abbott
 * Copyright 2015 Jared Abbott
 * Distributed under the MIT license
 */
var xhrBinary = function(url, fn) {

    // RESOURCES
    // http://mgran.blogspot.com/2006/08/downloading-binary-streams-with.html
    // http://miskun.com/javascript/internet-explorer-and-binary-files-data-access/
    // http://stackoverflow.com/questions/9267899/arraybuffer-to-base64-encoded-string
    // https://msdn.microsoft.com/en-us/library/y39d47w8(v=VS.85).aspx
    // https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Sending_and_Receiving_Binary_Data
    
    // USAGE
    // xhrBinary(url, function(data){
    //     // Encode the string w/ Base64
    //     var base64 = 'data:application/octet-stream;base64,'+btoa(data);
    // });

    var request = window.XMLHttpRequest || ActiveXObject;
    var xhr = new request("MSXML2.XMLHTTP.3.0");
    var data = '';
    var buffer = '';
    var bufferLength = 0;

    // Retrieve the file
    xhr.open('GET', url, true);

    // XMLHttpRequest2-compliant browsers
    if (!!window.Uint8Array) {
        xhr.responseType = "arraybuffer";
    }
    // XMLHttpRequest-compliant browsers
    else if ('overrideMimeType' in xhr) {
        xhr.overrideMimeType('text\/plain; charset=x-user-defined');
    }
    // Microsoft.XMLHTTP-compliant browsers [IE9]
    else {
        xhr.setRequestHeader('Accept-Charset', 'x-user-defined');
    }

    xhr.onreadystatechange = function(e) {

        if (this.readyState == 4 && this.status == 200) {

            // Handles most browsers with a fallback for IE9
            if (!!window.Uint8Array || !!window.VBArray) {

                // In IE9 and below the responseBody property returns a byte array in VBArray format 
                // VBArray is not directly usable 'as is' with JavaScript.
                // In IE9+ we can access the bytes by converting the VBArray to a JS-compatible array
                // with the VBArray.toArray() method.
                buffer = (!!window.Uint8Array) ? new Uint8Array(xhr.response) : xhr.responseBody.toArray();
                data = String.fromCharCode.apply(null, buffer);
            }
            // IE8
            else if (!!window.execScript) {

               buffer = xhr.responseBody;

               var parseVBArray = function (buffer) {

                    // VBScript
                    var VBScript = 
                         "Function binToArrByteStr(buffer)\r\n"+
                         "   binToArrByteStr = CStr(buffer)\r\n"+
                         "End Function\r\n"+
                         "Function binToArrByteStr_Last(buffer)\r\n"+
                         "   Dim lastIndex\r\n"+
                         "   lastIndex = LenB(buffer)\r\n"+
                         "   if lastIndex mod 2 Then\r\n"+
                         "       binToArrByteStr_Last = AscB( MidB( buffer, lastIndex, 1 ) )\r\n"+
                         "   Else\r\n"+
                         "       binToArrByteStr_Last = -1\r\n"+
                         "   End If\r\n"+
                         "End Function\r\n";

                    // Execute VBScript
                    window.execScript(VBScript);

                    // Mapper
                    var byteMapping = {};
                    for ( var i = 0; i < 256; i++ ) {
                         for ( var j = 0; j < 256; j++ ) {
                             byteMapping[String.fromCharCode(i + j * 256)] = String.fromCharCode(i) + String.fromCharCode(j);
                         }
                    }

                    // Data
                    var rawBytes = binToArrByteStr(buffer);
                    var lastChr = binToArrByteStr_Last(buffer);
                    return rawBytes.replace(/[\s\S]/g, function( match ) { return byteMapping[match]; }) + lastChr;
               }

               data = parseVBArray(buffer);
            }
            // Other browsers
            else {
                buffer = xhr.responseText;
                bufferLength = buffer.length;
                for (var i = 0, len = bufferLength; i < len; ++i) {

                    // Throw away high-order bytes at offset i (f7)
                    data += String.fromCharCode(buffer.charCodeAt(i) & 255) // 255 === 0xff
                }
            }

            // Return the binary
            if (typeof fn !== 'undefined') {
                fn(data);
            } else {
                return data;
            }
        }
    };

    xhr.send(null);
}