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    <p>Here's how I did it. </p> <p>This technique is provided in an answer to another SO question, but it's also relevant here. </p> <p>I didn't want to base64 encode anything. I wanted to download and parse binary files in the browser via Javascript, without modifying the server to encode them specially. I found that in Firefox, by coercing the mimetype of the response via <code>overrideMimeType()</code>, I could use <code>XMLHttpRequest.responseText</code>. On IE, it's different because:</p> <ul> <li><p><code>responseText</code> on IE truncates at the first zero. For binary streams this is a big problem.</p></li> <li><p>there is no <code>XMLHttpRequest.overrideMimeType()</code>, to force IE to treat binary streams as text.</p></li> <li><p>while there is a <code>XMLHttpRequest.responseBody</code> (IE only!) that is specifically designed to be used with binary data streams, maddeningly that property is not usable from Javascript. </p></li> </ul> <p>Therefore, the need is to convert IE's <code>responseBody</code> property into a thing that looks like <code>responseText</code> from FireFox, with the mime-type coercion. This is possible using injected VBScript. </p> <p>To make it cross-browser, you need to just pack up the browser-specific logic in a conditional. This is what I used:</p> <pre><code>// one-time code if(/msie/i.test(navigator.userAgent) &amp;&amp; !/opera/i.test(navigator.userAgent)) { var IEBinaryToArray_ByteStr_Script = "&lt;!-- IEBinaryToArray_ByteStr --&gt;\r\n"+ "&lt;script type='text/vbscript'&gt;\r\n"+ "Function IEBinaryToArray_ByteStr(Binary)\r\n"+ " IEBinaryToArray_ByteStr = CStr(Binary)\r\n"+ "End Function\r\n"+ "Function IEBinaryToArray_ByteStr_Last(Binary)\r\n"+ " Dim lastIndex\r\n"+ " lastIndex = LenB(Binary)\r\n"+ " if lastIndex mod 2 Then\r\n"+ " IEBinaryToArray_ByteStr_Last = Chr( AscB( MidB( Binary, lastIndex, 1 ) ) )\r\n"+ " Else\r\n"+ " IEBinaryToArray_ByteStr_Last = "+'""'+"\r\n"+ " End If\r\n"+ "End Function\r\n"+ "&lt;/script&gt;\r\n"; // inject VBScript document.write(IEBinaryToArray_ByteStr_Script); } // each time you make a request for a binary resource: var req = (function() { if (window.XMLHttpRequest) { return new window.XMLHttpRequest(); } else { try { return new ActiveXObject("MSXML2.XMLHTTP"); } catch(ex) { return null; } } })(); var fileContents = ""; var filesize = -1; var readByteAt = function(i){ return fileContents.charCodeAt(i) &amp; 0xff; }; req.open("GET", url, true); if(/msie/i.test(navigator.userAgent) &amp;&amp; !/opera/i.test(navigator.userAgent)) { // IE-specific logic here // helper to convert from responseBody to a "responseText" like thing var convertResponseBodyToText = function (binary) { var byteMapping = {}; for ( var i = 0; i &lt; 256; i++ ) { for ( var j = 0; j &lt; 256; j++ ) { byteMapping[ String.fromCharCode( i + j * 256 ) ] = String.fromCharCode(i) + String.fromCharCode(j); } } var rawBytes = IEBinaryToArray_ByteStr(binary); var lastChr = IEBinaryToArray_ByteStr_Last(binary); return rawBytes.replace(/[\s\S]/g, function( match ) { return byteMapping[match]; }) + lastChr; }; req.setRequestHeader("Accept-Charset", "x-user-defined"); req.onreadystatechange = function(event){ if (req.readyState == 4) { if (req.status == 200) { fileContents = convertResponseBodyToText(req.responseBody); fileSize = fileContents.length-1; // invoke a callback here, if you like... } else{ alert("download failed, status " + req.status); } } }; req.send(); } else { // ff/Gecko/Webkit specific stuff here req.onreadystatechange = function(aEvt) { if (req.readyState == 4) { // completed if(req.status == 200){ // status == OK fileContents = binStream.req.responseText; filesize = fileContents.length; // invoke a callback here, if you like... } else { alert("download failed, status " + req.status); } } }; // coerce response type req.overrideMimeType('text/plain; charset=x-user-defined'); req.send(null); } </code></pre> <p>...then call <code>readByte(i)</code> to get the byte at the <em>ith</em> position in the binary file.</p> <p>Good luck.</p> <p><a href="http://miskun.com/javascript/internet-explorer-and-binary-files-data-access/">Credit to Miskun</a> for the VBScript conversion logic.</p>
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