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    <p>So I'm answering to myself — and sorry for that — but I think it might be useful for someone as lost as I was ;)</p> <p>So you have to use <a href="https://developer.mozilla.org/en/JavaScript_typed_arrays/ArrayBuffer" rel="noreferrer">ArrayBuffer</a> and set the <code>responseType</code> property of your <code>XMLHttpRequest</code> object instance to <code>arraybuffer</code> for retrieving a native array of Bytes, which can be converted to base64 using the following convenient function (found <a href="https://gist.github.com/958841" rel="noreferrer">there</a>, author may be blessed here):</p> <pre><code>function base64ArrayBuffer(arrayBuffer) { var base64 = '' var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/' var bytes = new Uint8Array(arrayBuffer) var byteLength = bytes.byteLength var byteRemainder = byteLength % 3 var mainLength = byteLength - byteRemainder var a, b, c, d var chunk // Main loop deals with bytes in chunks of 3 for (var i = 0; i &lt; mainLength; i = i + 3) { // Combine the three bytes into a single integer chunk = (bytes[i] &lt;&lt; 16) | (bytes[i + 1] &lt;&lt; 8) | bytes[i + 2] // Use bitmasks to extract 6-bit segments from the triplet a = (chunk &amp; 16515072) &gt;&gt; 18 // 16515072 = (2^6 - 1) &lt;&lt; 18 b = (chunk &amp; 258048) &gt;&gt; 12 // 258048 = (2^6 - 1) &lt;&lt; 12 c = (chunk &amp; 4032) &gt;&gt; 6 // 4032 = (2^6 - 1) &lt;&lt; 6 d = chunk &amp; 63 // 63 = 2^6 - 1 // Convert the raw binary segments to the appropriate ASCII encoding base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d] } // Deal with the remaining bytes and padding if (byteRemainder == 1) { chunk = bytes[mainLength] a = (chunk &amp; 252) &gt;&gt; 2 // 252 = (2^6 - 1) &lt;&lt; 2 // Set the 4 least significant bits to zero b = (chunk &amp; 3) &lt;&lt; 4 // 3 = 2^2 - 1 base64 += encodings[a] + encodings[b] + '==' } else if (byteRemainder == 2) { chunk = (bytes[mainLength] &lt;&lt; 8) | bytes[mainLength + 1] a = (chunk &amp; 64512) &gt;&gt; 10 // 64512 = (2^6 - 1) &lt;&lt; 10 b = (chunk &amp; 1008) &gt;&gt; 4 // 1008 = (2^6 - 1) &lt;&lt; 4 // Set the 2 least significant bits to zero c = (chunk &amp; 15) &lt;&lt; 2 // 15 = 2^4 - 1 base64 += encodings[a] + encodings[b] + encodings[c] + '=' } return base64 } </code></pre> <p>So here's a working code:</p> <pre><code>var xhr = new XMLHttpRequest(); xhr.open('GET', 'http://some.tld/favicon.png', false); xhr.responseType = 'arraybuffer'; xhr.onload = function(e) { console.log(base64ArrayBuffer(e.currentTarget.response)); }; xhr.send(); </code></pre> <p>This will log a <em>valid</em> base64 encoded string representing the binary file contents.</p> <p><em>Edit:</em> For older browsers not having access to <code>ArrayBuffer</code> and having <code>btoa()</code> failing on encoding characters, here's another way to get a base64 encoded version of any binary:</p> <pre><code>function getBinary(file){ var xhr = new XMLHttpRequest(); xhr.open("GET", file, false); xhr.overrideMimeType("text/plain; charset=x-user-defined"); xhr.send(null); return xhr.responseText; } function base64Encode(str) { var CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; var out = "", i = 0, len = str.length, c1, c2, c3; while (i &lt; len) { c1 = str.charCodeAt(i++) &amp; 0xff; if (i == len) { out += CHARS.charAt(c1 &gt;&gt; 2); out += CHARS.charAt((c1 &amp; 0x3) &lt;&lt; 4); out += "=="; break; } c2 = str.charCodeAt(i++); if (i == len) { out += CHARS.charAt(c1 &gt;&gt; 2); out += CHARS.charAt(((c1 &amp; 0x3)&lt;&lt; 4) | ((c2 &amp; 0xF0) &gt;&gt; 4)); out += CHARS.charAt((c2 &amp; 0xF) &lt;&lt; 2); out += "="; break; } c3 = str.charCodeAt(i++); out += CHARS.charAt(c1 &gt;&gt; 2); out += CHARS.charAt(((c1 &amp; 0x3) &lt;&lt; 4) | ((c2 &amp; 0xF0) &gt;&gt; 4)); out += CHARS.charAt(((c2 &amp; 0xF) &lt;&lt; 2) | ((c3 &amp; 0xC0) &gt;&gt; 6)); out += CHARS.charAt(c3 &amp; 0x3F); } return out; } console.log(base64Encode(getBinary('http://www.google.fr/images/srpr/logo3w.png'))); </code></pre> <p>Hope this helps others as it did for me.</p>
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    1. COSecond solution ("For older browsers") works in phantomjs (webkit). Thanks a lot :)
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    2. COThank you, thank you, thank you, thank you for your edit regarding older browsers! I can assure you, the excitement grows with the time looking for an answer and then finding it! \*hugs\*
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