# Encoding XHR image data

## JavaScript performance comparison

Revision 6 of this test case created by John-David Dalton

## Preparation code

``<img id="image"><script>  function arrayBufferDataUri(raw) {   var base64 = '';   var encodings = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';   var bytes = new Uint8Array(raw);   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 < mainLength; i = i + 3) {    // Combine the three bytes into a single integer    chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2];    // Use bitmasks to extract 6-bit segments from the triplet    a = (chunk & 16515072) >> 18; // 16515072 = (2^6 - 1) << 18    b = (chunk & 258048) >> 12; // 258048 = (2^6 - 1) << 12    c = (chunk & 4032) >> 6; // 4032 = (2^6 - 1) << 6    d = chunk & 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 & 252) >> 2 // 252 = (2^6 - 1) << 2;    // Set the 4 least significant bits to zero    b = (chunk & 3) << 4 // 3 = 2^2 - 1;    base64 += encodings[a] + encodings[b] + '==';   }   else if (byteRemainder == 2) {    chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1];    a = (chunk & 16128) >> 8 // 16128 = (2^6 - 1) << 8;    b = (chunk & 1008) >> 4 // 1008 = (2^6 - 1) << 4;    // Set the 2 least significant bits to zero    c = (chunk & 15) << 2 // 15 = 2^4 - 1;    base64 += encodings[a] + encodings[b] + encodings[c] + '=';   }   return "data:image/jpeg;base64," + base64;  }    // Get the image into the cache as we are not testing the network speed  var req1 = new XMLHttpRequest;  req1.overrideMimeType('text/plain; charset=x-user-defined');  req1.open('GET', 'http://jonathanleighton.com/images/me.jpg', false);  req1.send(null);    var req2 = new XMLHttpRequest;  req2.overrideMimeType('text/plain; charset=x-user-defined');  req2.open('GET', 'http://jonathanleighton.com/images/me.jpg', false);  req2.responseType = 'arraybuffer';  req2.send(null);    Benchmark.prototype.setup = function() {   var req1 = window.req1,       req2 = window.req2,       image = document.getElementById('image');  };</script>``

## Test runner

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Testing in unknown unknown
Test Ops/sec
charCodeAt
``var binary = '';for (var i = 0, text = req1.responseText, l = text.length; i < l; i++) { binary += String.fromCharCode(text.charCodeAt(i) & 0xff);}image.src = "data:image/jpeg;base64," + window.btoa(binary);``
pending…
ArrayBuffer + window.btoa
``var binary = '';var buffer = req2.mozResponseArrayBuffer || req2.response;var bytes = new Uint8Array(buffer);for (var i = 0, l = bytes.byteLength; i < l; i++) { binary += String.fromCharCode(bytes[i]);}image.src = "data:image/jpeg;base64," + window.btoa(binary);``
pending…
ArrayBuffer direct to base64
``image.src = arrayBufferDataUri(req2.mozResponseArrayBuffer || req2.response);``
pending…

## Revisions

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# 1 comment

Chris Cinelli commented :

The if branch "else if (byteRemainder == 2) {" has a bug on variable "a"

This is one quick solution:

else if (byteRemainder == 2) { chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) ; // Use bitmasks to extract 6-bit segments from the triplet a = (chunk & 16515072) >> 18; // 16515072 = (2^6 - 1) << 18 b = (chunk & 258048) >> 12; // 258048 = (2^6 - 1) << 12 c = (chunk & 4032) >> 6; // 4032 = (2^6 - 1) << 6 base64 += encodings[a] + encodings[b] + encodings[c] + '='; }

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