# Encoding XHR image data

## JavaScript performance comparison

Revision 33 of this test case created by

## 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>
``````

<img id="image">

## Test runner

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Testing in CCBot 2.0.0 / Other 0.0.0
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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