gistfile1.js

// Converts an ArrayBuffer directly to base64, without any intermediate 'convert to string then
// use window.btoa' step. According to my tests, this appears to be a faster approach:
// http://jsperf.com/encoding-xhr-image-data/5

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;
    for (var i = 0; i < mainLength; i = i + 3) {
        chunk = bytes[i] << 16 | bytes[i + 1] << 8 | bytes[i + 2];
        a = (chunk & 16515072) >> 18;
        b = (chunk & 258048) >> 12;
        c = (chunk & 4032) >> 6;
        d = chunk & 63;
        base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d];
    }
    if (byteRemainder == 1) {
        chunk = bytes[mainLength];
        a = (chunk & 252) >> 2;
        b = (chunk & 3) << 4;
        base64 += encodings[a] + encodings[b] + '==';
    } else if (byteRemainder == 2) {
        chunk = bytes[mainLength] << 8 | bytes[mainLength + 1];
        a = (chunk & 64512) >> 10;
        b = (chunk & 1008) >> 4;
        c = (chunk & 15) << 2;
        base64 += encodings[a] + encodings[b] + encodings[c] + '=';
    }
    return base64;
}