Javascript implementation of UTF-8 decoding algorithm by Björn Höhrmann explained at http://bjoern.hoehrmann.de/utf-8/decoder/dfa/. To support codepoints outside the Unicode BMP, this algorithm uses the custom String.fromCodePoint method explained at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fromChar…

gistfile1.js

var UTF8_ACCEPT = 0;
var UTF8D = [
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
    0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,  0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
    1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,  9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,
    7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,  7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
    8,8,2,2,2,2,2,2,2,2,2,2,2,2,2,2,  2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,
    10,3,3,3,3,3,3,3,3,3,3,3,3,4,3,3, 11,6,6,6,5,8,8,8,8,8,8,8,8,8,8,8,
    0,12,24,36,60,96,84,12,12,12,48,72, 12,12,12,12,12,12,12,12,12,12,12,12,
    12, 0,12,12,12,12,12, 0,12, 0,12,12, 12,24,12,12,12,12,12,24,12,24,12,12,
    12,12,12,12,12,12,12,24,12,12,12,12, 12,24,12,12,12,12,12,12,12,24,12,12,
    12,12,12,12,12,12,12,36,12,36,12,12, 12,36,12,12,12,12,12,36,12,36,12,12,
    12,36,12,12,12,12,12,12,12,12,12,12,
];

function decode(utf8text) {
    var state = UTF8_ACCEPT;
    var codep = 0;
    var codepoints = [];

    for (var i=0; i < utf8text.length; i++) {
        byte = utf8text.charCodeAt(i);
        type = UTF8D[byte];
        codep = (state != UTF8_ACCEPT) ?
            (byte & 0x3f) | (codep << 6) : (0xff >> type) & (byte);
        state = UTF8D[256 + state + type];
        if (state == UTF8_ACCEPT)
            codepoints.push(codep);
    }
    return String.fromCodePoint.apply(null, codepoints);
}