pure JavaScript jpeg quality value recover

getJpegQuality.js

// javascript jpeg decoder: https://github.com/eugeneware/jpeg-js/blob/79c4c7ea876d2e7b46981a563fcc4ae8f9e19849/lib/decoder.js
// Determine the JPEG compression quality from the quantization tables: https://github.com/ImageMagick/ImageMagick/blob/59f28acff016f6ae5e8b7e6300bcdf97e94affc9/coders/jpeg.c#L879
// JPEG File Layout and Format: http://vip.sugovica.hu/Sardi/kepnezo/JPEG%20File%20Layout%20and%20Format.htm
// jpeg constants: https://github.com/LuaDist/libjpeg/blob/6c0fcb8ddee365e7abc4d332662b06900612e923/jpeglib.h#L45

function getQuantizationTables(data) {

	var quantizationTables = []

	var dctZigZag = new Int32Array([
		0,
		1,  8,
		16,  9,  2,
		3, 10, 17, 24,
		32, 25, 18, 11, 4,
		5, 12, 19, 26, 33, 40,
		48, 41, 34, 27, 20, 13,  6,
		7, 14, 21, 28, 35, 42, 49, 56,
		57, 50, 43, 36, 29, 22, 15,
		23, 30, 37, 44, 51, 58,
		59, 52, 45, 38, 31,
		39, 46, 53, 60,
		61, 54, 47,
		55, 62,
		63
	]);


	var offset = 0, length = data.length;

	function readUint16() {
		var value = (data[offset] << 8) | data[offset + 1];
		offset += 2;
		return value;
	}

	var fileMarker = readUint16();
	if (fileMarker != 0xFFD8) { // SOI (Start of Image)
		throw new Error("SOI not found");
	}

	fileMarker = readUint16();

	markerLoop:
	while (fileMarker != 0xFFD9) { // EOI (End of image)
		var i, j, l;
		switch(fileMarker) {
			case 0xFF00: break;
			case 0xFFE0: // APP0 (Application Specific)
			case 0xFFE1: // APP1
			case 0xFFE2: // APP2
			case 0xFFE3: // APP3
			case 0xFFE4: // APP4
			case 0xFFE5: // APP5
			case 0xFFE6: // APP6
			case 0xFFE7: // APP7
			case 0xFFE8: // APP8
			case 0xFFE9: // APP9
			case 0xFFEA: // APP10
			case 0xFFEB: // APP11
			case 0xFFEC: // APP12
			case 0xFFED: // APP13
			case 0xFFEE: // APP14
			case 0xFFEF: // APP15
			case 0xFFFE: // COM (Comment)
			// ---
			case 0xFFC0: // SOF0 (Start of Frame, Baseline DCT)
			case 0xFFC1: // SOF1 (Start of Frame, Extended DCT)
			case 0xFFC2: // SOF2 (Start of Frame, Progressive DCT)
			case 0xFFC4: // DHT (Define Huffman Tables)
			case 0xFFDD: // DRI (Define Restart Interval)

				offset += readUint16(); // skip
				break;

			case 0xFFDB: // DQT (Define Quantization Tables)
				var quantizationTablesLength = readUint16();
				var quantizationTablesEnd = quantizationTablesLength + offset - 2;
				while (offset < quantizationTablesEnd) {
					var quantizationTableSpec = data[offset++];
					var tableData = new Int32Array(64);
					if ((quantizationTableSpec >> 4) === 0) { // 8 bit values
						for (j = 0; j < 64; j++) {
							var z = dctZigZag[j];
							tableData[z] = data[offset++];
						}
					} else if ((quantizationTableSpec >> 4) === 1) { //16 bit
						for (j = 0; j < 64; j++) {
							var z = dctZigZag[j];
							tableData[z] = readUint16();
						}
					} else
						throw new Error("DQT: invalid table spec");
					quantizationTables[quantizationTableSpec & 15] = tableData;
				}
				break;

			case 0xFFDA: // SOS (Start of Scan)
				break markerLoop;

			case 0xFFFF: // Fill bytes
				if (data[offset] !== 0xFF) { // Avoid skipping a valid marker.
					offset--;
				}
				break;

			default:
				if (data[offset - 3] == 0xFF &&
						data[offset - 2] >= 0xC0 && data[offset - 2] <= 0xFE) {
					// could be incorrect encoding -- last 0xFF byte of the previous
					// block was eaten by the encoder
					offset -= 3;
					break;
				}
				throw new Error("unknown JPEG marker " + fileMarker.toString(16));
		}
		fileMarker = readUint16();
	}

	return quantizationTables;
}


const DCTSIZE2 = 64
const NUM_QUANT_TBLS = 4

const hash_q01 = [ 1020, 1015,  932,  848,  780,  735,  702,  679,  660,  645, 632,  623,  613,  607,  600,  594,  589,  585,  581,  571, 555,  542,  529,  514,  494,  474,  457,  439,  424,  410, 397,  386,  373,  364,  351,  341,  334,  324,  317,  309, 299,  294,  287,  279,  274,  267,  262,  257,  251,  247, 243,  237,  232,  227,  222,  217,  213,  207,  202,  198, 192,  188,  183,  177,  173,  168,  163,  157,  153,  148, 143,  139,  132,  128,  125,  119,  115,  108,  104,   99, 94,   90,   84,   79,   74,   70,   64,   59,   55,   49, 45,   40,   34,   30,   25,   20,   15,   11,    6,    4, 0 ];
const sums_q01 = [ 32640, 32635, 32266, 31495, 30665, 29804, 29146, 28599, 28104, 27670, 27225, 26725, 26210, 25716, 25240, 24789, 24373, 23946, 23572, 22846, 21801, 20842, 19949, 19121, 18386, 17651, 16998, 16349, 15800, 15247, 14783, 14321, 13859, 13535, 13081, 12702, 12423, 12056, 11779, 11513, 11135, 10955, 10676, 10392, 10208, 9928,  9747,  9564,  9369,  9193,  9017,  8822,  8639,  8458, 8270,  8084,  7896,  7710,  7527,  7347,  7156,  6977,  6788, 6607,  6422,  6236,  6054,  5867,  5684,  5495,  5305,  5128, 4945,  4751,  4638,  4442,  4248,  4065,  3888,  3698,  3509, 3326,  3139,  2957,  2775,  2586,  2405,  2216,  2037,  1846, 1666,  1483,  1297,  1109,   927,   735,   554,   375,   201, 128,     0 ];
const hash_q0 = [ 510,  505,  422,  380,  355,  338,  326,  318,  311,  305, 300,  297,  293,  291,  288,  286,  284,  283,  281,  280, 279,  278,  277,  273,  262,  251,  243,  233,  225,  218, 211,  205,  198,  193,  186,  181,  177,  172,  168,  164, 158,  156,  152,  148,  145,  142,  139,  136,  133,  131, 129,  126,  123,  120,  118,  115,  113,  110,  107,  105, 102,  100,   97,   94,   92,   89,   87,   83,   81,   79, 76,   74,   70,   68,   66,   63,   61,   57,   55,   52, 50,   48,   44,   42,   39,   37,   34,   31,   29,   26, 24,   21,   18,   16,   13,   11,    8,    6,    3,    2, 0 ];
const sums_q0 = [ 16320, 16315, 15946, 15277, 14655, 14073, 13623, 13230, 12859, 12560, 12240, 11861, 11456, 11081, 10714, 10360, 10027,  9679, 9368,  9056,  8680,  8331,  7995,  7668,  7376,  7084,  6823, 6562,  6345,  6125,  5939,  5756,  5571,  5421,  5240,  5086, 4976,  4829,  4719,  4616,  4463,  4393,  4280,  4166,  4092, 3980,  3909,  3835,  3755,  3688,  3621,  3541,  3467,  3396, 3323,  3247,  3170,  3096,  3021,  2952,  2874,  2804,  2727, 2657,  2583,  2509,  2437,  2362,  2290,  2211,  2136,  2068, 1996,  1915,  1858,  1773,  1692,  1620,  1552,  1477,  1398, 1326,  1251,  1179,  1109,  1031,   961,   884,   814,   736, 667,   592,   518,   441,   369,   292,   221,   151,    86, 64,     0 ]

function getQualityFromQuantizationTables(quantizationTables) {

	var i, j;
	var sum = 0;
	for ( i=0; i < NUM_QUANT_TBLS; i++ ) {

		if ( quantizationTables[i] )
			for ( j=0; j < DCTSIZE2; j++ )
				sum += quantizationTables[i][j];
	}

	if ( quantizationTables[0] && quantizationTables[1] ) {

		var qvalue = quantizationTables[0][2] + quantizationTables[0][53] + quantizationTables[1][0] + quantizationTables[1][DCTSIZE2-1];
		for ( i=0; i < 100; i++ ) {

			if ( (qvalue < hash_q01[i]) && (sum < sums_q01[i]) )
				continue;
			if ( ((qvalue <= hash_q01[i]) && (sum <= sums_q01[i])) || (i >= 50) )
				return i+1;
			break;
		}
	} else
	if ( quantizationTables[0] ) {

		var qvalue = quantizationTables[0][2] + quantizationTables[0][53];
		for ( i=0; i < 100; i++ ) {

			if ((qvalue < hash_q0[i]) && (sum < sums_q0[i]))
				continue;
			if (((qvalue <= hash_q0[i]) && (sum <= sums_q0[i])) || (i >= 50))
				return i+1;
			break;
		}
	}

	return undefined;
}


module.exports = function getJpegQuality(jpegData) {

	// the Buffer class implements the Uint8Array API.
	return getQualityFromQuantizationTables(getQuantizationTables(jpegData));
}

OMG thnx mate! =)

Really nice!