get a JSON array of mp3 tags. Also gets pictures out. Pretty hacky.

id3 v2 reader

var crypto = require("crypto");

var readId3 = function(buff){
    var out = {};
    if (String.fromCharCode(buff[0], buff[1], buff[2]) != "ID3"){
        throw ("no id3 v2 tag")
    }
    out.version = "v2."+ buff[3] + "." + buff[4];
    //flags
    var flags = buff[5]; 
    if (flags !== 0){throw ("tags with flags not supported");}
    
    var size = (buff[6] << 24) | (buff[7] << 16) | (buff[8] << 8) | (buff[9]);
    //console.log("id3 tag size is ", size);
    var i = 10;
    while (i-size<-255){
	var tn = buff.toString("utf8", i,i+4);
	if (tn.substring(0,1) != "T" && tn.substring(0,1) != "A"){break;}
	//clearly this can be much greater than the total tag size.
	//this is needed for the case where there is a picture. need to tidy this to identify accurately when we've finished reading tags.
	//by checking for footer.
        var framesize = (buff[i+4] << 24) | (buff[i+5] << 16) | (buff[i+6] << 8) | (buff[i+7]);
	if (tn != "APIC"){
	    var tv = buff.toString("utf8", i+11,i+10+framesize);
	    out[tn] = tv;
	}
	else{
	    //console.log("found picture tag");
	    out.APIC = pic(buff.slice(i+11, i+10+framesize));
	}
        i= i+10+framesize;
    }
    out = readmp3(buff, out);
    return out;
}

var pic = function(buff){
    var out = {};
    var i=0;
    while (buff[i] != 0){i++;}
    out.mime = buff.toString("utf8", 0, i);
    
    // i don't care about this value. but interestingly, according to the spec, if this value is 17 (or hex 11), the picture
    // is of type "A bright coloured fish" Who knew.
    out.picType = buff[i++]; 
    i++;
    var j = i;
    while (buff[i] != 0){i++;}
    out.description = buff.toString("utf8",j,i);
    out.picture = buff.slice(i+1);
    out.hash = crypto.createHash("md5").update(out.picture).digest("hex");
    return out;
};
var bitRates =[0,32000,40000,48000,56000,64000,80000,96000,112000,128000,160000,192000,224000,256000,320000,"bad"];
var sampleRates = [44100,48000,32000,"reserved"];
var channels = ["Stereo","Joint Stereo", "Dual", "Mono"]

var readmp3 = function(buff, out){
    for (var i=0; i<buff.length; i++){
	if (buff[i] == 255 && buff[i+1].toString(2).substring(0,7)=="1111101"){
	    var f = buff[i+1].toString(2);
	    if (f.substring(3,5) !="11" || f.substring(5,7) !="01"){
		throw ("not an mp3 file, in probability ")
	    }
	    var f = buff[i+2].toString(2);
	    out.SampleRate = sampleRates[parseInt(f.substring(4,6),2)]
	    out.VBR = false;
	    if (isVBR(buff, i)){
		out.VBR = true;
		var x = i  + (buff.toString('utf8', i, i+200).indexOf("Xing"))
		/*for (j=1; j<200; j++){
		    console.log(j, buff[x+j], String.fromCharCode(buff[x+j]));
		}*/
		/*
		var g = buff[x+7].toString(2);
		console.log("flags", g);
		var hasFrames = g.substring(0,1) == "1";
		var hasBytes = g.substring(1,2) == "1";
		var hasToc = g.substring(2,3) == "1";
		var hasScale = g.substring(3,4) == "1";
		var y = 0;
		*/
		x = x+8;
		var framecount = (buff[x] << 24) | (buff[x+1] << 16) | (buff[x+2] << 8) | (buff[x+3]);
		
		//nb this works for STEREO MP3
		out.Duration =  Math.floor((1152 * framecount)/out.SampleRate);
	    }
	    else{
		out.Bitrate = bitRates[parseInt(f.substring(0,4),2)]
		out.Duration = Math.floor((buff.length -i)/(out.Bitrate/8));
	    }
	    var f = buff[i+3].toString(2);
	    out.Channel = channels[parseInt(f.substring(0,2),2)];
	    break;
	}
    }
    
    return out;
}
//nb only works if Xing frame is found - which is normal.
var isVBR = function(buff, i){
    return buff.toString('utf8', i, i+100).indexOf("Xing") > -1;
}

//USAGE
var fs = require("fs");
fs.readFile("/Volumes/MEDIA/Music/Vocal/Bonnie Prince Billy/Little Joya/02 - Joya.mp3", function(err, data){
    var tags = readId3(data);
    if (tags.APIC){
	var ext=".jpg";
	switch (tags.APIC.mime){
	    case "image/png":
		ext = ".png";
		break;
	    case "image/gif":
		ext=".gif";
		break;
	}
	//i like writing filenames using the md5 hash - means i can guarantee uniqueness
	//you may wish to use tags.APIC.description which is normally the name of the file used to create the tag.
	fs.writeFile(tags.APIC.hash +".gif", tags.APIC.picture, function(err){
	    delete tags.APIC.picture;
	    console.log(JSON.stringify(tags));
	});
    }
});

Made my head hurt by going through MP3 header frames to dig out bit rate, sample rate, and calculate duration.
Needs some stress testing!