Determine the key of a song by its chords

Here's what I came up with. Still new with modern JS so apologies for messiness and bad use of map().

I looked around the internals of the tonal library, it has a function scales.detect(), but it was no good since it required every note present. Instead I used it as inspiration and flattened the progression into a simple note list and checked this in all transpositions as a subset of all the possible scales.

const _ = require('lodash');
const chord = require('tonal-chord');
const note = require('tonal-note');
const pcset = require('tonal-pcset');
const dictionary = require('tonal-dictionary');
const SCALES = require('tonal-scale/scales.json');
const dict = dictionary.dictionary(SCALES, function (str) { return str.split(' '); });

//dict is a dictionary of scales defined as intervals
//notes is a string of tonal notes eg 'c d eb'
//onlyMajorMinor if true restricts to the most common scales as the tonal dict has many rare ones
function keyDetect(dict, notes, onlyMajorMinor) {
    //create an array of pairs of chromas (see tonal docs) and scale names
    var chromaArray = dict.keys(false).map(function(e) { return [pcset.chroma(dict.get(e)), e]; });
    //filter only Major/Minor if requested
    if (onlyMajorMinor) { chromaArray = chromaArray.filter(function (e) { return e[1] === 'major' || e[1] === 'harmonic minor'; }); }
 //sets is an array of pitch classes transposed into every possibility with equivalent intervals
 var sets = pcset.modes(notes, false);

 //this block, for each scale, checks if any of 'sets' is a subset of any scale
 return chromaArray.reduce(function(acc, keyChroma) {
    sets.map(function(set, i) {
        if (pcset.isSubset(keyChroma[0], set)) {
            //the midi bit is a bit of a hack, i couldnt find how to turn an int from 0-11 into the repective note name. so i used the midi number where 60 is middle c
            //since the index corresponds to the transposition from 0-11 where c=0, it gives the tonic note of the key
            acc.push(note.pc(note.fromMidi(60+i)) + ' ' + keyChroma[1]);
            }
        });
        return acc;
    }, []);

    }

const p1 = [ chord.get('m','Bb'), chord.get('m', 'C'), chord.get('M', 'Eb') ];
const p2 = [ chord.get('M','F#'), chord.get('dim', 'B#'), chord.get('M', 'G#') ];
const p3 = [ chord.get('M','C'), chord.get('M','F') ];
const progressions = [ p1, p2, p3 ];

//turn the progression into a flat string of notes seperated by spaces
const notes = progressions.map(function(e) { return _.chain(e).flatten().uniq().value(); });
const possibleKeys = notes.map(function(e) { return keyDetect(dict, e, true); });

console.log(possibleKeys);
//[ [ 'Ab major' ], [ 'Db major' ], [ 'C major', 'F major' ] ]

Some drawbacks:
- doesn't give the enharmonic note you want necessarily. In p2, the more correct response is C# major, but this could be fixed by checking somehow with the original progression.
-‎ won't deal with 'decorations' to chords that are out of the key, which might occur in pop songs, eg. CMaj7 FMaj7 GMaj7 instead of C F G. Not sure how common this is, not too much I think.

If you're not opposed to switching languages, music21 (my library, disclaimer) in Python would do this:

from music21 import stream, harmony

chordSymbols = ['Cm', 'Dsus2', 'E-/C', 'G7', 'Fm', 'Cm']
s = stream.Stream()
for cs in chordSymbols:
    s.append(harmony.ChordSymbol(cs))
s.analyze('key')

Returns: <music21.key.Key of c minor>

The system will know the difference between, say C# major and Db major. It has a full vocabulary of chord names so things like "Dsus2" won't confuse it. The only thing that might bite a newcomer is that flats are written with minus signs so "E-/C" instead of "Eb/C"

There is an online free tool (MazMazika Songs Chord Analyzer), which analyzes and detects the chords of any song very fast. You can process the song through file upload (MP3/WAV) or by pasting YouTube / SoundCloud links. After processing the file, you can play the song while seeing all the chords playing along in-real time, as well as a table containing all the chords, each chord is assigned to a time-position & a number ID, which you can click to go directly to the corresponding chord and it`s time-position.

https://www.mazmazika.com/chordanalyzer