MusicTheoryJS

https://zachacious.github.io/MusicTheoryJS
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MusicTheoryJS is a fast, zero-dependency music theory library for JavaScript and TypeScript covering...
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### Import Scala tuning with scalaTuning

Source: https://zachacious.github.io/MusicTheoryJS

Imports a Scala tuning file (sclFileText) and shows how to access the number of notes in the scale and the cents for a given degree.

```javascript
const tuning = scalaTuning(sclFileText);
tuning.size;               // number of notes in the scale
tuning.centsForDegree(4);  // cents above the tonic
```

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### From chords to a track

Source: https://zachacious.github.io/MusicTheoryJS

Shows building a track from chords: comped hits over a walking bass, swung, then converting beats exactly to MIDI ticks and writing a real MIDI file.

```javascript
import { compProgression, bassline, swing,
         mergeStreams, sequenceToMidi, writeMidi } from "musictheoryjs";

// A played ii-V-I: comped hits over a walking bass, swung.
const comp = compProgression("C major", "ii-V-I", { rhythm: [1, 0, 0, 1, 0, 0, 0, 0] });
const track = mergeStreams(swing(comp), bassline(["Dm7", "G7", "Cmaj7"]));

// Beats convert exactly to ticks — straight into a real MIDI file.
writeMidi(sequenceToMidi(track, { bpm: 140 }));
```

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### MIDI and audio without extra packages

Source: https://zachacious.github.io/MusicTheoryJS

Shows reading and writing Standard MIDI Files with a small byte codec, and detecting pitch from audio samples with a dependency-free DSP layer (monophonic, YIN).

```javascript
import { parseMidi, midiToNoteStream, writeMidi,
         noteStreamToMidi, detectNote } from "musictheoryjs";

// Read a MIDI file, spell its notes, analyze it, write one back
const stream = midiToNoteStream(parseMidi(midiBytes));
const bytes  = writeMidi(noteStreamToMidi(stream, { tempo: 500000 }));

// Detect a pitch from audio samples you already have
detectNote(float32Samples, 44100)?.toString();   // "A4"  (monophonic, YIN)
```

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### Scales, modes, and detection

Source: https://zachacious.github.io/MusicTheoryJS

Demonstrates creating a scale from a tonic and mode name, extracting a mode from a scale, and detecting scales from a set of notes. The comments show the tonic of the sixth mode of C major and the detected scale names.

```javascript
Scale.from("D4", "dorian").noteNames();
mode(Scale.from("C4", "major"), 6).tonic.toString(); // "A4"
detectScales(["C4","D4","E4","G4","A4"]); // major pentatonic, …
```

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### Create scales, chords, and progressions with musictheoryjs

Source: https://zachacious.github.io/MusicTheoryJS

Shows the core v3 API for creating a major scale, a Cmaj7 chord, and a Roman-numeral progression, each returning note names or chord symbols. The output comments show the expected results for each call.

```javascript
import { Note, Scale, Chord, Key } from "musictheoryjs";

Scale.from("C4", "major").noteNames();
// ["C4", "D4", "E4", "F4", "G4", "A4", "B4"]

Chord.from("Cmaj7").noteNames();
// ["C4", "E4", "G4", "B4"]

Key.major("C").progression("ii7 V7 Imaj7").map(String);
// ["Dm7", "G7", "Cmaj7"]
```

--------------------------------

### Tunings beyond 12-TET

Source: https://zachacious.github.io/MusicTheoryJS

Shows how to use tunings beyond 12-TET: equal temperament of any size, just intonation, and a maqam defined in cents. The default is 12-TET, but it is not wired into the core.

```javascript
import { equalTemperament, centsTuning, justIntonation,
         scaleFromTuning, frequencyOfNote, Note } from "musictheoryjs";

frequencyOfNote(new Note("A4"));                 // 440   (12-TET default)
frequencyOfNote(new Note("E4"), justIntonation()); // pure third above C

// 24 equal divisions of the octave — quarter tones
scaleFromTuning(equalTemperament(24));

// A maqam Rast defined in cents, anchored to a real frequency
const rast = centsTuning([0, 204, 355, 498, 702, 906, 1057], { name: "Rast" });
scaleFromTuning(rast, { frequency: 264 }, true);
```

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### Key detection and harmonic analysis

Source: https://zachacious.github.io/MusicTheoryJS

Shows key detection with the Krumhansl–Schmuckler algorithm and harmonic analysis that segments a timed note stream into a chord timeline, labels Roman numerals, and marks cadences.

```javascript
import { detectKey, analyzeHarmony } from "musictheoryjs";

detectKey(["C4", "E4", "G4"])[0].key.toString();   // "C major"

// From a stream of timed notes (MIDI, a sequencer, a transcriber you supply):
const { key, timeline, cadences } = analyzeHarmony(noteStream);
// key: C major
// timeline: [{ …, roman: "ii" }, { …, roman: "V" }, { …, roman: "I" }]
// cadences: [{ type: "authentic", index: 1 }]
```

--------------------------------

### Roman numerals with Key.major

Source: https://zachacious.github.io/MusicTheoryJS

Shows how to work with Roman numerals in the key of C major: converting a chord to its Roman numeral, converting a Roman numeral to a chord, and generating a chord progression. The comments show the expected results.

```javascript
Key.major("C").romanNumeral(Chord.from("G7")); // "V7"
Key.major("C").chordFromRoman("bVII");           // Bb
Key.major("C").progression("ii7 V7 Imaj7");      // [Dm7, G7, Cmaj7]
```

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### Drop-2 voicing with drop2

Source: https://zachacious.github.io/MusicTheoryJS

Applies a drop-2 voicing to a Cmaj7 chord and converts the resulting notes to strings. The comment shows the expected voicing.

```javascript
drop2(Chord.from("Cmaj7")).map(String);
// ["G3", "C4", "E4", "B4"]
```

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### Detect chord from notes with detectChord

Source: https://zachacious.github.io/MusicTheoryJS

Uses the detectChord function to identify a chord from an array of note names. The optional chaining (?.) is used because the function may return null or undefined; the comments show the detected chord names.

```javascript
detectChord(["G4", "B4", "D5", "F5"])?.toString(); // "G7"
detectChord(["C4", "Eb4", "Gb4", "A4"])?.toString(); // "Cdim7"
```

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### Pitch class loses spelling

Source: https://zachacious.github.io/MusicTheoryJS

Shows that a bare pitch class (0–11) loses the spelling, so you cannot tell whether it is F, E#, or Gbb.

```javascript
// A note stored as a number 0–11
const pitchClass = 5;   // is this F? or E#? or Gbb?
// You can't tell. The spelling is gone.
```

--------------------------------

### Interval spelling with intervalTo and intervalName

Source: https://zachacious.github.io/MusicTheoryJS

Shows how intervals preserve their spelling: the interval from C4 to Fb4 is a diminished fourth, and intervalName returns "d4" rather than the enharmonically equivalent "M3".

```javascript
new Note("C4").intervalTo("Fb4");   // diminished fourth
intervalName(interval(4, "d"));      // "d4"  (not "M3")
```

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### Note keeps spelling

Source: https://zachacious.github.io/MusicTheoryJS

Shows that Note keeps the letter and accidental apart, so transposition and naming stay correct (e.g., E#4 transposed by a major third gives G##4, not A4).

```javascript
const eSharp = new Note("E#4");
eSharp.letter;             // "E"
eSharp.pitchClass;         // 5
eSharp.transpose(interval(3, "M")).toString(); // "G##4", not "A4"
```
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