frequency-to-notes.js

const NOTES = [
    "C",
    "C♯/D♭",
    "D",
    "D♯/E♭",
    "E",
    "F",
    "F♯/G♭",
    "G",
    "G♯/A♭",
    "A",
    "A♯/B♭",
    "B"
]

const frequencyToNote = frequency => {
    const midi = Math.round(69 + 12 * Math.log2(frequency / 440)) % 12;
    return NOTES[midi];
}

const useFlatsOrSharps = notes => notes.reduce((acc, current) => {
    let usedNote = current
    if(current.includes("/")){
        const [sharp, flat] = current.split("/")
        usedNote = notes.includes(sharp[0]) ? flat : sharp
    }
    return [
        ...acc,                                                                                 
        usedNote
    ]
}, [])

const getMusicalNotes = notesAsFrequencies => useFlatsOrSharps(notesAsFrequencies.map(freq => frequencyToNote(freq)))