Frequency to note name

(Assuming MIDI note names)


function frequencyToNoteNumber(f) {
return Math.round(12 * Math.log(f / 440.0) + 69);
}

function noteNumberToNote(n) {
var notes = ['C','C#','D','D#','E','F','F#','G','G#','A','A#','B'],
name = n % 12,
octave = Math.floor(n / 12) - 1;

return notes[name] + octave;
}

function frequencyToNote(f) {
return noteNumberToNote(frequencyToNoteNumber(f));
}

Lots more information on sengpiel audio