Music Helper 
You can do the following conversions:
var Music = require('music-helper');
Music.noteNameToNoteNumber(A4); // 69
Music.noteNameToFrequency(A4); // 440
Music.noteNumberToFrequency(69); // 440
Music.noteNumberToName(69); // A4
Music.frequencyToNoteNumber(440); // 69
Music.frequencyToNoteName(440); // A4