noteToHz.js

/*
    Function that converts scientific pitch notation (American standard pitch notation or international pitch notation)
    to a frequency. Input must pass noteToPitch.validate else function may not work as intended.
    First character must be an uppercase letter of the note A-G. Follows is
    '#' for sharp and 'b' for flat. Although not standard, these flat and sharp 
    modifiers may be stacked infinitely. The octave number follows; it can be any integer.
    All pitch is equal temperament at A440. The pitch of standard A can be trivially
    changed.

    Example:
    'A4' returns 440, standard A
    'Ab#4' returns 440, A flat sharp 4, equivlent standard A
    'G##4' returns 440, G double sharp 4, equivlent standard A
    'C4' returns 261.6255653005986, middle C
    'Bb6' returns 1864.6550460723597, B flat 6
    'C-1' returns 8.175798915643707, it is one octave lower than C0
*/
const noteToHz = note => 440 * 2 ** (
    ("C-D-EF-G-A-B".indexOf(note[0]) + (note.match(/#/g) || []).length - (note.match(/b/g) || []).length - 9) / 12 
    + (Number((note.match(/\d|-/g) || ['4']).join('')) - 4)
)

noteToHz.validate = s => /^[A-G][#b]*-?\d*$/g.test(s)