This is a simple tool to explore negative harmony
findOppositeChord('C',['E','Ab','B','D']);
That will give you the opposite chord of an E7 chord (in the 'C' tonality).
This is the index.js output:
Find all the diatonic chords' opposites (in C major key)
[ 'C', 'E', 'G', 'B' ] ' => ' [ 'G', 'Eb', 'C', 'Ab' ]
[ 'D', 'F', 'A', 'C' ] ' => ' [ 'F', 'D', 'Bb', 'G' ]
[ 'E', 'G', 'B', 'D' ] ' => ' [ 'Eb', 'C', 'Ab', 'F' ]
[ 'F', 'A', 'C', 'E' ] ' => ' [ 'D', 'Bb', 'G', 'Eb' ]
[ 'G', 'B', 'D', 'F' ] ' => ' [ 'C', 'Ab', 'F', 'D' ]
[ 'A', 'C', 'E', 'G' ] ' => ' [ 'Bb', 'G', 'Eb', 'C' ]
[ 'B', 'D', 'F', 'A' ] ' => ' [ 'Ab', 'F', 'D', 'Bb' ]