I was wondering how would one create a javascript function for transposing music chords.
Since I don't expect everyone to be a musician here, I'll try to explain how it works in music theory. I hope I don't forget something. If yes, musicians, please, correct me.
1) The simple chords
The simple chords are almost as simple as an alphabet and it goes like this:
C, C#, D, D#, E, F, F#, G, G#, A, A# B
From B it loops all over again to C. Therefore, If the original chord is E
and we want to transpose +1, the resulting chord is F
. If we transpose +4, the resulting chord is G#
.
2) Expanded chords
They work almost like the simple chords, but contain a few more characters, which can safely be ignored when transposing. For example:
Cmi, C#7, Dsus7, Emi, Fsus4, F#mi, G ...
So again, as with the simple chords, if we transpose Dsus7
+ 3 = Fsus7
3) Non-root bass tone
A problem arises when the bass plays a different tone than the chord root tone. This is marked by a slash after the chord and also needs to be transposed. Examples:
C/G, Dmi/A, F#sus7/A#
As with examples 1 and 2, everything is the same, but the part after the slash needs transpose too, therefore:
C/G
+ 5 = F/C
F#sus7/A#
+ 1 = Gsus7/B
I think this should be all, unless I forgot something.
So basically, imagine you have a javascript variable called chord
and the transpose value transpose
. What code would transpose the chord?
Example:
var chord = 'F#sus7/C#';
var transpose = 3; // remember this value also may be negative, like "-4"
... code here ...
var result; // expected result = 'Asus7/E';
How about a little somethin' like this:
Note that I threw in the "F#maj9#11" example just to give you more to think about with regard to what makes up a valid chord name: you may find a "#" sharp symbol that doesn't follow a letter (in this case it belongs to the "11").
And, obviously, my function only understands sharps, not flats, and doesn't understand keys, so, e.g.,
transposeChord("C/E", 1)
will give "C#/F" when it really should be "C#/E#".