The glyphs themselves are written in PatEl: see https://github.com/be5invis/Iosevka/blob/main/packages/font-glyphs/src/letter/latin/lower-a.ptl https://github.c

The glyphs themselves are written in PatEl: see https://github.com/be5invis/Iosevka/blob/main/packages/font-glyphs/src/letter/latin/lower-a.ptl https://github.com/be5invis/Iosevka/blob/main/packages/font-... for instance. If you look through the commit history, you can see Be5invis gradually elaborating the font itself by editing these files. Javascript is used as the build language, but not to define the characters.