This is because beepbox's script: https://cdn.jsdelivr.net/npm/beepbox@4.2.0/global/beepbox_synth.min.js has a sourceMappingURL comment at the bottom for whatever reason, causing it to become //# sourceMappingURL=beepbox_synth.min.js.map;return beepbox; This is obviously going to break and might also be the case with other libraries too, hot just beepbox. Ported from AmpMod: https://codeberg.org/ampmod/ampmod/commit/0daf1021bdc8e67a1f0d08b91ce500605fcaa711