You could also directly call those functions with the necessary variable:
async function layer_pictures(mode){...}
function layer_pictures_caller(mode) {
try {
require('photoshop').core.executeAsModal(() => layer_pictures(mode));
}
...
}
document.getElementById("btn_dropin_normal").addEventListener("click", () => layer_pictures_caller("normal"));
document.getElementById("btn_dropin_screen").addEventListener("click", () => layer_pictures_caller("screen"));
If you want to unsubscribe from that eventListener at a later point you’d have to move those inline handlers to a variable like
const handleNormalDropin = () => layer_pictures_caller("normal")
document.getElementById("btn_dropin_normal").addEventListener("click", handleNormalDropin);