JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" name="incompatibility[]" value="1" checked>
<input type="checkbox" name="incompatibility[]" value="2">
<input type="checkbox" name="incompatibility[]" value="3" checked>
<input type="checkbox" name="incompatibility[]" value="4" checked>

JavaScript

var elementy = document.getElementsByName('incompatibility[]');
for(var i=0;i<elementy.length;i++){
	if(elementy[i].checked == true) console.log("Zaznaczony: "+i);
}