JSFiddle - React, Tailwind, and code Playground
HTML
<form name="formName">
<input name="m" value="foo">
<input name="p" value="bar">
<input name="prop.0" value="111">
<input name=".prop.1" value="222">
</form>JavaScript
var i = 0;
var s = 'prop.'+i;
var v = document.formName[s].value;
alert(v);