JSFiddle - React, Tailwind, and code Playground

HTML

<form  method="get"  name="myforma1" id="myforma1">
<input name="txtfield" type="text" id="txtfield" onchange="detect_it(this);" value="name of person" size="20" maxlength="25" />
<input name="txtfield" type="text" id="txtfield" onchange="detect_it(this);" value="name of person" size="20" maxlength="25" />
<input name="txtfield" type="text" id="txtfield" onchange="detect_it(this);" value="name of person" size="20" maxlength="25" />
</form>

JavaScript

var getIndex = function (node) {
    for (var i =0;i<node.parentNode.children.length;i++) {
        if (node.parentNode.children[i] === node) {
            return i;
        }
    }
}
function detect_it(oo) {
    alert('Index of triggered element is: ' + getIndex(oo));
}