JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a1">

</div>

JavaScript

function spliceSlice(str, index, count, add) {
  return str.slice(0, index) + (add || "") + str.slice(index + count);
}

function spliceSplit(str, index, count, add) {
  var ar = str.split('');
  ar.splice(index, count, add);
  return ar.join('');
}

a1 = document.getElementById("a1")
a1.innerHTML += "slice('abc', -1, 1): "
a1.innerHTML += spliceSlice("abc", -1, 1);
a1.innerHTML += "<br>";
a1.innerHTML += "split('abc', -1, 1): "
a1.innerHTML += spliceSplit("abc", -1, 1);