JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="Last Universe"  id="input">
<div id="result">

</div>

JavaScript

var input  = document.getElementById("input");
var result  = document.getElementById("result");

var parseUserName = userName => {
    const splitted = userName.split(/\s+/);
    return splitted[0]+" "+splitted[1][0]+".";
};

result.innerText = parseUserName(input.value);

input.oninput = function() {
	result.innerText = parseUserName(input.value);
}