JSFiddle - React, Tailwind, and code Playground

HTML

<span id="username"></span><br/><br/>
<input type="text" name="firstname" /><br/><br/>
<input type="text" name="lastname" />

JavaScript

var $username = $("#username");

$("input[name='firstname']").keyup(function() {
	$("#username").text( this.value + ".");
});

$("input[name='lastname']").keyup(function() {
	$("#username").text( this.value );
});