JSFiddle - React, Tailwind, and code Playground

HTML

<form id="reservation">
    Valid Email:    <input type=text id="validemail" value="abc@def.com">
    <label id=Result1>Result is: </label>
       <br>
Invalid Email: <input type=text id="invalidemail" value="abc@def.">
    <label id=Result2>Result is: </label>
</form>

JavaScript

var emailRE = /^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/;

function formChecker(oElement) {
    if (!emailRE.test(oElement)) {
        window.alert("Your e-mail address is invalid");
        return false;
    } else {
        window.alert("Your e-mail address is valid");
        return true;
    }

}

document.getElementById("Result1").innerHTML = "Result is: " + formChecker(document.forms["reservation"].validemail.value);

document.getElementById("Result2").innerHTML = "Result is: " + formChecker(document.forms["reservation"].invalidemail.value);