JSFiddle - React, Tailwind, and code Playground

HTML

<form onsubmit="return proceed();">
    <input type="text" name="email" id="email">
    <input type="button" text="submit" onclick="proceed()" />
</form>

JavaScript

window.proceed = function () {
    var email = document.getElementById('email').value;

    if (email == "") {
        alert("Empty variable \n");
    } else if (email.indexOf("@") == -1) {
        alert("No @ \n");
    } else return true;
}