JSFiddle - React, Tailwind, and code Playground

HTML

<section class="signup">
        <h1>Keep up to date.</h1>
        <p>Sign up to our newsletter to hear about improvements and additions to Electric Dreams.</p>
        <form class="newsletterForm">
            <label for="name" class="label">Full Name</label>
            <input type="text" id="name" name="name" class="input" placeholder="Full Name"> <br>
            <label for="email" class="label">Email Address</label>
            <input type="email" id="email" name="email" class="inout" placeholder="Email Address"> <br>
            <button type="submit" class="btn" onclick="submitNewsletter()">Submit</button>
        </form>
    </section>

JavaScript

function submitNewsletter(e) {
		e.preventDefault();
    var name = document.getElementById("name");
    var email = document.getElementById("email");
    if (name.value === "" || email.value === "" || !name.value || !email.value) {
        alert("Please fill in your details.");
        return false;
    }
}

document.querySelector(".newsletterForm")
	.addEventListener('submit', submitNewsletter);