JSFiddle - React, Tailwind, and code Playground

HTML

<form action="" id="myForm">
    Use <code>return false</code> (if not checked, <code>e.preventDefault()</code> will be used):
    <input type="checkbox" id="myCheckbox" checked />
    <br/>
    <input type="submit" value="Submit me" />
</form>

JavaScript

document.getElementById("myForm").addEventListener("submit", function (e) {
    if (document.getElementById("myCheckbox").checked) {
        alert('checked (returnValue = false)');
        e.returnValue = false;
    }
    else {
        alert('not checked (preventDefault)');
        e.preventDefault();
    }
});