JSFiddle - React, Tailwind, and code Playground

HTML

<form action='bla.php' method='post' name="form">
    <input type='text' name='someTextField1'>
    <input type='text' name='someTextField2'>
    <input type='text' name='someTextField3'>
        <br/>
    <input type='submit' name='submit' onclick="return window.CheckFields()">
    <input type='submit' name='cancel' value="cancel" onclick="return false;">
</form>

JavaScript

window.CheckFields = function() {
    var formname = "form";
    var x = document.forms[formname]["someTextField1"].value;
    var result = true;
    var text = "";
    if (x == null || x == "") {
        text += "Dont forget about the someTextField1.\n";
        result =  false;
    }

    if(!result)
        alert(text);
    return result;
}