JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" value="test">
<textarea>test</textarea>
<select name="">
    <option value="test">test</option>
</select>
    
    <a href="" id="convert">Click me to convert to uppercase :-)</a>

JavaScript

function formFieldsToUpperCase() {
    var inputElements = document.querySelectorAll('input, select, textarea');
    
    for (var i = 0, l = inputElements.length; i < l; i++) {
        if (inputElements[i].tagName == 'SELECT') {
            inputElements[i].options[inputElements[i].selectedIndex].value = inputElements[i].options[inputElements[i].selectedIndex].value.toUpperCase();
        } else {
            inputElements[i].value = inputElements[i].value.toUpperCase();
        }
    }    
}

(function() {
    document.getElementById('convert').addEventListener('click', function(e) {
        formFieldsToUpperCase();
        
        e.preventDefault();
        e.stopPropagation();
    });
}());