JSFiddle - React, Tailwind, and code Playground

HTML

<div class="data">    
    <span>
    <input name="employee" type="checkbox" value="Alex"/>
    <label for="employee">Alex</label>
    </span> 

    <span>
    <input name="employee" type="checkbox" value="Frank"/>
    <label for="employee">Frank</label>
    </span>    

    <span>
    <input name="employee" type="checkbox" value="Mark"/>
    <label for="employee">Mark</label>
    </span>
    
    <input type="button" onclick="alert(getCheckedCheckboxesFor('employee'));" value="Get Values" />
</div>

JavaScript

function getCheckedCheckboxesFor(checkboxName) {
    var checkboxes = document.querySelectorAll('input[name="' + checkboxName + '"]:checked'), values = [];
    Array.prototype.forEach.call(checkboxes, function(el) {
        values.push(el.value);
    });
    return values;
}