JSFiddle - React, Tailwind, and code Playground

HTML

<select id="project_billing_code_id">
    <option value="">Pick one</option>
    <option value="1">1-Hourly</option>
    <option value="2">2-Per Diem</option>
    <option value="3">3-Fixed</option>
</select>

<div id="hourly">Hourly</div>
<div id="per_diem">Per Diem</div>
<div id="fixed">Fixed</div>

JavaScript

hideAllDivs = function () {
    $("#hourly").hide();
    $("#per_diem").hide();
    $("#fixed").hide();
};

handleNewSelection = function () {

    hideAllDivs();
    
    switch ($(this).val()) {
        case '1':
            $("#hourly").show();
        break;
        case '2':
            $("#per_diem").show();
        break;
        case '3':
            $("#fixed").show();
        break;
    }
};

$(document).ready(function() {
    
    $("#project_billing_code_id").change(handleNewSelection);
    handleNewSelection.apply($("#project_billing_code_id"));
    
});