JSFiddle - React, Tailwind, and code Playground

HTML

<script>
    var vehList = [];
    const VehOptions = document.getElementById("options");
    const categoryTitles = [
        "1",
        "2", // for example
    ]
    alert(vehList); // SHOW EMPTY ARRAY

    vehList.forEach(function (item) {
        var signElement = document.createElement('div');
        signElement.className = "vehopt";
        signElement.innerHTML = categoryTitles[item];
        VehOptions.appendChild(signElement);
    })
</script>

JavaScript

mp.events.add("List", function () {
    for (let i = 0; i < categoryTitles.length; i++) {
        let numMods = player.vehicle.getNumMods(i);

        if (numMods > 0 && categoryTitles[i].length > 0) {
            CarTuning.browser.execute(`vehList.push('${i.toString()}');`);
        }
    }
})