JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="list">
    <li>1-1</li>
    <li>1-2</li>
    <li>1-3</li>
</ul>

CSS

ul.list li.item1 {font-weight: bold;}

JavaScript

$(".list").each(function(curListIdx, curListEntity) {
    $(curListEntity).children("li").each(function(curLiIdx, curLiEntity) {
        var idxPlus1 = curLiIdx+1;
        $(curLiEntity).addClass("item-" + idxPlus1);
    });
});