JSFiddle - React, Tailwind, and code Playground

HTML

<div id="block-new40">&nbsp;</div>

CSS

#block-new40{
    background: red;
    width: 100px;
    height: 100px;
    opacity: .5;
}

JavaScript

var startOpacity = 0;
$('#block-new40').hover(function(){
    startOpacity = parseFloat($(this).css('opacity'));
    $(this).css('opacity', '1');
}, function(){
    $(this).css('opacity', startOpacity);
});