JSFiddle - React, Tailwind, and code Playground

HTML

<div class="item">a
    </div>

    <div class="item">b
    </div>

    <div class="item">c
    </div>

    <div class="item">d
    </div>

JavaScript

$('.item').mouseover(function(){
    $('.item').not(this).css('opacity','0.4');
}).mouseout(function(){
    $('.item').not(this).css('opacity','1');
});