JSFiddle - React, Tailwind, and code Playground

HTML

<ul class="test">
    <li>0</li>
    <li>1</li>
    <li>2</li>
</ul>

JavaScript

/*

//This is what is looks like in core:

is: function( selector ) {
        return !!selector && jQuery.filter( selector, this ).length > 0;
    },

*/

console.log($('li:eq(0)').html()); //0
console.log($('li:eq(1)').html());//1
console.log($('li:eq(2)').html());//2

console.log($('li:first').html());//0
console.log($('li:last').html()); //2

var $x = $('li:first')
var $y = $('li:last');


console.log($('li:eq(0)').is('li:eq(2)'));  //false, correct
console.log($('.test li:first').is('.test li:last'));   //true, incorrect
console.log($x.is($y)); //false, correct.