JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="ul1">
  <li id="1">ul1-1</li>
  <li id="2">ul1-2</li>
  <li id="3">ul1-3</li>
</ul>

<ul id="ul2">
  <li id="1">ul2-1.1</li>
  <li id="2">ul2-2.2</li>
</ul>

JavaScript

const array1 = $( document ).find( '#ul1' ).find( 'li' ).val();
const array2 = $( document ).find( '#ul2' ).find( 'li' ).get();

const array3 = [];

if($( array1 ).length > $( array2 ).length) {
	$.each($( array1 ), (index) => {
  	const element = $( array1 ).get(index);
    console.log('index ar 1: ', element );
  	const id = $( element ).attr('id');
    if(!$( array2 ).find('li#'+id).get()){
    	array3.push(id);
    }
  });
}
console.log(array1);
console.log(array2);
console.log(array3);
console.log('---------');