JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div>
        <div class="block-1">1</div>
    </div>
    <div>
        <div>
            <div class="block-45">2</div>
        </div>
    </div>
    <div>
        <div>
            <div>
                <div class="block-312">3</div>
            </div>
        </div>
    </div>
</div>

JavaScript

var $el = $('[class ^= block-]');

function func(a){
	var nextEl = $el.eq($el.index($(a)) + 1),
    text = nextEl.text();
    
    alert(text);
};

func('.block-1')