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 class="block-1">4</div>
  <div class="block-2">5</div>
</div>

JavaScript

function findNextElement(_className) {
    var items = document.querySelectorAll('[class^=block-]');
    var result = [];

    [].forEach.call(items, function(item, i) {
        if (_className === item.className && items[i+1]) {
            result.push( items[i+1] );
        }
    });

    return result.length
        ? result
        : -1
    ;
}

findNextElement('block-1').forEach(function(elem) {
	alert(elem.innerHTML);
});