JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post">
  <label>Wert1</label><input type="checkbox" value="wert1" class="checkboxen">
  <label>Wert2</label><input type="checkbox" value="wert2" class="checkboxen">
  <label>Wert3</label><input type="checkbox" value="wert3" class="checkboxen">
  <label>Wert4</label><input type="checkbox" value="wert4" class="checkboxen">
  
  <input type="reset" value="Checkboxen reseten">
  <button id="checkTheCheckboxes">Alle Checkboxen auswählen</button>
  
  <button name="submitted">Formular absenden</button>
</form>

JavaScript

var setButton = document.getElementById("checkTheCheckboxes");

setButton.addEventListener("click", function(event){
		event.preventDefault();
		var checkboxen = document.getElementsByClassName("checkboxen");
    
    for(var i = 0; i < checkboxen.length; i++){
    		checkboxen[i].checked = true;
    }
 
});