Alternatively, you could use the id-attribute or any data-* attribute to select the hidden inputfield, e.g. <input type="hidden" data-id="myHiddenField" /> and then select it via document.querySelector( '[data-id="myHiddenField"]' ).
<input type="hidden" data-id="myHiddenField" />
document.querySelector( '[data-id="myHiddenField"]' )