How to check if a field is filled [duplicate]

I need to check if the fields and radio buttons are filled using

parentNode, childNodes, firstChild, lastChild, previousSibling, nextSibling.
 parentElement, children, firstElementChild, lastElementChild, previousElementSibling, nextElementSibling.

After checking the completeness of the fields, send a signal to second section.

I am trying but cannot get to getting the values.

  <div id="firstsection" class="Section">
        <label for="age"
          >Возраст пациента<input
            type="number"
            class="stepTwo-profile-item-textAge"
            name="age"
            min="0"
            max="80"
            maxlength="2"
            id="age"
        /></label>
        <p class="stepTwo-profile-item-smTitle">Пол</p>
        <label for="male">
          <input type="radio" name="gender" id="male" value="male" />Мужской
          <span class="stepTwo-profile-item-radionbtn"></span
        ></label>

        <label for="female">
          <input type="radio" name="gender" id="female" value="female" />Женский
          <span class="stepTwo-profile-item-radionbtn"></span>
        </label>
      </div>
    </div>
 
      

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