我已經(jīng)完成了帶有即時(shí)字段驗(yàn)證的代碼，但也希望在字段沒(méi)有值的情況下防止提交表單。我已經(jīng)設(shè)法阻止提交表單，但是當(dāng)我返回并提供一個(gè)值時(shí)，它仍然不允許它提交。知道我在這里做錯(cuò)了什么嗎？這是 HTML：<form id="contactForm" action="https://formspree.io/xrggbejj" method="POST" onsubmit="return formValidation(contactForm);">  <div class="row">  <div class="col">    <label for="fName">First Name</label>  </div>  <div class="col">    <input type="text" id="fName" name="firstname" onblur="checkFirstName(firstName)" autofocus/>    <img src="../images/alert-icon.png" class="alert" id="firstNameMsg" alt="alert icon" width="40" height="35"/>  </div>  </div> <!-- End of row -->  <div class="row">  <div class="col">    <label for="lName">Last Name</label>  </div>  <div class="col">    <input type="text" id="lName" name="lastname" onblur="checkLastName(lastName)"/>    <img src="../images/alert-icon.png" class="alert" id="lastNameMsg" alt="alert icon" width="40" height="35"/>  </div>  </div> <!-- End of row -->  <div class="row">  <div class="col">    <label for="subject">Questions and/or Comments</label>  </div>  <div class="col">    <textarea id="subject" name="subject" onblur="checkSubject(subject)" style="height:200px"></textarea>    <img src="../images/alert-icon.png" class="alert" id="subjectMsg" alt="alert icon" width="40" height="35"/>  </div>  </div> <!-- End of row -->  <div class="row">    <input type="submit" value="SEND" />  </div> <!-- End of row --></form>