我已经完成了具有立即字段验证的代码，但是如果字段没有值，我还想阻止表单提交。我设法阻止了表单的提交，但是当我返回并提供一个值时，它仍然不允许它提交。知道我在做什么错吗？

这是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>

这是JavaScript：

联系表单字段验证

var alertStyle;
var alertImg;
const firstName = document.forms['contactForm']['fName'];
const lastName = document.forms['contactForm']['lName'];
const subject = document.forms['contactForm']['subject'];

function checkFirstName(firstName) {

  if (firstName.value === '') {
    alertStyle = document.getElementById('fName').style.border = '2px solid rgba(255,6,0)';
    alertImg = document.getElementById('firstNameMsg').style.display = 'inline-block';
    return false;
  } else if (firstName.value !== '') {
    alertStyle = document.getElementById('fName').style.border = '1px solid #ccc';
   alertImg = document.getElementById('firstNameMsg').style.display = 'none';
   return true;
 }
}

function checkLastName(lastName) {

  if (lastName.value === '') {
    alertStyle = document.getElementById('lName').style.border = '2px solid rgba(255,0)';
    alertImg = document.getElementById('lastNameMsg').style.display = 'inline-block';
    return false;
  } else if (lastName.value !== '') {
    alertStyle = document.getElementById('lName').style.border = '1px solid #ccc';
    alertImg = document.getElementById('lastNameMsg').style.display = 'none';
    return true;
 }
}

function checkSubject(subject) {

  if (subject.value === '') {
    alertStyle = document.getElementById('subject').style.border = '2px solid rgba(255,0)';
    alertImg = document.getElementById('subjectMsg').style.display = 'inline-block';
    return false;
  } else if (firstName.value !== '') {
    alertStyle = document.getElementById('subject').style.border = '1px solid #ccc';
    alertImg = document.getElementById('subjectMsg').style.display = 'none';
    return true;
}}

提交时联系表单验证

function formValidation(contactForm) {

  if (firstName && lastName && subject !== true) {
    alert('Please fill out required fields.')
    event.preventDefault()
    return false;
  } else if (firstName && lastName && subject === true) {
    return true;
 }
}