多个步骤形式的复选框验证

Question

我的网站上有续订表格，但我的复选框验证有问题。这是代码：

var currentTab = 0; // Current tab is set to be the first tab (0)
showTab(currentTab); // Display the current tab

function showTab(n) {
  // This function will display the specified tab of the form ...
  var x = document.getElementsByClassName("tab");
  x[n].style.display = "block";
  // ... and fix the Previous/Next buttons:
  if (n == 0) {
    document.getElementById("prevBtn").style.display = "none";
  } else {
    document.getElementById("prevBtn").style.display = "inline";
  }
  if (n == (x.length - 1)) {
    document.getElementById("nextBtn").innerHTML = "Submit";
  } else {
    document.getElementById("nextBtn").innerHTML = "Next";
  }
  // ... and run a function that displays the correct step indicator:
  fixStepIndicator(n)
}

function nextPrev(n) {
  // This function will figure out which tab to display
  var x = document.getElementsByClassName("tab");
  // Exit the function if any field in the current tab is invalid:
  if (n == 1 && !validateForm()) return false;
  // Hide the current tab:
  x[currentTab].style.display = "none";
  // Increase or decrease the current tab by 1:
  currentTab = currentTab + n;
  // if you have reached the end of the form... :
  if (currentTab >= x.length) {
    //...the form gets submitted:
    document.getElementById("regForm").submit();
    return false;
  }
  // Otherwise, display the correct tab:
  showTab(currentTab);
}

function validateForm() {
  // This function deals with validation of the form fields
  var x, y, i, valid = true;
  x = document.getElementsByClassName("tab");
  y = x[currentTab].getElementsByTagName("input");
  // A loop that checks every input field in the current tab:
  for (i = 0; i < y.length; i++) {
    // If a field is empty...
    if (y[i].value == "") {
      // add an "invalid" class to the field:
      y[i].className += " invalid";
      // and set the current valid status to false:
      valid = false;
    }
  }
  // If the valid status is true, mark the step as finished and valid:
  if (valid) {
    document.getElementsByClassName("step")[currentTab].className += " finish";
  }
  return valid; // return the valid status
}

function fixStepIndicator(n) {
  // This function removes the "active" class of all steps...
  var i, x = document.getElementsByClassName("step");
  for (i = 0; i < x.length; i++) {
    x[i].className = x[i].className.replace(" active", "");
  }
  //... and adds the "active" class to the current step:
  x[n].className += " active";
}

 /* Style the form */
#regForm {
  background-color: #ffffff;
  margin: 100px auto;
  padding: 40px;
  width: 70%;
  min-width: 300px;
}

/* Style the input fields */
input {
  padding: 10px;
  width: 100%;
  font-size: 17px;
  font-family: Raleway;
  border: 1px solid #aaaaaa;
}

/* Mark input boxes that gets an error on validation: */
input.invalid {
  background-color: #ffdddd;
}

/* Hide all steps by default: */
.tab {
  display: none;
}

/* Make circles that indicate the steps of the form: */
.step {
  height: 15px;
  width: 15px;
  margin: 0 2px;
  background-color: #bbbbbb;
  border: none;
  border-radius: 50%;
  display: inline-block;
  opacity: 0.5;
}

/* Mark the active step: */
.step.active {
  opacity: 1;
}

/* Mark the steps that are finished and valid: */
.step.finish {
  background-color: #4CAF50;
} 

 <form id="regForm" action="">

<h1>Register:</h1>

<!-- One "tab" for each step in the form: -->
<div class="tab">Name:
  <p><input placeholder="First name..." oninput="this.className = ''"></p>
  <p><input placeholder="Last name..." oninput="this.className = ''"></p>
  <p><input type="checkbox" required>Agree</p>
</div>

<div class="tab">Contact Info:
  <p><input placeholder="E-mail..." oninput="this.className = ''"></p>
  <p><input placeholder="Phone..." oninput="this.className = ''"></p>
</div>

<div class="tab">Birthday:
  <p><input placeholder="dd" oninput="this.className = ''"></p>
  <p><input placeholder="mm" oninput="this.className = ''"></p>
  <p><input placeholder="yyyy" oninput="this.className = ''"></p>
</div>

<div class="tab">Login Info:
  <p><input placeholder="Username..." oninput="this.className = ''"></p>
  <p><input placeholder="Password..." oninput="this.className = ''"></p>
</div>

<div style="overflow:auto;">
  <div style="float:right;">
    <button type="button" id="prevBtn" onclick="nextPrev(-1)">Previous</button>
    <button type="button" id="nextBtn" onclick="nextPrev(1)">Next</button>
  </div>
</div>

<!-- Circles which indicates the steps of the form: -->
<div style="text-align:center;margin-top:40px;">
  <span class="step"></span>
  <span class="step"></span>
  <span class="step"></span>
  <span class="step"></span>
</div>

</form> 

表单无法验证复选框，单选按钮或输入文本字段。