Uncaught TypeError：无法读取某些页面上未定义的属性“样式”

Question

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").style.display = "none";
  } else {
    document.getElementById("nextBtn").style.display = "inline";
    document.getElementById("nextBtn").innerHTML = "Next";
    document.body.scrollTop = document.documentElement.scrollTop = 0;
  }
  // ... 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 == 0 && !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 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";
}

* {
  box-sizing: border-box;
}

body {
  background-color: #f1f1f1;
}

#regForm {
  background-color: #ffffff;
  margin: 100px auto;
  font-family: Raleway;
  padding: 40px;
  width: 70%;
  min-width: 300px;
}

h1 {
  text-align: center;  
}

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;
}

button {
  background-color: #4CAF50;
  color: #ffffff;
  border: none;
  padding: 10px 20px;
  font-size: 17px;
  font-family: Raleway;
  cursor: pointer;
}

button:hover {
  opacity: 0.8;
}

#prevBtn {
  background-color: #bbbbbb;
}

/* 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;
}

.step.active {
  opacity: 1;
}

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

<form id="regForm" action="/action_page.php">
  <h1>Register:</h1>
  <!-- One "tab" for each step in the form: -->
  <div class="tab">Name:
    <p><input placeholder="First name..." oninput="this.className = ''" name="fname"></p>
    <p><input placeholder="Last name..." oninput="this.className = ''" name="lname"></p>
  </div>
  <div class="tab">Contact Info:
    <p><input placeholder="E-mail..." oninput="this.className = ''" name="email"></p>
    <p><input placeholder="Phone..." oninput="this.className = ''" name="phone"></p>
  </div>
  <div class="tab">Birthday:
    <p><input placeholder="dd" oninput="this.className = ''" name="dd"></p>
    <p><input placeholder="mm" oninput="this.className = ''" name="nn"></p>
    <p><input placeholder="yyyy" oninput="this.className = ''" name="yyyy"></p>
  </div>
  <div class="tab">Login Info:
    <p><input placeholder="Username..." oninput="this.className = ''" name="uname"></p>
    <p><input placeholder="Password..." oninput="this.className = ''" name="pword" type="password"></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>

此脚本在我运行它的页面上运行良好，但是在其他页面上，我收到下一个错误：“未捕获的TypeError：无法读取未定义的属性'style'”。我认为出现此问题是因为该脚本在其他页面中没有找到necessaire元素（变量），但是在使用该脚本的页面中，由于necessaire元素不在此页面中，因此不会出现该问题。如何解决该问题以避免我描述的问题？

Answer 1

您需要检查是否定义了x并包含类名称为tab的元素：

var x = document.getElementsByClassName("tab");
//check here if x is defined and it has elements
x[n].style.display = "block";

因此，您的代码将包含一个if块以防止该运行时错误：

var x = document.getElementsByClassName("tab");
if(x.length){
  x[n].style.display = "block";
  ...
  ...
}

Answer 2

我添加了错误处理并检查他的代码中是否存在未定义的内容。这样可以更好地处理边缘情况。另外，建议将诸如document.getElementById之类的操作存储在变量中，以备将来使用。

var currentTab = 0;
showTab(currentTab);

function showTab(n) {
  var x = document.getElementsByClassName("tab");
  if (x && x[n]) {
    x[n].style.display = "block"; //checking for undefined
  }
  var prevBtn = document.getElementById("prevBtn")
  if (prevBtn) {
    if (n == 0) {
      prevBtn.style.display = "none"; //Checking for undefined
    } else {
      prevBtn.style.display = "inline";
    }
  }
  var nextBtn = document.getElementById("nextBtn");
  if (nextBtn) {
    if (n == (x.length - 1)) {
      nextBtn.style.display = "none";
    } else {
      nextBtn.style.display = "inline";
      nextBtn.innerHTML = "Next";
      document.body.scrollTop = document.documentElement.scrollTop = 0;
    }
  }
  fixStepIndicator(n)
}

function nextPrev(n) {
  var x = document.getElementsByClassName("tab");
  if (x && x[currentTab]) {
    x[currentTab].style.display = "none";
  }
  currentTab = currentTab + n;
  showTab(currentTab);
}

function fixStepIndicator(n) {
  var x = document.getElementsByClassName("step");
  for (var i = 0; i < x.length; i++) {
    if (!x || !x[i]) {
      continue;
    }
    x[i].className = x[i].className.replace(" active", "");
  }
  if (x && x[n]) {
    x[n].className += " active";
  }
}

* {
  box-sizing: border-box;
}

body {
  background-color: #f1f1f1;
}

#regForm {
  background-color: #ffffff;
  margin: 100px auto;
  font-family: Raleway;
  padding: 40px;
  width: 70%;
  min-width: 300px;
}

h1 {
  text-align: center;
}

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

input.invalid {
  background-color: #ffdddd;
}

.tab {
  display: none;
}

button {
  background-color: #4CAF50;
  color: #ffffff;
  border: none;
  padding: 10px 20px;
  font-size: 17px;
  font-family: Raleway;
  cursor: pointer;
}

button:hover {
  opacity: 0.8;
}

#prevBtn {
  background-color: #bbbbbb;
}

.step {
  height: 15px;
  width: 15px;
  margin: 0 2px;
  background-color: #bbbbbb;
  border: none;
  border-radius: 50%;
  display: inline-block;
  opacity: 0.5;
}

.step.active {
  opacity: 1;
}

.step.finish {
  background-color: #4CAF50;
}

<form id="regForm" action="/action_page.php">
  <h1>Register:</h1>
  <!-- One "tab" for each step in the form: -->
  <div class="tab">Name:
    <p><input placeholder="First name..." oninput="this.className = ''" name="fname"></p>
    <p><input placeholder="Last name..." oninput="this.className = ''" name="lname"></p>
  </div>
  <div class="tab">Contact Info:
    <p><input placeholder="E-mail..." oninput="this.className = ''" name="email"></p>
    <p><input placeholder="Phone..." oninput="this.className = ''" name="phone"></p>
  </div>
  <div class="tab">Birthday:
    <p><input placeholder="dd" oninput="this.className = ''" name="dd"></p>
    <p><input placeholder="mm" oninput="this.className = ''" name="nn"></p>
    <p><input placeholder="yyyy" oninput="this.className = ''" name="yyyy"></p>
  </div>
  <div class="tab">Login Info:
    <p><input placeholder="Username..." oninput="this.className = ''" name="uname"></p>
    <p><input placeholder="Password..." oninput="this.className = ''" name="pword" type="password"></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>

  <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>