How to create: Cascading dropdown lists

Create three dropdown lists

Create three dropdown lists within the HTML form.

The second and third dropdown lists will display different options, depending on the value selected in the parent dropdown list.

第一步 - 添加 HTML：

<form name="form1" id="form1" action="/action_page.php">
  Subjects: <select name="subject" id="subject">
    <option value="" selected="selected">Select subject</option>
  </select>
  <br><br>
  Topics: <select name="topic" id="topic">
    <option value="" selected="selected">Please select subject first</option>
  </select>
  <br><br>
  Chapters: <select name="chapter" id="chapter">
    <option value="" selected="selected">Please select topic first</option>
  </select>
  <br><br>
  <input type="submit" value="Submit">
</form>

第二步 - 添加 JavaScript：

var subjectObject = {
  "Front-end": {
    "HTML": ["Links", "Images", "Tables", "Lists"],
    "CSS": ["Borders", "Margins", "Backgrounds", "Float"]
    "JavaScript": ["Variables", "Operators", "Functions", "Conditions"]
  },
  "Back-end": {
    "PHP": ["Variables", "Strings", "Arrays"],
    "SQL": ["SELECT", "UPDATE", "DELETE"]
  }
}
window.onload = function() {
  var subjectSel = document.getElementById("subject");
  var topicSel = document.getElementById("topic");
  var chapterSel = document.getElementById("chapter");
  for (var x in subjectObject) {
    subjectSel.options[subjectSel.options.length] = new Option(x, x);
  }
  subjectSel.onchange = function() {
    // Clear the 'Chapter' and 'Topic' dropdown menus
    chapterSel.length = 1;
    topicSel.length = 1;
    // Display the correct value
    for (var y in subjectObject[this.value]) {
      topicSel.options[topicSel.options.length] = new Option(y, y);
    }
  }
  topicSel.onchange = function() {
    // Clear the 'Chapter' dropdown menu
    chapterSel.length = 1;
    // Display the correct value
    var z = subjectObject[subjectSel.value][this.value];
    for (var i = 0; i < z.length; i++) {
      chapterSel.options[chapterSel.options.length] = new Option(z[i], z[i]);
    }
  }
}

Try it yourself

Related page

Tutorial:CSS Dropdown Menu

Tutorial:How to Create: Hoverable Dropdown Menu