zig Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script>
  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 () {
��� //empty Chapters- and Topics- dropdowns
��� chapterSel.length = 1;
��� topicSel.length = 1;
    //display correct values
    for (var y in subjectObject[this.value]) {
      topicSel.options[topicSel.options.length] = new Option(y, y);
    }
  }
  topicSel.onchange = function() {
��� //empty Chapters dropdown
��� chapterSel.length = 1;
    //display correct values
    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]);
    }
  }
}
</script>
</head>   
<body>
<h1>Cascading Dropdown Example</h1>
<form name="form1" id="form1" action="https://www.w3schools.com/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>
</body>

</html>