dependent dropdown 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> 

  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() {
    //display correct values 
    for (var y in subjectObject[this.value]) 
  {
      topicSel.options[topicSel.options.length] 
  = new Option(y, y);
    }
  }
  
  topicSel.onchange = function() {
    //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]);
    
  }
  }
}