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To do list in JavaScript / HTML

I installed this to-do list and almost everything works fine. However, if I delete everything in the list. This will not allow me to add a new entry, and I cannot understand why. It will work fine if I have at least one entry in the list.

Any help is greatly appreciated.

< head >
  < script type = "text/javascript" >
  /*<![CDATA[*/
  function addTask() {
    if (document.forms[0].newtask.value == "")
      window.alert("You must enter a value in the New Task field.");
    else {
      if (document.forms[0].tasks.options[0].value == "tasks")
        document.forms[0].tasks.options[0] = null;
      var newTask = new Option();
      newTask.value = document.forms[0].newtask.value;
      newTask.text = document.forms[0].newtask.value;
      var numTasks = document.forms[0].tasks.options.length;
      document.forms[0].tasks.options[numTasks] = newTask;
      document.forms[0].newtask.value = "";
    }
  }

function deleteTask() {
  var selectedTask = 0;
  var taskSelected = false;
  while (selectedTask < document.forms[0].tasks.length) {
    if (document.forms[0].tasks.options[selectedTask].selected == true) {
      taskSelected = true;
      break;
    }
    ++selectedTask;
  }
  if (taskSelected == true)
    document.forms[0].tasks.options[selectedTask] = null;
  else
    window.alert("You must select a task in the list.");
}

function ascendingSort() {
  var newTasks = new Array();
  for (var i = 0; i < document.forms[0].tasks.length; ++i) {
    newTasks[i] = document.forms[0].tasks.options[i].value;
  }
  newTasks.sort();
  for (var j = 0; j < document.forms[0].tasks.length; ++j) {
    document.forms[0].tasks.options[j].value = newTasks[j];
    document.forms[0].tasks.options[j].text = newTasks[j];
  }
}

/*]]>*/
< /script>

    <title>To Do List</title >
< /head>
<body>
  <h1>To Do List</h1>
  <form action="">
    <p>New Task
      <input type="text" size="68" name="newtask" />
    </p>
    <p>
      <input type="button" value="Add Task" onclick="addTask()" style="width: 150px" />
      <input type="button" value="Delete Selected Task" onclick="deleteTask()" style="width: 150px" />
      <br />
      <input type="button" value="Ascending Sort" onclick="ascendingSort()" style="width: 150px" />
    </p>
    <p>
      <select name="tasks" size="10" style="width: 500px">
        <option value="tasks">Tasks</option>
      </select>
    </p>
  </form>
</body>
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+4
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3 answers

When you delete all your tasks, you have a blank select, without options.

You need to protect your state in your addTaskfunction with

document.forms[0].tasks.options.length > 0

Something like that:

if (document.forms[0].tasks.options.length > 0 && document.forms[0].tasks.options[0].value == "tasks")
    document.forms[0].tasks.options[0] = null;

See updated code

+1
source

,

            document.forms[0].tasks.options[0].value //throw an error as 

            document.forms[0].tasks.options[0] //is undefiend

,

             if (typeof document.forms[0].tasks.options[0] != 'undefined' && document.forms[0].tasks.options[0].value == "tasks")
                    document.forms[0].tasks.options[0] = null;
+1

, jsconsole

 if (document.forms[0].tasks.options[0].value == "tasks")
You try to read the parameters [0] .value after the settings [0] are zero in

document.forms[0].tasks.options[selectedTask] = null;
Run code
Try not to destroy the data structure when deleting an item
0
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