Validating User Input To Prevent Duplicate Id

I'm trying to create new 'student' objects each time a user inputs User ID, first name, last name, and clicks a button. I think I've made it to that point so far, but I'm trying to

Solution 1:

Here is a working snippet. Your ìds was always set to empty array because it was inside your function

var ids = [];

  functionmyFunction(list) {
    var student = newObject();
    student.idNo = document.getElementById("idNumber").value;
    student.firstName = document.getElementById("fName").value;
    student.lastName = document.getElementById("lName").value;

    if (ids.indexOf(student.idNo) == -1) {
      
      ids.push(student.idNo);
      var text = " " + student.idNo + " " + student.firstName + " " + student.lastName;
      var li = document.createElement("li");
      var node = document.createTextNode(text);
      li.appendChild(node);
      document.getElementById("list").appendChild(li);
      
    } else {
      
      alert("Duplicated ID");
      
    }


  }

<form>
  ID Number:
  <br><inputtype="text"id="idNumber"><br>First name:
  <br><inputtype="text"name="firstName"id="fName"><br>Last name:
  <br><inputtype="text"name="lastName"id="lName"></form><br><buttontype="submit"onclick="myFunction(list)">Submit</button><divid="container"><ulid="list"></ul></div>