Issue with passing property through function

First off my apologies if I did something incorrectly with asking a question, I'm very new to stackoverflow and javascript as well. I am having an issue with passing a property through my getPassword function and I've searched around and couldn't truly pinpoint an answer. The code I created is designed for an object called "student" with two properties; FirstName and LastName. Using a couple of dialogue boxes the information related to each student is entered. At the end, a prompt should display and asks the user "Do you want to add more student?" If the answer is "Yes", It asks the next student's information. If the answer is anything else, It stops asking. Then the information is displayed on the webpage. I want to have a property called "UID" The format of UID is FirstName_PSWD. For calculating the "PSWD" the function called "generatePassword" is used. This function randomly creates a 6-digit password including characters and numbers. For example: if username is John, then UID may be "John_X12bn231". I can not seem to get this password function to work, what might I be doing wrong? I am also aware that there might be other errors in my code, which I do apologize for i am very much a beginner.

var student={FirstName:""; LastName:""};
var studentlist=[];

var i==0;


function generatePassword() {
    var length = 4,
        charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789",
        retVal = "";
    for (var i = 0, n = charset.length; i < length; ++i) {
        retVal += charset.charAt(Math.floor(Math.random() * n));
    }
    return retVal;
} 




  function Register() {
    var next="Yes";
    while (next="Yes")(
        student.FirstName=prompt("Please enter the name");
        student.LastName=prompt("Please enter the last name");


        studentlist.push(student);

        document.getElementById("demo").innerHTML += "<li> <b>First Name:</b> "+ studentlist[i].FirstName + "," +
        "<b>Last Name: </b>"+ studentlist[i].LastName + "," + "</li>";
        next = prompt ("Do you want to add more data?", "Yes")
         i++;
    }

}