Untitled

<!DOCTYPE html>
<html>
    <head>
        <title>Oppgave 1-3</title>
        <meta charset="utf-8">

        <script>
            // Register all variables
            var username, password, passwordAgain, age;
            var tid = new Date();

            window.onload = oppstart;

            function getAge(elem) {
                var birthdate = Date.parse(elem.value)

                //Date difference is in milliseconds...
                var dateDifference = Date.now() - birthdate

                //...so we need to convert it to years
                return dateDifference / 1000 / 60 / 60 / 24 / 365
            }

            function oppstart(){
                document.getElementById("btnRegister").onclick = visMelding;
            }

            function visMelding(){
                // Set input variables
                username = document.querySelector("input[name='Username']")
                password = document.querySelector("input[name='Password']")
                passwordAgain = document.querySelector("input[name='Passwordagain']");
                age = document.querySelector("input[name='birthdate']")

                // If username input has no value, return error message
                if (!username.value) {
                    return alert("Du må fylle inn feltet for brukernavn");
                }

                // If the password fields have no value, return error message
                if (!password.value || !passwordAgain.value) {
                    return alert("Du må fylle inn passordfeltene");
                }

                // If the password fields have a value, check if their given values are equal
                else if (password.value !== passwordAgain.value) {
                    return alert("Passordet må være likt i begge felt");
                }

                //If age is less than 18, return an error message
                if (getAge(age) < 18) {
                    return alert("Du er ikke gammel nok")
                }
               }

               alert("Du er nå registrert. Velkommen");
           }
       </script>
        <style>
            .addCSS {
                margin-bottom: 5px;
                padding:5px;
            }
        </style>
    </head>
   <body>
        <h1>Velkommen til registrering:</h1>
        <div class="container">
            <div class="addCSS">
                <label><b>Skriv inn brukernavn:</b></label><br/>
                <input type="text" placeholder="Skriv brukernavn" name="Username">
            </div>
            <div class="addCSS"">
                <label><b>Skrive inn passord:</b></label><br/>
                <input type="password" placeholder="Skriv passord" name="Password" min="5"><br/>
            </div>
            <div class="addCSS"">
                <label><b>Bekreft passord:</b></label><br/>
                <input type="password" placeholder="skriv passord på nytt" name="Passwordagain"><br/>
            </div>
            <div class="addCSS"">
                <label><b>Fødselsdag:</b></label><br/>
                <input type="number" placeholder="Skriv fødselsdato" name="birthdate">
            </div>

            <p class="addCSS" type="checkbox" checked="checked"> Husk passord </p>

            <div class="knapper addCSS">
                <button type="button" id="btnRegister">Trykk her for å registrere deg</button>
                <button type="button" id="btnCancel">Avbryt</button>
            </div>
        </div>
    </body>
</html>