Javascript variable comes null even though it has one

Hello im using javascript to add parameters to an url, this is my code what i have done until now:

var startdate = document.getElementById("startdateOutlay"); var enddate = document.getElementById("enddateOutlay");

    alert(startdate.value);
    const apiUrl = `http://127.0.0.1:8000/api/outlay?startdate=${startdate.value}&enddate=${enddate.value}`;

the alert brings me the value correct but then on the url i get this error:

Uncaught (in promise) TypeError: startdate is null