You can use SharePoint's javascript API to retrieve the current user's information.

_spPageContextInfo.userId

see: https://social.technet.microsoft.com/wiki/contents/articles/29766.sharepoint-understanding-the-sppagecontextinfo-object.aspx

Another suggestion is to use the API:

<script type="text/javascript">
ExecuteOrDelayUntilScriptLoaded(getCurrentUser, "sp.js");
 
var currentUser;
 
function getCurrentUser(){
var ctx= new SP.ClientContext.get_current();
var web = ctx.get_web();
currentUser = web.get_currentUser();
ctx.load(currentUser);
ctx.executeQueryAsync(onSuccess, onFailure);
}
 
function onSuccess() {
alert(currentUser.get_title()); // Domain\Account 
alert(currentUser.get_email());
document.getElementById('userLogin').innerHTML = currentUser.get_loginName(); 
}
 
function onFailure() {
alert('request failed' + args.get_message() + '\n' + args.get_stackTrace());
}
 
</script>
 
<div>Currently Logged User:
    <span id="userLogin"></span>
</div>

MS reference: https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/complete-basic-operations-using-sharepoint-client-library-code

If someone is still interested in this I found a way to get the user principal name with microsoft graph API https://docs.microsoft.com/en-us/graph/api/user-get?view=graph-rest-1.0&tabs=http

https://graph.microsoft.com/v1.0/me?$select=mailnickname,onpremisesUserprincipalName was the actual request I needed