Get Current login User details

var UserID= _spPageContextInfo.userId;

function GetCurrentloginUser()
{
 var requestUri = _spPageContextInfo.webAbsoluteUrl + "/_api/web/getuserbyid(" + UserID+ ")";
 var requestHeaders = { "accept" : "application/json;odata=verbose" };
 $.ajax({
   url : requestUri,
   contentType : "application/json;odata=verbose",
   headers : requestHeaders,
   success : onSuccess,
   error : onError
 });
}

/* Function to get currently logged on user name for On Success */

function onSuccess(data, request){

  var name = data.d.LoginName.split('|')[2];
  loginusername=name.split('@')[0]

}

/* Function to alert an error message on Error */

function onError(error) {
  alert(error);
}