How to get server data

Template.registerHelper('getUserNameById',(a)=>{
    return Meteor.users.findOne(a).username;
});

if user login ,return username
if user no login,return undefind

i try Meteor.call(‘user’)
but Meteor.call(‘user’) is async
i cant get server data with other name

how to get server data?

thanks

thanks
:grinning::grinning::grinning::grinning::grinning: