JavaScript wait for asynchronous function in if statement

I do this exact thing using async/await in my games code here

Assuming req.isLoggedIn() returns a boolean, it's as simple as:

const isLoggedIn = await req.isLoggedIn();
if (isLoggedIn) {
    // do login stuff
}

Or shorthand it to:

if (await req.isLoggedIn()) {
    // do stuff
} 

Make sure you have that inside an async function though!

You could promisify your function, like this:

req.isLoggedin = () => new Promise((resolve, reject) => {
    //passport-local
    if(req.isAuthenticated()) return resolve(true);

    //http-bearer
   passport.authenticate('bearer-login', (err, user) => {
       if (err) return reject(err);
       resolve(!!user);
   })(req, res);
});

And then you can do:

req.isLoggedin().then( isLoggedin => {
    if (isLoggedin) {
        console.log('user is logged in');
    }
}).catch( err => {
    console.log('there was an error:', err); 
});

Do not try to keep the synchronous pattern (if (req.isLoggeedin())), as it will lead to poorly designed code. Instead, embrace fully the asynchronous coding patterns: anything is possible with it.