This example route for validation.

example.js

async function loginUser(userObject, callback) {
  // some validation for the request body or data supplied to function
  const { error, value } = Joi.validate(userObject, userObjectSchema);
  if (error) {
    callback(error);
  } else {
    try {
      // check for validate of the access torken the code for it is given in pervious example
      const isTokenValid = await fbHelper.validateToken(
        userObject.access_token,
        userObject.ffid
      );
      if (isTokenValid) {
        // find the user in db
        const userDetials = await User.findOne({ ffid: userObject.ffid });
        // if yes then just issue a token for it
        if (userDetials) {
          try {
            // code of this function is given in next example
            const token = await jwtHelper.sign({
              userId: userDetials._id
            });
            callback(null, token);
            // update new access token
            User.update(
              { ffid: passwordAndMail.ffid },
              { access_token: passwordAndMail.access_token }
            );
          } catch (errorInSign) {
            callback(errorInSign);
          }
        } else {
          // if not create new using your user model
          const newUser = new User(userObject);
          await newUser.save();
          // issue him a new token
          const token = await jwtHelper.sign({ userId: newUser._id });
          callback(null, token);
        }
      } else {
        callback("Invalid Facebook Token");
      }
    } catch (errorInSave) {
      callback(errorInSave);
    }
  }
}