Validate username available using express-validator

Writing custom validators for express-validator is easy, but to validate that username is not used we should do call to database which is asynchronous. Async validation is a little bit tricky so here we consider it.

First, add a custom validator which returns Promise:

router.use(expressValidator({
    customValidators: {
      isUsernameAvailable(username) {
        return new Promise((resolve, reject) => {
          User.findOne({ username: username }, (err, user) => {
            if (err) throw err;
            if(user == null) {
              resolve();
            } else {
              reject();
            }
          });
        });
      }
    }
  })
);

Now registration route for the rest endpoint looks next:

// Register User
router.post('/register/', (req, res) => {
  const username = req.body.username;
  const password = req.body.password;

  // Validation
  req.checkBody('username', 'Username is required').notEmpty();
  req.checkBody('username', 'Username already in use').isUsernameAvailable();
  req.checkBody('password', 'Password is required').notEmpty();

  req.asyncValidationErrors().then(() => {
    //no errors, create user
    const newUser = new User({
      username: username,
      password: password,
    });

    User.createUser(newUser, (err, user) => {
      res.json({
        status: 'success',
        errors: null,
      })
      console.log("New user:", newUser);
    });
  }).catch((errors) => {

      if(errors) {
          return res.json({
              success: false,
              errors: errors
          });
      };
  });
});