1.js

const schema = Joi.object().keys({
  isRegister: Joi.any(),
  username: Joi.string(),
  password: Joi.string(),
  email: Joi.string()
}).with('isRegister', 'username', 'password');

app.post('/register', function (req, res) {
  const isValid = schema.validate(Object.assign({ isRegister: true }, req.params));
});

app.post('/update_user', function (req, res) {
  const isValid = schema.validate(req.params);
});

2.js

const schema = Joi.object().keys({
  username: Joi.string(),
  password: Joi.string(),
  email: Joi.string()
});

app.post('/register', function (req, res) {
  const isValid = schema.requiredKeys('username', 'password').validate(req.params);
});

app.post('/update_user', function (req, res) {
  const isValid = schema.validate(req.params);
});