schm-joi.js

const schema = require("schm");
const Joi = require("joi");

const joiValidate = constraints => prevSchema => prevSchema.merge({
  validate(values) {
    const parsed = prevSchema.parse(values);
    return Joi.validate(parsed, constraints);
  }
});

const userSchema = schema(
  { username: String },
  joiValidate({
    username: Joi.string().alphanum().min(3).max(30).required()
  })
);

userSchema.validate({ username: 'ab' });