.js

const Joi = require('joi');
const util = require('util');

const schema = Joi.object().keys({
    a: Joi.number().meta('bad a'),
    b: Joi.number().meta('bad b')
}).error(errs => {
    let obj = { };
    errs.forEach(err => {
        obj[err.path] = Joi.reach(schema, err.path).describe().meta[0];
    });
    return JSON.stringify(obj);
});

const value = {
    a: 'AAA',
    b: "BBB"
};

const result = Joi.validate(value, schema, { abortEarly: false });
console.log(JSON.parse(result.error.message));