Node.js + Joi how to display a custom error messages?

Extending on Ashish Kadam's answer, if you have many different error types, you can check which type of error is present, and set its message accordingly:

Joi v17.4.0

v17.4.0 uses err.code

var schema = Joi.object().keys({
  firstName: Joi.string().min(5).max(10).required().error(errors => {
    errors.forEach(err => {
      switch (err.code) {
        case "any.empty":
          err.message = "Value should not be empty!";
          break;
        case "string.min":
          err.message = `Value should have at least ${err.local.limit} characters!`;
          break;
        case "string.max":
          err.message = `Value should have at most ${err.local.limit} characters!`;
          break;
        default:
          break;
      }
    });
    return errors;
  }),
  // ...
});

You can check the list of errors here: Joi 17.4.0 API Reference > Errors > List of errors

Joi v14.3.1

v14.3.1 uses err.type

var schema = Joi.object().keys({
  firstName: Joi.string().min(5).max(10).required().error(errors => {
    errors.forEach(err => {
      switch (err.type) {
        case "any.empty":
          err.message = "Value should not be empty!";
          break;
        case "string.min":
          err.message = `Value should have at least ${err.context.limit} characters!`;
          break;
        case "string.max":
          err.message = `Value should have at most ${err.context.limit} characters!`;
          break;
        default:
          break;
      }
    });
    return errors;
  }),
  // ...
});

You can check the list of errors here: Joi 14.3.1 API Reference > Errors > List of errors

Also you can check the any.error reference for more information. Quoting the docs:

Overrides the default joi error with a custom error if the rule fails where:

• err can be:
  • an instance of Error - the override error.
  • a function(errors), taking an array of errors as argument, where it must either:
    • return a string - substitutes the error message with this text
    • return a single object or an Array of it, where:
      • type - optional parameter providing the type of the error (eg. number.min).
      • message - optional parameter if template is provided, containing the text of the error.
      • template - optional parameter if message is provided, containing a template string, using the same format as usual joi language errors.
      • context - optional parameter, to provide context to your error if you are using the template.
    • return an Error - same as when you directly provide an Error, but you can customize the error message based on the errors.
• options:
  • self - Boolean value indicating whether the error handler should be used for all errors or only for errors occurring on this property (true value). This concept only makes sense for array or object schemas as other values don't have children. Defaults to false.

Original answer:

The current way (I personally find it better) is to use .messages() (or .prefs({messages})).

const Joi = require('@hapi/joi');

const joiSchema = Joi.object({
  a: Joi.string()
    .min(2)
    .max(10)
    .required()
    .messages({
      'string.base': `"a" should be a type of 'text'`,
      'string.empty': `"a" cannot be an empty field`,
      'string.min': `"a" should have a minimum length of {#limit}`,
      'any.required': `"a" is a required field`
    })
});

const validationResult = joiSchema.validate({ a: 2 }, { abortEarly: false });
console.log(validationResult.error); // expecting ValidationError: "a" should be a type of 'text'

Usage of .errors() is not recommended just to update default message with custom message.

.prefs({ messages }) is an elaborate way to provide more options as preferences. The other options for prefs are taken directly from options of .validate()

Further read: https://github.com/hapijs/joi/issues/2158

---

Update 1: I saw that the above explanation did not work out for some folks, so I have put up some code to test yourself. Check it here: https://runkit.com/embed/fnfaq3j0z9l2

Also updated the code snippet shared previously to have details from package inclusion, to usage, to calling the actual validation method.

---

Update 2: The list of joi error types and their description (for .messages() - like string.base, array.unique, date.min etc..) is available here.

---

Update 3: Joi has moved from hapi project to standalone: https://www.npmjs.com/package/joi. Make sure you are using latest version (or at least above v17).

Joi Version 14.0.0

const SchemaValidation = {
  coins: Joi.number()
    .required()
    .error(() => {
      return {
        message: 'Coins is required.',
      };
    }),
  challenge_name: Joi.string()
    .required()
    .error(() => {
      return {
        message: 'Challenge name is required.',
      };
    }),
  challengeType: Joi.string()
    .required()
    .error(() => {
      return {
        message: 'Challenge type is required.',
      };
    }),
  challengeDescription: Joi.string()
    .required()
    .error(() => {
      return {
        message: 'Challenge description is required.',
      };
    }),
};

In errors object you can get, error type and change message according.