Custom Validation with Joi

Joi is a library that lets us validate an object’s structure with ease.

In this article, we’ll look at how to validate objects with Joi.

Getting Started

We can install Joi by running:

npm i joi

Then we can use it by writing:

const Joi = require("joi");

const schema = Joi.object({
  username: Joi.string().alphanum().min(3).max(30).required(),
  password: Joi.string().pattern(new RegExp("^[a-zA-Z0-9]+$"))
});

const value = schema.validate({ username: "abc", password: "abc123" });
console.log(value);

We create a schema with Joi by calling its built-in methods.

The keys are the property names,

string makes sure it’s a string.

alphanum checks for alphanumeric characters.

min checks for a minimum amount of characters.

max checks for a max amount of characters.

required makes the property value required.

pattern checks for a given regex pattern.

Once we created a schema, we called schema.validate to validate the property values.

It returns an object with the value and error properties.

value has the argument we passed in.

error has any error that are found.

If we call:

const value = schema.validate({});

then error would be an object with the message:

'ValidationError: "username" is required'

The details property has more details.

We can do the validation asynchronously with the validateAsync method.

For example, we can write:

const Joi = require("joi");

const schema = Joi.object({
  username: Joi.string().alphanum().min(3).max(30).required(),
  password: Joi.string().pattern(new RegExp("^[a-zA-Z0-9]+$"))
});

(async () => {
  try {
    const value = await schema.validateAsync({
      username: "abc",
      password: "abc123"
    });
    console.log(value);
  } catch (err) {
    console.log(err);
  }
})();

to do the validation.

value has the value we passed into validateAsync .

And err has validation errors if there are any.

Assertions

The assert mthod lets us validate against a schema and throws an error if validation fails.

For example, we can write:

Joi.assert("x", Joi.number());

Then we would get:

"value" must be a number

error displayed.

The first argument is the value to validate.

The 2nd argument is the data type to validate against.

The attempt method validates against a schema and returns a valid object.

It’ll throw an error if validation fails.

For example, we can write:

const result = Joi.attempt("10", Joi.number());
console.log(result);

and result is 10.

Defaults

The defaults method returns a new joi instance that applies the provided modifier function to every new schema.

For example, if we have:

const Joi = require("joi");
const custom = Joi.defaults((schema) => {
  switch (schema.type) {
    case "string":
      return schema.allow("");
    case "object":
      return schema.min(1);
    default:
      return schema;
  }
});

const schema = custom.object();

Then schema is schema.min(1) since 'object' is the value of schema.type .

in

The in method returns a reference to another schema.

For example, we can write:

const Joi = require("joi");
const schema = Joi.object({
  a: Joi.array().items(Joi.number()),
  b: Joi.number().valid(Joi.in("a"))
});

Then we check against the schema for property a , which is an array that have numbers.

So we check that it’s a number that’s in the a array with the in method.

Conclusion

We can check for object properties and values with Joi.