Discussion on: Joi — awesome code validation for Node.js and Express

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Replies for: Would TypeScript eliminate the need for Joi (I've used Joi before at work)? I know Flow is similar to TypeScript so usually, people won't use both.

You would get the greatest benefit by using a static typechecker like TypeScript or Flow in combination with a dynamic validator like Joi or Ajv. It works roughly like this:

Define a static type
Define a validator function for that type, using Joi or Ajv as the underlying validation tool
Make the validator function return the input value cast to the static type (if validation succeeded) or the validation error (if validation failed)

For example:

interface Person {
  id: string;
  name: string;
}

const personValidator: Validator<Person> = Validator(
  Joi.object().keys({id: Joi.string(), name: Joi.string()}),
);

...

const result = personValidator.validate(personObj);
if (result instanceof ValidationError) {...}
else {
  // Now we know 100% that result is a valid Person with id and name
}