How to validate array length with io-ts?

Question

I am working on an io-ts validation where I would like to validate the list length (it has to be between min and max). I am wondering if there's a way to achieve this behavior since it could come quite handy at runtime for API endpoint validation.

What I have so far is

interface IMinMaxArray {
  readonly minMaxArray: unique symbol // use `unique symbol` here to ensure uniqueness across modules / packages
}

const minMaxArray = (min: number, max: number) => t.brand(
  t.array,
  (n: Array): n is t.Branded<Array, IMinMaxArray> => min < n.length && n.length < max,
  'minMaxArray'
);

The code above does not work, it requires an argument for the Array-s and t.array is also not accepted. How could I make this work in a generic way?

Answer 1

Your definition is missing the typing and the codec for the array. You could make this work with a few modifications on the interface definition and extending the branded type with a codec:

interface IMinMaxArray<T> extends Array<T> {
  readonly minMaxArray: unique symbol
}

const minMaxArray = <C extends t.Mixed>(min: number, max: number, a: C) => t.brand(
  t.array(a),
  (n: Array<C>): n is t.Branded<Array<C>, IMinMaxArray<C>> => min < n.length && n.length < max,
  'minMaxArray'
);

Now you can create a definition like

minMaxArray(3,5, t.number)

If you want the definition to be more generic and composable, you could write a generic branded type that accepts a predicate:

interface RestrictedArray<T> extends Array<T> {
  readonly restrictedArray: unique symbol
}

const restrictedArray = <C>(predicate: Refinement<C[], ArrayOfLength<C>>) => <C extends t.Mixed>(a: C) => t.brand(
  t.array(a), // a codec representing the type to be refined
  (n): n is t.Branded<C[], RestrictedArray<C>> => predicate(n), // a custom type guard using the build-in helper `Branded`
  'restrictedArray' // the name must match the readonly field in the brand
)

interface IRestrictedArrayPredicate<C extends t.Mixed> {
  (array: C[]): array is ArrayOfLength<C>
}

Now you can define your restrictions. It's probably a good idea to define min and max separately, since they can be useful on their own too:

const minArray = <C extends t.Mixed>(min: number) 
  => restrictedArray(<IRestrictedArrayPredicate<C>>((array) => array.length >= min));
const maxArray = <C extends t.Mixed>(max: number)
  => restrictedArray(<IRestrictedArrayPredicate<C>>((array) => array.length <= max));

And combining these two you can define minMaxArray:

export const minMaxArray = <C extends t.Mixed>(min: number, max: number, a: C) => t.intersection([minArray(min)(a), maxArray(max)(a)])

Hope this helps.