What is the best approach to implementing self-referencing type validation in TypeScript?

Question

What is the best approach to implementing self-referencing type validation in TypeScript?

I am working on creating a specific type that guarantees all fields listed in the requiredFields array are included in the defaults object.

The goal is to have defaults trigger type errors if any of the names from requiredFields are missing, and if the values do not match the specified options.

The requiredFields array should be adaptable. While the entire object will be known statically and not assessed at runtime, I want to set up various versions with the same type logic to ensure no omissions in registering defaults.

const configuration: FormConfiguration = {
  defaults: { // should throw error for missing `numbers`
    letters: 'f', // should throw error because 'f' is not an option
  },
  requiredFields: [
    {
      name: 'letters',
      options: ['a', 'b', 'c'],
    },
    {
      name: 'numbers',
      options: [1, 2, 3],
    },
  ],
}

While there are other methods to define defaults for form items, this example serves as a model for a more intricate type that I am striving to build.

This is my current approach to defining types:

type Names = FormConfiguration['requiredFields'][number]['name']
// returns:
// type Names = string

type Options =
  FormConfiguration['requiredFields'][number]['options'][number]
// returns:
// type Options = string | number

type FormConfiguration = {
  requiredFields: Array<{
    name: string
    options: (string | number)[]
  }>
  defaults: Record<Names, Options>
}

Based on the results, it appears there is no self-referencing or correct per-item options retrieval happening.

There must be a way to achieve this, right?

typescript

Answer 1

Answer №1

Certain TypeScript types like FormConfiguration have limitations in terms of self-referencing. To address this, utilizing generics is recommended, such as defining the type as generic through FormConfiguration<F>. Another approach involves using the polymorphic this type, although it may not offer significant benefits and could complicate the process.

To implement a FormConfiguration<F> where F represents the type of the requiredFields array elements:

interface FormConfiguration<F extends { name: string, options: any[] }> {
  defaults: { [T in F as T["name"]]: T["options"][number] };
  requiredFields: F[]
}

The defaults type in the above code snippet utilizes key-remapped mapped types, mapping keys from the name property of F to corresponding values from the options property.

In practical usage, manually specifying the type can result in redundancy. An alternative method involves creating a helper function that allows for type inference. For example, instead of explicitly stating

const configuration: FormConfiguration<infer> = { ⋯ };

, you could use

const configuration = formConfiguration({ ⋯ });

, which achieves a similar outcome without much difference in effort.

Here's an example of the helper function implementation:

const formConfiguration = <const F extends { name: string, options: any[] }>(
  f: FormConfiguration<F>) => f;

By leveraging a const type parameter, literal types for the name and options fields are inferred, enhancing precision during type evaluation.

To test the functionality:

const configuration = formConfiguration({
  defaults: {
    numbers: 1,
    letters: "b"
  },
  requiredFields: [
    {
      name: 'letters',
      options: ['a', 'b', 'c'],
    },
    {
      name: 'numbers',
      options: [1, 2, 3],
    },
  ],
}); // works correctly

This test compilation validates the correctness of the configuration setup, ensuring proper error handling when mistakes occur in defining the fields or their values within the configuration object.

Playground link for the code

Answer 2

Certain TypeScript types like FormConfiguration have limitations in terms of self-referencing. To address this, utilizing generics is recommended, such as defining the type as generic through FormConfiguration<F>. Another approach involves using the polymorphic this type, although it may not offer significant benefits and could complicate the process.

To implement a FormConfiguration<F> where F represents the type of the requiredFields array elements:

interface FormConfiguration<F extends { name: string, options: any[] }> {
  defaults: { [T in F as T["name"]]: T["options"][number] };
  requiredFields: F[]
}

The defaults type in the above code snippet utilizes key-remapped mapped types, mapping keys from the name property of F to corresponding values from the options property.

In practical usage, manually specifying the type can result in redundancy. An alternative method involves creating a helper function that allows for type inference. For example, instead of explicitly stating

const configuration: FormConfiguration<infer> = { ⋯ };

, you could use

const configuration = formConfiguration({ ⋯ });

, which achieves a similar outcome without much difference in effort.

Here's an example of the helper function implementation:

const formConfiguration = <const F extends { name: string, options: any[] }>(
  f: FormConfiguration<F>) => f;

By leveraging a const type parameter, literal types for the name and options fields are inferred, enhancing precision during type evaluation.

To test the functionality:

const configuration = formConfiguration({
  defaults: {
    numbers: 1,
    letters: "b"
  },
  requiredFields: [
    {
      name: 'letters',
      options: ['a', 'b', 'c'],
    },
    {
      name: 'numbers',
      options: [1, 2, 3],
    },
  ],
}); // works correctly

This test compilation validates the correctness of the configuration setup, ensuring proper error handling when mistakes occur in defining the fields or their values within the configuration object.

Playground link for the code

What is the best approach to implementing self-referencing type validation in TypeScript?

Answer №1

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