@check-type: Generate TypeScript functions that check that a runtime value conforms to a given TypeScript type

This library automatically generates TypeScript functions for a given TypeScript type, which can dynamically check if a particular value conforms to that type. This is useful whenever you pass values across program boundaries, e.g. through a REST API, retrieving a value from a database or reading a file from disk.

For example, consider this type:

// @check-type
export type Person = {
  name: string;
  age: number;
}

We can run the code generator using yarn generate-schema --config '[{"root":"./src"}]', which generates a function assertPerson: (value: unknown) => Person¹ that takes any value and either validates that the value conforms to Person (and returns that value fully typed), or throws an exception describing why the value is not conforming to Person.

For instance, here are some examples:

const person = { name: "Peter"; age: 22 }
const notAPerson = { name: "Marry" }

assertPerson(person) // returns person (but typed now as Person)
assertPerson(notAPerson) // throws an exception, which the following message:
/*
value (aka. `{"person":"Peter"}`) does not conform to Person!
Field 'age' is not optional but missing from value
*/

Footnotes

1. Technically, the value argument has type NotPromise<T> which is defined as type NotPromise<T> = T extends Promise<unknown> ? never : T;. This helps prevent accidentally passing an unresolved promise to the assertion functions, which is basically never right. ↩