How to enforce array entries to be keys in a record in TypeScript?

Question

I want to define a TypeScript interface like this:

interface ObjectSchema {
  properties: Record<string, any>;
  required: Array<string>;
}

but with the added constraint that entries of required should be keys of properties. How do I go about this?

A validly typed object would be:

let schemaA = {
  properties: {
    foo: {},
    bar: {},
  },
  required: ["foo"]
}

A invalidly typed object would be:

let schemaA = {
  properties: {
    foo: {},
    bar: {},
  },
  required: ["baz"] // "baz" isn't a key in properties.
}

Answer 1

Two type variables must be used, one for the keys of properties, one for the subset of said keys for required.

asObjectSchema is just a convenient function to exploit the inference, so we don't have to annotate the type variables.

interface ObjectSchema<A extends string, B extends A> {
  properties: Record<A, any>
  required: Array<B>
}

const asObjectSchema = <A extends string, B extends A>(
  schema: ObjectSchema<A, B>
): ObjectSchema<A, B> => schema

const schemaA = asObjectSchema({
  properties: {
    foo: {},
    bar: {},
  },
  required: ['foo'],
})

const schemaB = asObjectSchema({
  properties: {
    foo: {},
    bar: {},
  },
  required: ['baz'], // Type '"baz"' is not assignable to type '"foo" | "bar"'
})

Playground