Typescript: impossible to use JSONSchemaType with mixed index signatures and defined properties

ajv version: 8.2.0

interface T {
  a: number;
  [key: string]: unknown;
}

const b: JSONSchemaType<T> = {
  type: 'object',
  properties: { a: { type: 'number' } },
  required: ['a'],
  additionalProperties: true, // doesn't affect anything
};

ts error

Type '{ type: "object"; properties: { a: { type: "number"; }; }; required: string[]; }' is not assignable to type 'JSONSchemaType<T, false>'.
  Type '{ type: "object"; properties: { a: { type: "number"; }; }; required: string[]; }' is not assignable to type '{ type: "object"; additionalProperties?: boolean | JSONSchemaType<unknown, false> | undefined; unevaluatedProperties?: boolean | JSONSchemaType<unknown, false> | undefined; ... 7 more ...; maxProperties?: number | undefined; } & { ...; } & { ...; } & { ...; }'.
    Type '{ type: "object"; properties: { a: { type: "number"; }; }; required: string[]; }' is not assignable to type '{ type: "object"; additionalProperties?: boolean | JSONSchemaType<unknown, false> | undefined; unevaluatedProperties?: boolean | JSONSchemaType<unknown, false> | undefined; ... 7 more ...; maxProperties?: number | undefined; }'.
      Types of property 'properties' are incompatible.
        Type '{ a: { type: "number"; }; }' is not assignable to type 'PropertiesSchema<T>'.
          Property 'a' is incompatible with index signature.
            Type '{ type: "number"; }' is not assignable to type '{ $ref: string; } | (JSONSchemaType<unknown, false> & { nullable: true; const?: undefined; enum?: readonly unknown[] | undefined; default?: unknown; })'.
              Type '{ type: "number"; }' is not assignable to type '{ oneOf: readonly JSONSchemaType<unknown, false>[]; } & { [keyword: string]: any; $id?: string | undefined; $ref?: string | undefined; $defs?: { [x: string]: JSONSchemaType<Known, true> | undefined; } | undefined; definitions?: { ...; } | undefined; } & { ...; }'.
                Property 'oneOf' is missing in type '{ type: "number"; }' but required in type '{ oneOf: readonly JSONSchemaType<unknown, false>[]; }'.

I’ve tried just about everything to make it work, only solution right now is to pass a different, less defined, type to JSONSchemaType.

Things I’ve tried

• typecasting the schemas
• defining the additional properties as { type: ‘string’ }
• all manners of redefining the index signature

[epoberezkin]

Going to close it.

The main reason to use JSONSchemaType is to avoid typos in large but common schemas. It doesn’t have to be used for all schemas, and some typescript types would always remain out of scope.

[erikbrinkman]

I’m intrigued by this, however there are typescript limitations with implementing it. If you define type Keys = keyof T what do you get? string | number. I’m not going to go into why, but from an inspection perspective, it’s not possible for JSONSchemaType to determine the explicitly named properties afaik.

The reverse should be possible. I’m not sure if JTDDataType supports it, but it probably should.