Nullable doesn't cover unions with null in it

[SMJSGaming]

What version of Ajv are you using? Does the issue happen if you use the latest version?

8.12.0

Your typescript code

export const UUID_PARAM: JSONSchemaType<{ uuid: string | null }> = {
    type: "object",
    additionalProperties: false,
    properties: {
        uuid: {
            nullable: true,
            type: "string",
            format: "uuid"
        }
    },
    required: [ "uuid" ]
};

Typescript compiler error messages

Type '{ type: "object"; additionalProperties: false; properties: { uuid: { nullable: true; type: "string"; format: string; }; }; required: "uuid"[]; }' is not assignable to type 'JSONSchemaType<{ uuid: string | null; }>'.
  The types of 'properties.uuid' are incompatible between these types.
    Type '{ nullable: true; type: "string"; format: string; }' is not assignable to type '{ $ref: string; } | ({ anyOf: readonly UncheckedJSONSchemaType<string | null, false>[]; } & { [keyword: string]: any; $id?: string | undefined; $ref?: string | undefined; $defs?: Record<...> | undefined; definitions?: Record<...> | undefined; } & { ...; }) | ({ ...; } & ... 1 more ... & { ...; }) | ({ ...; } & ... 2...'.
      Types of property 'nullable' are incompatible.
        Type 'true' is not assignable to type 'false'.

Describe the change that should be made to address the issue?

Nullable should cover unions which include null.

Are you going to resolve the issue?

I would love for this to be resolved but I don't have enough experience with the internals of AJV to solve this type error myself. I would prefer it if someone who maintains this library would take a look at this.

[oyamauchi]

I believe this is the same as: #1664

This is the workaround I'm using:

type MyClass = {
  field: number | null;
}

const nullable = <T>(input: T): T => {
  return { anyOf: [input, { type: 'null' }] } as T;
};

const schema: JSONSchemaType<MyClass> = {
  // ...
  properties: {
    field: nullable({ type: 'number' })
  }
}

The nullable function adds the right element to the schema, so that the right thing happens in runtime validation, but hides it from tsc.

[SMJSGaming]

The story you mentioned isn't exactly the same. Also I get that you can trick TS into accepting the values, but my point is that null unions and nullable should be considered the same thing rather than being a type error (especially considering it doesn't handle unions properly in genera making it impossible to properly validate them without tricking TS).

[Renegade334]

Duplicate: #2163

[jasoniangreen]

Closing as it is a duplicate of #2163