JSONSchemaType fails wiht type when a field is declared nullable

[gpad]

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

8.12.0

Ajv options object

export const ajv = new Ajv({ strict: true })

I'm using Typescript

interface MyType {
  id: string
  value: string | null
}

const MySchema: JSONSchemaType<MyType> = {
  type: "object",
  properties: {
    id: { type: "string" },
    value: { type: "string", nullable: true },
  },
}
export const validateMySchema = ajv.compile<MyType>(MySchema)

I'm getting this issue:

src/user/validation.ts:13:7 - error TS2322: Type '{ type: "object"; properties: { id: { type: "string"; }; value: { type: "string"; nullable: true; }; }; }' is not assignable to type 'JSONSchemaType<MyType>'.
  The types of 'properties.value' are incompatible between these types.
    Type '{ type: "string"; nullable: true; }' 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'.

13 const MySchema: JSONSchemaType<MyType> = {

Also adding null as type:

const MySchema: JSONSchemaType<MyType> = {
  type: "object",
  properties: {
    id: { type: "string" },
    value: { type: ["string", "null"], nullable: true },
  },
}
export const validateMySchema = ajv.compile<MyType>(MySchema)

I have the same error:

src/user/validation.ts:13:7 - error TS2322: Type '{ type: "object"; properties: { id: { type: "string"; }; value: { type: ("string" | "null")[]; nullable: true; }; }; }' is not assignable to type 'JSONSchemaType<MyType>'.
  The types of 'properties.value' are incompatible between these types.
    Type '{ type: ("string" | "null")[]; nullable: true; }' 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'.

13 const MySchema: JSONSchemaType<MyType> = {

also removing the nullable, same issue:

const MySchema: JSONSchemaType<MyType> = {
  type: "object",
  properties: {
    id: { type: "string" },
    value: { type: ["string", "null"] },
  },
}
export const validateMySchema = ajv.compile<MyType>(MySchema)

Type '{ type: "object"; properties: { id: { type: "string"; }; value: { type: ("string" | "null")[]; }; }; }' is not assignable to type 'JSONSchemaType<MyType>'.
  The types of 'properties.value' are incompatible between these types.
    Type '{ type: ("string" | "null")[]; }' 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...'.
      Type '{ type: ("string" | "null")[]; }' is not assignable to type '{ type: "string"; } & StringKeywords & { allOf?: readonly UncheckedPartialSchema<string | null>[] | undefined; anyOf?: readonly UncheckedPartialSchema<string | null>[] | undefined; ... 4 more ...; not?: UncheckedPartialSchema<...> | undefined; } & { ...; } & { ...; }'.
        Type '{ type: ("string" | "null")[]; }' is not assignable to type '{ type: "string"; }'.
          Types of property 'type' are incompatible.
            Type '("string" | "null")[]' is not assignable to type '"string"'.

13 const MySchema: JSONSchemaType<MyType> = {

[epoberezkin]

JSONSchemaType equates "nullable" to optional property definition, string | null is not supported (I think)

cc @erikbrinkman am I right?

[erikbrinkman]

Yes, for ajv 8, nullable actually means undefined not null. the v9 branch has a version of the schema capable of recognizing the difference that you can patch in if you want if you really want. Also note, that JSONSchemaType requires that you use the required keyword, which you probably want to specify in this case.

See:

ajv/lib/types/json-schema.ts

Lines 175 to 187 in 490eb8c

	type Nullable<T> = undefined extends T
	? {
	nullable: true
	const?: null // any non-null value would fail `const: null`, `null` would fail any other value in const
	enum?: Readonly<(T | null)[]> // `null` must be explicitly included in "enum" for `null` to pass
	default?: T | null
	}
	: {
	nullable?: false
	const?: T
	enum?: Readonly<T[]>
	default?: T
	}