JsonSchema.ts

import Ajv, { JSONSchemaType, ErrorObject } from 'ajv'
import { FromSchema } from 'json-schema-to-ts'

interface AggregatedValidationError extends AggregateError {
    errors: ErrorObject[]
}

/**
 * ajvは全スキーマをキャッシュするので、インスタンスごとにnewせず、使い回す
 */
const ajv = new Ajv({ strict: true })

type SafeParseResult<T> =
    | { success: true, data: T, error: null }
    | { success: false, data: null, error: AggregatedValidationError }

export class JsonSchema<T> {
    private readonly validate

    /**
     * JSON Schemaを使ってパースしてくれるクラス。
     * 
     * NOTICE: ajvの`JSONSchemaType`は、
     * optionalなpropertyは必ずnullableだし、
     * nullableなものは必ずoptionalでもあるという仮定がある
     * 
     * @see https://github.com/ajv-validator/ajv/issues/1375
     */
    constructor(readonly schema: JSONSchemaType<T>, ajvInstance: Ajv = ajv) {
        this.validate = ajvInstance.compile(schema)
    }

    private get lastError(): AggregatedValidationError {
        return new AggregateError(this.validate.errors ?? [], `JSON did not match the schema: ${this.schema.$id ?? 'Unknown'}`)
    }

    /**
     * @throws {AggregatedValidationError}
     */
    parse(data: unknown): T {
        const valid = this.validate(data)
        if (valid) {
            return data
        } else {
            throw this.lastError
        }
    }

    safeParse(data: unknown): SafeParseResult<T> {
        const valid = this.validate(data)
        if (valid) {
            return { success: true, data, error: null }
        } else {
            return { success: false, data: null, error: this.lastError }
        }
    }
}

const SCHEMA = {
    $id: 'User',
    type: 'object',
    properties: {
        id: { type: 'number', nullable: true },
        name: { type: 'string' }
    },
    required: ['name'],
    additionalProperties: false
} as const

type User = FromSchema<typeof SCHEMA>
const User = new JsonSchema<User>(SCHEMA)

const user = User.parse({ id: 1, foo: 'John' })