NestJs JSON schema validator

is-json-schema.validator.ts

import Ajv from 'ajv/dist/2020';

import {
  ValidationArguments,
  ValidatorConstraint,
  ValidatorConstraintInterface,
  ValidatorOptions,
  registerDecorator,
} from 'class-validator';

@ValidatorConstraint({ name: 'isJsonSchema', async: false })
export class IsJsonSchemaConstraint implements ValidatorConstraintInterface {
  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  validate(schema: object, args: ValidationArguments) {
    // ensure that the string is parsable json
    try {
      JSON.parse(JSON.stringify(schema));
      // eslint-disable-next-line @typescript-eslint/no-unused-vars
    } catch (e) {
      return false;
    }

    // ensure that the json is a valid json schema
    const ajv = new Ajv({ strict: false });
    const validate = ajv.compile(JSON.parse(JSON.stringify(schema)));
    if (validate.errors?.length) return false;

    return true;
  }

  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  defaultMessage(args?: ValidationArguments): string {
    return 'Schema is not a valid JSON schema';
  }
}

/**
 * Allows the user to validate that a string is a valid JSON schema as a custom decorator
 * @example
 *  @IsJsonSchema()
 *  schema: Object;
 *
 * @param validationOptions the validation options
 */
export function IsJsonSchema(validationOptions?: ValidatorOptions) {
  return function (object: any, propertyName: string) {
    registerDecorator({
      target: object.constructor,
      propertyName: propertyName,
      options: validationOptions,
      constraints: [],
      name: 'isJsonSchema',
      validator: IsJsonSchemaConstraint,
    });
  };
}