NestJS Custom Decorator for Validating Objects

A custom class-validator Decorator to validate if a particular property is object or not

Data Transfer Objects (DTOs) are commonly used for validating request body. Storing metadata as a JSON type column in PostgreSQL is a common and flexible approach, especially when dealing with unstructured or semi-structured data. To validate if the property is an object, we can use the following decorator.

is-json-object.ts
import { ValidationArguments, registerDecorator } from "class-validator";
 
function IsJsonObject() {
  return function (object: object, propertyName: string) {
    registerDecorator({
      name: "isJsonObject",
      target: object.constructor,
      propertyName,
      validator: {
        validate(value: unknown) {
          return (
            typeof value === "object" && value !== null && !Array.isArray(value)
          );
        },
        defaultMessage(validationArguments?: ValidationArguments): string {
          return `${validationArguments.property} must be a valid object`;
        }
      }
    });
  };
}
 
export default IsJsonObject;