Request bodies, cache values and queue messages arrive as plain JSON. How do you turn them into class instances, and how do you control what a class instance looks like in API responses?

JSON holds only data. Prototypes, methods, getters, Date objects, Maps and #private fields are lost, or become strings or {}. So you must convert both ways on purpose.

JSON to instance:

• Static factory: Order.fromJSON(data) validates the input, turns strings back into Dates and builds nested objects. Clear, and no library needed.
• class-transformer: plainToInstance(CreateOrderDto, body) creates the class. Nested classes need @Type(() => LineDto). NestJS's ValidationPipe with transform: true does this, then runs class-validator.

Instance to JSON:

• Add a toJSON() method. JSON.stringify calls it and uses its return value.
• Or map to a response DTO in the controller.
• In NestJS, ClassSerializerInterceptor with @Exclude() on passwordHash removes it from every response.

Watch out:

• structuredClone also drops the prototype.
• Values in a cache or queue may come from older code. Version them, or validate them when you read them.