In Angular 14, how do I apply a function to a model field if HttpClient won't invoke its constructor?

I'm using Angular 14. I have a number of calls in my application that return my model via HttpClient GET and POST requests:

  constructor(
    http: HttpClient, 
	...
  ) {}

	...
	return this.http.post<MyObject[]>(`${this.entitiesUrl}search`, searchObj)

I'm noticing that when these requests are made, the constructor of the model isn't called. I'm looking for a way to apply a function to some of the fields of my newly-built object before it is used by other parts of the application. Applying a "map" to the result of HttpClient calls is certainly an option, but there are so many, I would have to apply the same map dozens of times, which seems wasteful and poor coding practice.

In your example MyObject[] is just a hint, in reality there is something like JSON.parse(body) as MyObject[]. It also means that you can not have any methods in your model because they will not exist on deserialized objects.

It really depends on what exactly you want to achieve.

If you want to alter some fields by its name or type or value pattern, or inject the same set of methods then interceptor is ok. You can even have several interceptors and enable/disable them using fake headers in the request. If you don't use interceptors and don't plan to use interceptors then you can wrap HttpClient into custom service that simply appends pipe(map(x => ...)) to post/get methods.

Example:
https://stackoverflow.com/questions/46559268/parse-date-with-angular-4-3-httpclient

If you want to convert deserialized objects into instances of concrete classes then it is not possible in some generic way, you will have to implement all the mapping, custom constructors, class registry/factory, discriminators and etc.

Alternative option is to use some sort of client code generation (e.g. OpenApi or Swagger). Usually there is a way to alter template that is used to generate code, so you can append custom code that does dates conversion and it will be automatically applied to all services.

What you are asking for is to get the benefits of TypeScript type safety, but you do not want to implement the heavy lift required to gain those benefits.

In TypeScript, types are not strictly enforced without extra steps. You can take an interface and apply it to a response to gain the benefits of dot notation, and "implied" type checking via the IDE during development, however, you will quickly realize that the types of that interface can be violated by the API... this is a "good faith" data contract implementation, and, you are using it as an implied shape to your data response and trusting the API to not violate it.

If you want to go a step beyond this, you will need to use a class pattern that implements an interface via constructor to "enforce" explicit type checking as the class instance is created... there is no other way that I know of to to gain this benefit... and it requires you to use a map implementation to loop over the response and map the data as you have already pointed out.

There is information you will know at the time of the API call that can be used during the response for classifying the response shape into the respective model class... you can certainly "abstract" this away in to "several services" by making the services API/model specific, but in the end, no matter the implementation you choose, you will have to implement classification/mapping logic in some way/at some level to gain type safety for your API responses.

True type safety in TypeScript only exists at runtime by mapping responses to class objects (run time) that implement explicit data type checking via the constructor.

Additional info on how TypeScript behaves:
https://stackoverflow.com/questions/59135732/runtime-typesafety-in-typescript/59136620#59136620