Angular httpClient: mapping response to model without losing non-specified properties

Question

Suppose I have the class

export class DummyClass {
   name: string;
   value: number = 10;
}

So a class that holds only name and value with a default value of 10.

If I now want to get data from the server and I initiate my call.

this.httpClient.post<DummyClass>(...)

And the json response is

{
   "name": "john doe"
}

Then my class also looks like this. This implies I have complete lost my property value with its default value as wel. And this was the whole point of my default value -> such that the server does not need to send default values.

How can I correctly map the json response to a model and not lose default values.

user17740968 · Accepted Answer · 2022-04-15 10:24:10Z

1

First of all, when you fetch data from an API, you don't get a class instance. You get a plain object.

This means that your API response follows the class interface, but does not instanciate it.

There might be some libraries around to do that, haven't checked.

To resolve your issue, you can use a pipe in your API call :

this.http.post<DummyClass>(...).pipe(
  map(res => {
    const instance = new DummyClass();
    Object.assign(instance, res);
    return instance;
  })
);

There a LOTS of ways to do it, with each having their pros and cons, but this one is the most broad and simple one I think.

answered Apr 15, 2022 at 10:24

user17740968

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2 Comments

Wouter Vandenputte Over a year ago

I have been thinking about that solution as well (to pipe-map them) but the problem with this is with nesting. if the class with default value isn't the root object but some child object of the root object than this Object.assign solution wont work either.

user17740968 Over a year ago

That's why I said there is multiple solutions to it. I found a library that could do it for you : github.com/typestack/class-transformer

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