Angular 6 HttpClient return instance of class

Question

Before angular's new HttpClient was introduced, our objects returned from the http api call could be validated with the instanceof keyword. they no longer can with the HttpClient Module. I'm trying some simple methods but the type checks return false every time. the desired behavior of:

```

getCow() {
    return this.http.get<Cow>(ApiRoute.GET_COW, options)
        .map(res => res as Cow)
        .toPromise()
        .then((c: Cow) => {
            console.log(c instanceof Cow); //this is false
        })
}

```

would return true. does anyone know of a simple way to new up an instance behind the scenes of the http client?

Answer 1

TypeScript uses structural typing, i.e. c object doesn't have to be an instance of Cow class to conform to Cow type.

TypeScript types exist only at compilation time and don't affect JS output in any way (with the exception of emitted types which are used for Angular DI). as Cow asserts that res conforms to Cow type, while instanceof Cow expects that c is an instance of Cow class. Since Cow wasn't instantiated, cow instanceof Cow is false.

A class should be designed to support hydration (possibly via constructor parameters) and be instantiated explicitly:

class Cow {
  sound: string;
}

return this.http.get<Cow>(ApiRoute.GET_COW, options)
    .map(res => Object.assign(new Cow(), res as Cow))
    .toPromise()
    .then((c: Cow) => {
        console.log(c instanceof Cow);
    })

If some logic is needed to construct Cow instance from plain object (validation, nested object construction), this can be done in class constructor or separate helper function (e.g. Cow static method).