Typescript - set default value for class members

Question

I have a simple model:

export class Profile extends ServerData {
  name: string;
  email: string;
  age: number;
}

When I am make a call to the server (Angular 4, $http) I often get the following response:

{
  name: string;
  email: string;
}

The age property is missing.

Is there any way to use my model and create a default age in case it is missing? I would prefer not to have to create 2 separate models if possible.

I don't want to create the age as an optional property - I need it, even if it is just with an incorrect default.

UPDATE:

This is the call I making to the server:

results-manager.component.ts:

this.resultsManagerService.getResults(this.config.request.action, this.pagingAndSorting, this.args).subscribe(
  results => {
    this.results = this.results.concat(results as Profile[]);

results-manager.service.ts:

getResults(action: string, pagingAndSorting: PagingAndSorting, args: string[]): Observable<Profile[]> {
return this.apiService.callServer(
  pagingAndSorting,
  action,
  ...args );

}

The request works and I receive the response, but even if I define the default values (as suggested by @msanford's answer) they get removed when I receive the response back in the component. Likewise if i add a constructor to the model (as per Mike Tung's answer).

It seems like the backend response is completely overwriting the model - not just assigning the values.

How can I get it to just assign the values to the model and not remove the values it does not return?

Answer 1

Yes, easily, and you don't need to add a class constructor.

export class Profile extends ServerData {
  name: string;
  email: string;
  age: number = 0;
}

The ability to define default values is one of the main things that differentiates a class from an interface.

For this to work you need to call new Profile() somewhere in your code, otherwise a class instance won't be created and you won't have defaults set, because the above TypeScript will compile to the following JavaScript:

var Profile = /** @class */ (function () {
    function Profile() {
        this.age = 0;
    }
    return Profile;
}());

So just using it for type assertion at compile-time isn't sufficient to set a default at run-time.

See it in action in the TypeScript Playground.