An error should not be thrown if the object is potentially 'undefined', as there is a filter in place from rxjs

During my service refactoring process, I encountered a frustrating issue. The API I am working with returns an object with various optional properties. To work with these properties, I need to check if they are undefined or not using an if statement, otherwise, errors occur. I attempted to simplify this process by incorporating the check into the filter operator after refactoring. However, despite my efforts, I still receive linting errors.

interface OptionalObject {
  values?: number[]
}

const obs$$ = new BehaviorSubject<OptionalObject>({});
obs$$.pipe(
  filter((val) => !!val.values),
  map((val) => val.values.length) // Object is possibly 'undefined'
).subscribe(() => {

});

I believe ESLint is being utilized in my project.

Even though checking with an if statement works, I prefer to use Rxjs's filter method for my implementation.

Answer №1

To solve this issue, one can utilize a TypeScript TypeGuard:

interface OptionalObject {
  items?: string[];
}

const objects$ = new BehaviorSubject<OptionalObject>({});
objects$
  .pipe(
    filter((value): value is {items: string[]} => !!value.items),
    map((value) => value.items.length)
  )
  .subscribe(() => {});

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