Add additional property to Observable array based on another array

Question

I have two Observable arrays type of Observable<MyType>

export interface MyType{
  title: string;
  id: string;
  other: [];
}

I want to add additional property exists to the first array and set it to true if the item is exists in the second array:

  const combined$ = combineLatest(this.first$, this.second$);
    this.output$ = combined.pipe(
      map((x, y) => {
        return x.map(a => {
          a.title = a.title;
          a.id = a.id;
          a.other = a.other;
          a.exists = y.find(b => b.id === a.id )
        });
      })
    );

Always getting [...undefined] result if subscribing to the output observable this.output$.subscribe(console.log);

Any ideas how to fix?

Answer 1

Please be aware that find returns the element found in the array (otherwise undefined). Better use some. Plus, when you return the object on the map, you should use either a regular return statement or an object enclosed in parenthesis.

const combined$ = combineLatest(this.first$, this.second$);
this.output$ = combined.pipe(
    map(([x, y]) => {
        return x.map(a => {
            return { 
                ...a,
                exists: y.some(b => b.id === a.id)
            };
         });
     })
);