订阅多个可观察项，条件匹配后仅获得一个

Question

我有一个可观察的物品阵列和一个可观察的物品。我想同时订阅两者，并从可观察项数组中获得可观察项的索引

我已经尝试过switchmap，concatmap和mergemap，但没有得到任何结果

selectedcategory$ = this.store.pipe(select(selectSelectedCategory));
```observable of array of items
allcategories$ = this.store.pipe(select(selectCategories));

```i am working with ngrx 

I an expecting index of item

Answer 1

import { combineLatest } from 'rxjs';
import { map } from 'rxjs/operators';

...

combineLatest(
  selectedcategory$,
  allcategories$,
).pipe(
  map(([selected, categories]) => categories.findIndex(/* whatever logic you need */),
).subscribe(...);