How to use the output of an observable to filter another
You need to join these two streams e.g. with combineLatest:
let category$ = Observable.of({id: 1});let products$ = Observable.from([{name: 'will be included', cat_ids: [1, 5]}, {name: 'nope', cat_ids: [2, 3]}, {name: 'also yep', cat_ids: [1, 7]}]);return Observable.combineLatest(products$, category$) .map(([products, category]) => { return products.filter(prod => prod.cat_id.some(id => id === category.id); });Update
As @olsn pointed out with Observable.from you get stream of events not stream of array of events. Hence the solution should be:
let category$ = Observable.of({id: 1});let products$ = Observable.from([{name: 'will be included', cat_ids: [1, 5]}, {name: 'nope', cat_ids: [2, 3]}, {name: 'also yep', cat_ids: [1, 7]}]);return Observable.combineLatest(products$, category$) .filter(([product, category]) => { return product.cat_id.some(id => id === category.id); }) .map(([product, category]) => product);