The RxJS function combineLatest() accepts a selector function as last parameter. This selector function receives the latest value of each observable as argument and can be used to return a new value that will be emitted.

// Array of observables
[stream1$, stream2$, stream3$],

// Selector function
(val1, val2, val3) => val1 + val2 + val3 // Return value will be emitted

Try the example above in the online playground.