Kitty Quinn Thanks a lot for the response. 👍👏

You are right, it would be nice to get an object instead of an array. This would improve readability and Typesafety.

We can do it with scan. Although I am not sure if scan is the correct operator to use. The signature of scan is shown as following:

scan(accumulator: function, seed: any): Observable

Scan takes an accumulator function — so it basically combines together all emitted values.

In our case we are not interested in an accumulation but only in the comparison between the pervious and the current.

Comparison example

I created a small Stackblitz that tries to show the difference. Feel free to edit it, maybe you have some better solution 😉

Pairwise

const routeEvents = ['RouteEvent 1', 'RouteEvent 2', 'RouteEvent 3', 'RouteEvent 4', 'RouteEvent 5']from(routeEvents).pipe(
   pairwise(),
   map((values) => ({previous: values[0], current: values[1]}))
)
.subscribe(e => console.log(e))

Outputs:

Scan

const routeEvents = ['RouteEvent 1', 'RouteEvent 2', 'RouteEvent 3', 'RouteEvent 4', 'RouteEvent 5']from(routeEvents).pipe(
   scan((acc: any, current: string) => {
      return {
          previous: acc.current || current,  
          current: current
      }
   }, {}))
)
.subscribe(e => console.log(e))

Outputs:

Scan also emits the first event so we could basically get rid of the startWith operator too.

I see your point with the object which is quite good. I would still prefer to use pairwise with an additional map function.

Using scan works but feels kind of using the wrong operator for our purpose.