Rxjs: The scan operator

Question

I have been reading this code if I will replace scan with map I get can not get property "getTime" of undefined, why is it happening I assume that both operator takes an item emits from observable and apply some function on it

this.clock = Observable.merge(
        this.click$,
        Observable.interval(5000)
    )
        .startWith(new Date())
        .map((acc : Date)=> {
            const date = new Date(acc.getTime());
             date.setSeconds(date.getSeconds() + 1);
            return date;
        });

Please have a look at the following suggestions for debugging questions : http://stackoverflow.com/help/mcve — user3743222, Jul 04 '16 at 15:37
Second advice is to not assume something, but to read the documentation of the functions you are using : https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/scan.md for rxjs v4. Note that for Rxjs v2, the seed comes first — user3743222, Jul 04 '16 at 15:38

score 0 · Answer 1 · answered Jul 04 '16 at 15:39

0

Because you merge two streams into a single one. You will receive events from click$ or interval. In these cases, they aren't of type Date so you can use the getTime method.

The scan operator allows keeping a state between events. The map one simply converts an input into an output. In the case of the last one, you will receive the event itself...

answered Jul 04 '16 at 15:39

Thierry Templier

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What does it mean scan operator keeps a state between events – blackHawk Jul 04 '16 at 15:45
I assume that .startWith(new Date()) emitting a date object and map will receive it so getTime should be read on acc – blackHawk Jul 04 '16 at 15:46
Yes but it's only for the first event... So it will initialize the value received first by the scan callback. – Thierry Templier Jul 04 '16 at 15:54
What Im assuming that code will start execution from startwith operator this will emit a date object which will go to map function(actually there was scan operator instead of map i replaced just for sake of understanding) because its type of Date, it should read getTime() (but there is error can not read property) – blackHawk Jul 04 '16 at 16:07
and the merge operator merging existing subject observable and interval(I assume that interval is used only for delaying it to 5 sec) and this will received by map function on second iteration – blackHawk Jul 04 '16 at 16:11
Perhaps this link could help you to understand: https://gist.github.com/staltz/868e7e9bc2a7b8c1f754. – Thierry Templier Jul 04 '16 at 16:22
When you merge streams, the new / resulting stream will receive events from them... You will receive corresponding values within operator callbacks. With scan, you have an additional parameter to handle a kind of context across events. – Thierry Templier Jul 04 '16 at 16:24
Okay so with scan we will have a context that acc is of type Date otherwise threre will be no way to recognize this, e.g with map – blackHawk Jul 04 '16 at 16:41
It means that we have maintained the state of acc as Date object, M i right? – blackHawk Jul 04 '16 at 16:42

Rxjs: The scan operator

1 Answers1