reduce().map() ? ex20

[t]

Issue 1 Description

During ex 20, when I checked the solution I noticed something weird, why using map() after this reduce ? this reduce doesn't return an array so it doesn't compile. (I included concatMap just to be sure).

Screenshoot

Issue 2 Description

My solution doesn't valid though I got the right answer.

Screenshoot

My Code

return movieLists
    .concatMap(movie => movie.videos
        .map(v => {
            v.boxarts = v.boxarts
                .reduce((a, b) => a.width * a.height < b.width * b.height ? a : b)
            return { id: v.id, title: v.title, boxart: v.boxarts.url}
        })
    );

Browser Information

• Browser Name, Version: Firefox 47.0.1 and Google chrome
• Operating System: win 7
• Mobile, Desktop, or Tablet: Desktop

Hope this will help despite my bad english.
Have a nice day.

[rotexhawk]

Yes there seems to be a bug in exercise 20. It passes if you run it in the website but if you run it externally it will give you an error where that last .map() is.

[Sean Poulter]

CC: You can probably close this as a duplicate of #44 @morenoh149.

Good catch! It seems like a bug if you run reduce(...).map outside of the site but this is actually intentional. In Exercise 16 we write our own implementation of Array.prototype.reduce that returns the result in an Array. It has this little note:

Let's add a reduce() function to the Array type. Like map. Take note this is different from the reduce in ES5, which returns a value instead of an Array!

I found it confusing too. This was an intentional design choice that is discussed in #44. I'd summarize #44 by saying "we" decided to use the modified version of Array.prototype.reduce that returns an Array because it is similar to Observable.prototype.reduce returning an Observable. It's "more clear" to chain calls like reduce().map then it is to explain why the result types are different. (Personally I disagree with this approach. It'd be straightforward to explain we need to return an Array so we can chain the calls. 🤷‍♂️ )