Why use Object.create in exercise 19?

[prashcr]

In the exercise, Object.create is used to "create a fast copy of the accumulated map".
I don't see what's fast or useful about using Object.create instead of simply mutating the map.
I ran tests, and I see no reason why Object.create should be used instead.

http://jsperf.com/object-create-vs-mutation-in-reduce/2

[ghost]

I'm no expert, but I figured from reading it that they were addressing mutable vs. immutable data constructs. They even admit in the explanation paragraph (currently anyway) that it is slower than mutating the object:

// Object.create() is perfect for functional programming because it
// makes creating a new object with a different member value almost
// as cheap as changing the member on the original object!

Edit: You can see a slightly more real world example of Object.create in exercise 37:

function(queue, movieListsMessage) {
  var copyOfMovieLists = Object.create(movieListsMessage.list);
  if (queue !== undefined) {
    copyOfMovieLists.push(queue);
  }
  return copyOfMovieLists;
});

They call the variable copyOfMovieLists, so it looks like it's a nifty way to do an immutable copy of a list.

[kurzninja]

As @bill-mybiz said, I'm not expert but I think he's right: mutating the original copy through out the call stack would be considered a "side effect" and not very "functional". Creating and returning a new copy would preserve state of the input of each call while still giving you the correctly mutated copy on return.

[prashcr]

Hmm, I understand
90% speed penalty is definitely too much for real world use though.

I was curious as to how a typical immutable JS library would perform, check it out
http://jsperf.com/object-create-vs-mutation-in-reduce/4
50% penalty this time.

Would love to see even better perf

[gabrielkunkel]

Aside from performance, it makes testing a pain, because Jasmine won't look at the prototype properties.

[varungupta85]

@gabrielkunkel In that case, would Object.assign be a better alternative and it turns out that Object.assign is faster than Object.create at least according to the below test which is edited from test from @prashcr
http://jsperf.com/object-create-vs-mutation-in-reduce/6

[morenoh149]

would both create and assign achieve the desired effect?

[gabrielkunkel]

@varungupta85 Yes! Aside from being a little bit faster than Object.create(), Object.assign() is more testable with Jasmine. (Solves the issue I ran into while going through the tutorial.)

@morenoh149 They do different things, but for the sake of the tutorial they do achieve the same desired effect.

See my pull request.