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I think the second one isn't really a valid example. There are very few situations where you both want to have state mutations discard what the user has already entered and reflect user-provided state in real time. In other words even if it's supposedly "two-way" you don't actually want multiple inputs to share the same model and you likely don't want to represent the data the same way the form control needs to represent it internally.

As an example, let's say you have a formatted text input field that can be used to enter a decimal number. If the input contains no decimal separator, the decimal part is implicitly zero. But if you just use naive two-way binding with an actual decimal value this means you'll get in the way of the user trying to manually enter a decimal value (especially if the user tries to delete the decimal part starting with the separator).

As soon as any information is lost during the bind in either direction, you still need to explicitly think about where you want the data to flow into and out of the form field.

The archetypical example of two-way binding is definitely the auto-generated CRUD form that lets you edit a model in place, but IMO this is a tiny niche in practice because it falls apart as soon as you want to do anything non-trivial. It's great for prototypes though.

EDIT: To clarify, I think you're talking about two-way binding a form model to form inputs. But calling that two-way binding from an application developer's POV is kinda redundant because you still need changes from the form model to propagate outside the form in a more controllable and predictable way than two-way binding offers, so the two-way binding of the actual form fields becomes an implementation detail.

Redux is actually (at least partially) inspired by the Elm Architecture[0]

[0] http://redux.js.org/docs/introduction/PriorArt.html#elm

> I followed the link to the documentation, which looks fantastic

It is fantastic, and engrossing.

After I said that this morning I tabbed back to the docs and read through 'cover' to 'cover'.

Great docs, and looks like a great framework - I think that I could read through and understand the whole thing with very little JS knowledge is testament to that.

Simply put: because it's magic.

Two-way bindings are great for prototypes because you no longer need to think about how your state gets from one place in your application to another, you just shove your mutable state blob into every last part of your application and the framework does the rest.

But what you lose in return is the ability to control when state should change and sight of where changes come from. In the case of POJO state implementations like AngularJS 1 it also means you have to constantly diff the state against what you've last rendered to be able to respond to changes nobody told you about.

AngularJS 1 actually tried to optimise this a bit but ultimately it needed to pretty much control everything asynchronous in order for that optimisation to work and more often than not that resulted in hapless beginners wondering why their changes aren't always reflected in the UI (answer: because nobody told Angular it should check).

So in other words, your state consists of big balls of mud that may change shape without notice and if you pass one of them to any piece of code you must expect it to do just that without any way to tell until it happens (and good luck trying to figure out where it happened exactly).

Compare this to the "one-way data flow" React made popular: your state is a bunch of impenetrable rocks that roll into your application, which can't modify them but can do whatever it wants in response to them (e.g. push pixels around on the screen). If the application wants different rocks, it needs to explicitly tell wherever they came from that they should be different, at which point the application will be fed a different (or maybe identical) set of impenetrable rocks again.

In a word this is basically the difference between mutability and immutability, taken to the extreme. Although there's nothing in React forcing you to use immutable data structures, the one-way data flow assumes everything it's fed is immutable unless you say otherwise, and you pass in callbacks to be notified when state should change.

On the other hand, in two-way binding your state is mutable and extremely malleable by definition, and thus the implicit expectation for every operation is that it may have changed the state.

As a side-note: React actually has a dirty little secret called "context" which is the equivalent of thinking with portals: instead of passing ALL the state into your ENTIRE application's root, you pass the state container to a magical Pez dispenser that wraps your application root and then each component can be wrapped in a container that knows about the dispenser and asks it for a specific part of the state. That's how React-Redux works among other things and it's not entirely unlike dependency injection (except it doesn't happen globally but only in the context of the specific instance of the application).

You seem to think that because Facebook's website uses React it's a React app. Facebook is no different from other continuously developed applications that evolve organically. In other words: it's probably not a good example for a "best of breed" pure React app.

I'm guessing Facebook uses React more on facebook.com than it uses React Native in their mobile app (I think there was an article a while back on HN about the ridiculous amount of packages in their mobile app, which is a testament to their policy of bolting on new code instead of rewriting the old unless actually necessary) but it's a safe guess to say that there's more to facebook.com's frontend than just React.