Once you've written the imperative library/util code once, your components are super slim and completely reactive/declarative. Wow.

It works if you always want b to be the value deriving from a. However in the example above, we want the value of b to be temporarily out of sync of a.

My mental model has always been that a reactive declaration like $: b = a * 2 defines b in it's entirety ("Here's my recipe for b and all the ingredients, please Svelte make b always up to date"). And outside of this declaration b is immutable.

First of all, here is what I meant by updating reactive declared variable

The intention of the issue #2444 was to propagate the changes of the reactive declared variables back to its dependencies. in the context of this example, would meant, updating b should update a as well.

You declare a reactive variable and it's dependencies ("recipe").

Flow Debugger First up is a Flow Debugger. This acts like regular code debuggers, but at the flow level. You can set breakpoints on node ports, either inputs or outputs. Then, whenever a message arrives at a breakpoint, it pauses either at that node, or the entire runtime.

Svelte's advantage here is that it indicates the need for an update at the place where the associated data is updated, instead of at each place the data is used. Then each template expression of reactive statement is able to check very quickly if it needs to rerender or not.

So how do we declare the variable y to be the value of x plus 5? That's where reactive programming comes in. Reactive programming is a way of programming that makes it possible solve this problem

follows the reactive development architecture, but with a twist. The main thing to know about reactive programming is that it updates UI contents automatically when you update the variables in the code

reactions bridge reactive and imperative programming

situation

Netflix JavaScript Talks - Async JavaScript with Reactive Extensions