"Reactive"

Reactive websites (layouting)

Reactive websites, responsive design, adaptive websites, and mobile friendly websites, are all different takes on very similar themes

largely (but not only) being about layouting for different shapes and sizes of screen, and dealing with different kinds of input (e.g. touch verses mouse).

In webby context,

'responsive' often means that the same thing will naturally layout in a different sensible way on a different-shaped or different-sized screen

(regardless of why it is larger/smaller)

(and you'll get people resizing the browser and saying "see?")

'reactive' (and 'adaptive') often means you get sent a somewhat different site between desktop and mobile.

(regardless of whether it is larger/smaller)

...yet there are no hard boundaries here - the 'regardness' lines are not as hard as you'ld think.

Reactive programming (state management)

Reactive programming is programming style where state changes are automatically propagated to where they need to go.

This tends to mean a relatively declarative style - where you declare the flow and transformation of data, e.g. thinking in terms of

events (rather than things as ordered by code)

sources of those events (e.g. interactable items)

things that process those events

...and possibly...

• a fairly asynchronous system of execution (in terms of event ordering)

• grouping those events in event streams

(It's also a lot closer to functional programming than people seem to realize.)

This concept is used anywhere from

the model model that makesof GUI updates go, particularly on the web (where we may want things to be more redefinable than MVC easily gives us)

see also e.g. Vue, Svelte, where what looks like regular JS

program language design

e.g. languages that add features like coroutines

a fully reactive languages have proven to be useful for specific purposes but not as a generic language