Reactivity in app development means the user interface (UI) automatically updates whenever the underlying data changes, without the developer having to manually manipulate the DOM. This makes applications more dynamic and responsive.
In Svelte, reactivity is built into the language. Instead of using complex state management or a virtual DOM (like React or Vue), Svelte tracks changes to variables and directly updates the DOM where needed.
When a variable changes, the UI updates automatically wherever that variable is used.
You don’t need special APIs like
setState(React) orref(Vue) — just reassigning a variable is enough.Svelte uses a special
$:syntax for reactive statements, which re-run whenever their dependencies change.
In this example, whenever count changes, both the button label and the doubled value update automatically — no extra code is needed.