Worklet

A worklet is a JavaScript function that has been marked to run on the UI thread rather than the main JavaScript thread. It is the core idea behind Reanimated.

You mark one with the 'worklet' directive, though in practice the Babel plugin adds it for you inside hooks like useAnimatedStyle:

const style = useAnimatedStyle(() => {
  'worklet';
  return { opacity: progress.value };
});

Because the worklet executes on the UI thread, it can compute a new style every frame even while JavaScript is blocked parsing a large API response. That is why a Reanimated gesture keeps tracking your finger when a setState-driven animation would freeze.

The trade-off is isolation: a worklet runs in a separate context, so it can only use values explicitly captured from the enclosing scope, plus shared values. Calling back into normal JS needs runOnJS. See the Reanimated glossary.