Reactive signals
Programming model for reactivity — observable values + auto-tracking of dependencies. Knockout (2010) → MobX → SolidJS made them hip.
By 2024 "signals" have become nearly universal in frontend: Angular 17, Vue 3.4 (Vue 3 had ref/reactive since 2020), Svelte 5 (Runes), Solid (signals are "the" primitive), Preact, Qwik. Differs from React useState: no re-rendering the whole component, just the parts that actually read the signal.