What are Vue lifecycle hooks?

Vue lifecycle hooks let you execute code at specific stages of a component's life. Vue 3 hooks (Composition API): onBeforeMount — before the component is mounted (DOM not yet created); onMounted — component is mounted (DOM available, use for DOM access, fetch data, subscriptions); onBeforeUpdate — before the component re-renders due to state change; onUpdated — after re-render (DOM updated); onBeforeUnmount — before the component is unmounted (cleanup: clear timers, cancel subscriptions); onUnmounted — component is unmounted (DOM removed); onErrorCaptured — catches errors from descendant components; onActivated / onDeactivated — when a component inside <KeepAlive> is activated/deactivated. Usage in script setup: import { onMounted, onUnmounted } from "vue"; onMounted(() => { document.addEventListener("click", handler); }); onUnmounted(() => { document.removeEventListener("click", handler); });. Options API hooks: created (instance created, data/computed available, no DOM), mounted, updated, beforeDestroy → Vue 3 renamed to beforeUnmount, destroyed → unmounted. Setup() runs before beforeCreate and created — in Composition API, setup() replaces both. Most commonly used: onMounted (init, fetch data, DOM setup) and onUnmounted (cleanup).