Hydration

The step where JavaScript in the browser attaches to server-rendered HTML and makes it interactive.

The server sends ready HTML, so the page shows fast; then the framework downloads the component code, renders it again in memory and attaches event handlers. Until that finishes, buttons can look ready and do nothing. A hydration mismatch error means the browser rendered something different from the server, often because of dates, random values or browser-only APIs.

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Related terms

React Server Components (RSC): React components that run only on the server, send rendered output instead of JavaScript, and can read data directly.
SSR, SSG and ISR: Three ways to render pages: on every request (SSR), once at build time (SSG), or at build time and refreshed later (ISR).
Core Web Vitals (LCP, INP, CLS): Google’s three metrics of real user experience: loading (LCP), responsiveness (INP) and visual stability (CLS).

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