What is hydration and what issues can occur?
The hydration process in modern web development based on the React library is a crucial stage of processing static content that was previously generated on the server during server-side rendering. When a user requests a page, the server generates the ready-made HTML code and sends it to the client, which ensures instant visualization of the interface before the JavaScript scripts are fully loaded. However, this HTML is static and non-interactive because it does not yet contain attached event handlers and the internal state of the application.
At the second stage, the hydration or "revival" mechanism comes into play, running in the browser after the JavaScript bundle is downloaded and executed. React traverses the component tree again, computes the virtual DOM, compares it with the existing server markup, and carefully attaches the necessary event listeners, turning the "picture" into a live application. This process significantly improves performance metrics, such as First Contentful Paint, and positively affects the website's SEO by allowing search engine crawlers to index the ready content.
The main problem during hydration is the occurrence of so-called markup mismatch or hydration mismatch, which arises in situations where the HTML code generated on the server differs from the markup received during the first client-side render. Such discrepancies force React to interrupt smooth hydration, discard the mismatched tree fragment, and recreate it from scratch using client-side rendering, which leads to visible UI flickering and a drop in performance. To prevent such situations, developers must strictly ensure that the initial rendering is deterministic.
The primary rule to avoid hydration errors is to refrain from using random values, dynamic dates (such as calls to `Date.now()` or `Math.random()`), and browser-specific objects when forming the initial server markup. It is also recommended to use modern architectural approaches, such as separating server and client components using directives like `'use client'` in Next.js-tier frameworks. Proper design of the application lifecycle guarantees stable UI operation without unnecessary overhead for fixing rendering errors on the fly.