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Computed Truth

React Hydration Errors occur because the **Initial UI** (HTML) sent by the server must exactly match the **First Render** commit of the client-side JavaScript. If `Math.random()`, `new Date()`, or `window.localStorage` is accessed during the initial render pass, the "Virtual DOM" differs from the "Real DOM", causing React to bail out of hydration and repaint the entire tree (Slow!).

Next.js Hydration Error Debugger

Simulate Hydration Mismatch

The Technical Proof

React's `hydrateRoot` function expects the DOM structure to remain invariant. The algorithm is strict:

  1. Server sends string: `
    Hello
    `
  2. Client downloads JS bundle.
  3. React runs component logic (Render Phase).
  4. React compares result with `innerHTML`.
  5. Mismatch? Throw Error #418 / #423 and switch to Client-Side Rendering (CSR), discarding the HTML benefits.

Fix strategy: Use `useEffect` to defer client-specific logic until **after** the first paint.

Step-by-Step Debugging

  1. Identify the Diff: Look at the error message "Expected server HTML to contain...".
  2. Isolate Component: Are you using `typeof window !== 'undefined'`? Only use this inside `useEffect`.
  3. Check Nesting: Ensure `<p>` tags do not contain `<div>` or `<section>` tags. Browsers auto-close the `p`, confusing React.
  4. Suppress (Last Resort): Use `suppressHydrationWarning={true}` on the specific element (e.g., for Timestamps), but never on the body.