1. What is Hydration?

Hydration is the process of taking a pre-rendered HTML page sent to the client
and making it interactive by attaching event listeners,
usually after rebuilding framework and app state on the client

2. How does Hydration work? - in SSR

How does Hydration work?
  1. first the client sends a get request to the server
  2. the server executes all app logic
  3. in order to render the complete HTML page
  4. which is sent back to the client
  5. the client renders the HTML page
  6. and sends a request for the JavaScript bundle
  7. once it receives the JavaScript bundle
  8. the client executes all app logic again
  9. so as to hydrate the page

3. What are the Pros and Cons of Hydration?


  • enable interactive client-side apps rendered on the server
  • quick First Paint (FP) and First Contentful Paint (FCP)


  • execute all app logic twice
    • once on the server and once on the client
  • uncanny valley (fast First Contentful Paint, but slow Time To Interactive)
    • page looks ready, but isn't (no event handlers attached)
  • JS bundle bigger than with just Client-Side Rendering (CSR)
    • as now it needs to do both hydration and CSR

4. When should you use Hydration?

  • when you want to have interactive client-side apps rendered on the server

Useful for e-commerce, social media and big content websites

see also Progressive Hydration, Partial Hydration, Resumability and React Server Components


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Website created for the FrontendMasters course on JavaScript Patterns by Lydia Hallie