reactjs - How does Next.js hydration work? Component tree comparison vs HTML string comparison - Stack Overflow

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I'm trying to understand the technical details behind Next.js hydration process. Specifically, I'm confused about how React/Next.js detects hydration mismatches:

Does Next.js compare an internal data structure (like a virtual DOM) with the actual DOM, performing a component-by-component comparison? Or is it simply comparing the server-generated HTML string with what would be rendered on the client?

When hydration errors occur, I'd like to understand exactly what's being compared and how the framework identifies these differences. This would help me better debug hydration issues in my application. I've read the Next.js documentation on hydration, but I couldn't find details on the exact comparison mechanism. Any insights or references to technical documentation that explains this would be appreciated.

I'm trying to understand the technical details behind Next.js hydration process. Specifically, I'm confused about how React/Next.js detects hydration mismatches:

Does Next.js compare an internal data structure (like a virtual DOM) with the actual DOM, performing a component-by-component comparison? Or is it simply comparing the server-generated HTML string with what would be rendered on the client?

When hydration errors occur, I'd like to understand exactly what's being compared and how the framework identifies these differences. This would help me better debug hydration issues in my application. I've read the Next.js documentation on hydration, but I couldn't find details on the exact comparison mechanism. Any insights or references to technical documentation that explains this would be appreciated.

• reactjs
• next.js

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Hydration is react process, next sets it up and handles reporting hydration errors, but the hydration process itself is wholly a react concept.

Does Next.js compare an internal data structure (like a virtual DOM) with the actual DOM, performing a component-by-component comparison? Or is it simply comparing the server-generated HTML string with what would be rendered on the client?

The former, as part of the normal rendering process (not hydration) react renders components into a virtual DOM, and then diffs that VDOM against the actual DOM and where a difference is found, mutates the actual DOM to be consistent with the VDOM.

Hydration is essentially the same process: the component tree is rendered resulting in a VDOM which is diff'd with the actual dom. Only when detecting a difference, an error is reported as it indicates that the server rendered HTML doesn't represent the same DOM that the client side render produced.

This documentation may be helpful in understanding that diffing process. It doesn't pertain to hydration specifically, but hydration is ultimately the same process, except with the expectation that no diff is produced. As to the actual data structures involved, the VDOM output is a javascript structure, whereas the actual DOM is a browser dependent data structure that presents a standardized javascript interface that you might be familiar with if you've ever queried the DOM, the sort of thing returned by querySelector.

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