Viewing github.com/facebook/react/packages/react-devtools-extensions/popups/deadcode.html

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  <b>This page includes an extra development build of React. &#x1f6a7;</b>

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  The React build on this page includes both development and production versions because dead code elimination has not been applied correctly.

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  This makes its size larger, and causes React to run slower.

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  Make sure to <a href="https://reactjs.org/docs/optimizing-performance.html#use-the-production-build">set up dead code elimination</a> before deployment.

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  Open the developer tools, and  "Components" and "Profiler" tabs will appear to the right.

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