Commits on Apr 5, 2024

1. Fast JSX: Don't clone props object …

   (Unless "key" is spread onto the element.)
   
   Historically, the JSX runtime clones the props object that is passed in.
   We've done this for two reasons.
   
   One reason is that there are certain prop names that are reserved by
   React, like `key` and (before React 19) `ref`. These are not actual
   props and are not observable by the target component; React uses them
   internally but removes them from the props object before passing them
   to userspace.
   
   The second reason is that the classic JSX runtime, `createElement`, is
   both a compiler target _and_ a public API that can be called manually.
   Therefore, we can't assume that the props object that is passed into
   `createElement` won't be mutated by userspace code after it is
   passed in.
   
   However, the new JSX runtime, `jsx``, is not a public API — it's solely
   a compiler target, and the compiler _will_ always pass a fresh, inline
   object. So the only reason to clone the props is if a reserved prop name
   is used.
   
   In React 19, `ref` is no longer a reserved prop name, and `key` will
   only appear in the props object if it is spread onto the element.
   (Because if `key` is statically defined, the compiler will pass it as a
   separate argument to the `jsx` function.) So the only remaining reason
   to clone the props object is if `key` is spread onto the element, which
   is a rare case, and also triggers a warning in development.
   
   In a future release, we will not remove a spread key from the props
   object. (But we'll still warn.) We'll always pass the object
   straight through.
   
   The expected impact is much faster JSX element creation, which in many
   apps is a significant slice of the overall runtime cost of rendering.

   

   acdlite committed Apr 5, 2024
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