React 19

[imjordanxd]

Overview

Adds support for React 19. I don't have a lot of time to add this and would appreciate any help 🙏

New context rendering

In React 19, we can render a context as follows:

const Context = React.createContext();
function Foo({ children }) {
	return <Context>{children}</Context>;
}

The React Element produced by this is identical (as far as I can see!) to the React Element produced if we were to render:

const Context = React.createContext();
function Foo({ children }) {
	return <Context.Provider>{children}</Context.Provider>;
}

Because of this change, serialisation is broken. We don't know how a context was rendered. Therefore, we can't produce a JSX string that accurately reflects the original JSX that was written. To support a wide range of React versions, we should serialise the new context rendering (<Context>...</Context>) as <Context.Provider>...</Context.Provider>. This isn't exactly a 1:1 serialisation but React will still treat the JSX the same.

Furthermore, the element type symbols have changed between React major versions. For example:

// React 18:
<Context.Provider /> // Symbol.for('react.provider');

// React 19:
<Context /> /*or*/ <Context.Provider />  // Symbol.for("react.context");

This means we cannot bundle react-is as a dependency and allow a wide range or React peer dependencies. We should expect consumers of this package to provide react-is as a dependency which is aligned to their React version. This is a breaking change.

[oscarcarlstrom]

Great! This would be much appreciated! 🙏

[imjordanxd]

this import crashes the test runner for some reason

update: it's because it depends on React internals and they've changed between majors. I wonder can the tests that depend on this be deleted. Not entirely sure what value they provide