Allow JSX element tags to be symbols

Search Terms

es6 symbols JSX createElement element tag type intrinsic host

Suggestion

Allow JSX element tags to be ES6 symbols instead of just strings, functions or classes.

Use Cases

JSX-based libraries like React have adopted the convention of using special upper-cased symbols as the tag of the JSX expression (<Fragment />, <Suspense />) as a way to provide intrinsic elements which aren’t callable and whose behavior are defined by the library rather than the tag.

Currently, TypeScript only allows JSX tags to be strings, functions or classes as defined by the JSX interface, and will throw errors like the following if Symbols are used:

error TS2604: JSX element type 'Fragment' does not have any construct or call signatures.

Libraries are therefore forced to lie about the actual typings of Symbol tags as something other than a unique symbol. For instance, React defines Symbol tags as ExoticComponent and pretends that the symbols are callable (https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L358-L364). The framework I work on (Crank.js) just defines symbols meant to be used as tags as any but this then has further repercussions when I attempt to, for instance, use these symbol types in interfaces.

Symbols are useful because they:

1. Allow intrinsic elements to be upper-cased. This allows people to provide renderer-provided element APIs which mirror XML languages which are case-sensitive.
2. Not globally scoped, so there is no possibility of namespace clashes, and can be imported/exported.
3. Can still work with the global IntrinsicElements interfaces to be strongly typed.

Examples

const Fragment = Symbol.for("crank.Fragment");
type Fragment = typeof Fragment;

const Portal = Symbol.for("crank.Portal");
type Portal = typeof Portal;

// should not error
const el = (
  <Fragment>{children}</Fragment>
);

declare namespace JSX {
    interface IntrinsicElements {
        foo: any;
        [Portal]: {root: any};
    }
}

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

The issue's entry points are JSX element type checking and the JSX namespace's IntrinsicElements declarations; begin by tracing how existing string, function, and class cases are validated. Compare the Fragment and Portal examples with the current TS2604 diagnostic and define tests for symbol-tag JSX and symbol IntrinsicElements keys; done means valid symbol tags are accepted without changing emitted JavaScript.