Status: refused by design
const String is not FunctionalScript. const String = 1 is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
Array, Number, Object and String name built-ins the language recognizes: Number(x) is a conversion and Object.is a static function. A module that binds one of these names would make the same text mean something else, so the names are reserved: binding one, or reading one as a value (export default String), is refused.
The row name.intrinsic of refused constructs registers the refusal, and built-ins gives its reason.
/** @type {(x: number) => string} */
const String = x => "s" + x;
export default String(1);
The demo on this page compiles sample.mjs, the module above, with this build's fjs compile, and shows the line it prints and the token it refuses. Below that it shows what JavaScript answers for the same module: s1.
Give the function a name of its own, as alternative.f.js does:
/** @type {(x: number) => string} */
const s = x => "s" + x;
export default s(1);
It answers s1.
Today fjs compile prints reserved word at the refused token. The demo proposes what it should print instead: the reason at the same token, what JavaScript does there, and the allowed spelling, the line alternative.f.js writes, as help. The page compiles the alternative too, so the suggestion is one the compiler accepts.