Status: refused by design
export let is not FunctionalScript. export let a = 1; is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
FunctionalScript admits local and module-level variables, but exports expose immutable values rather than live mutable bindings. Export a const snapshot of the selected value; export let remains refused.
The row mod.export-let of refused constructs registers the refusal; exports describes the admitted forms.
export let a = 1;
export default a;
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: 1.
Export a const, as alternative.f.js does:
export const a = 1;
export default a;
It answers 1.
Today fjs compile prints unexpected token 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.