Status: refused by design
eval is not FunctionalScript. eval("1") is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
eval runs source text it receives at run time. That text is no part of the module the compiler reads, so nothing about it can be checked before it runs: not that it is FunctionalScript, not what it reads, not that it is pure. The built-ins register never admits it.
The row op.eval of refused constructs registers the refusal, and built-ins gives its reason.
export default eval("1 + 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: 2.
Write the code itself, where the compiler can check it, as alternative.f.js does:
export default 1 + 1;
It answers 2. The code is part of the module, so the compiler checks it as it checks the rest.
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.