Status: refused by design
instanceof is not FunctionalScript. [] instanceof Map is valid JavaScript, and fjs compile refuses it by design: the demo below shows what it prints.
x instanceof Array is in the language (instanceof): an array is the one kind of value with a class of its own. Any other right operand is refused, since JavaScript answers it by walking the prototype chain of x for that class's prototype, and FunctionalScript has neither the chain nor other classes to name.
The row op.instanceof of refused constructs registers the refusal, and instanceof gives its reason.
export default [] instanceof Map;
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: false.
Ask whether a value is an array with instanceof Array, as alternative.f.js does:
export default [] instanceof Array;
It answers true.
Today fjs compile prints the right operand of instanceof must be Array 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.