Status: implemented
export default (...a) => [Number(a[0]), Number("0x10"), Number(1n)];
Number(exp) converts its operand to a number, exactly as JavaScript's Number does when called. A number is itself. A string is read as a numeric literal, the whitespace around it ignored: Number("0x10") is 16, Number(" 4 ") is 4, Number("") is 0, and Number("x") is NaN. null is 0, undefined is NaN, and a boolean is 1 or 0. A bigint is the nearest number — Number(1n) is 1, and Number(2n ** 64n) is 18446744073709552000, rounded as JavaScript rounds it. An array and an object are made primitive as the operators make them: an array by joining its elements, so Number([7]) is 7 and Number([1, 2]) is NaN; an object through its own valueOf or toString, called as JavaScript calls them, and "[object Object]" otherwise, so Number({}) is NaN. A function converts to its text, which is no number. Where a valueOf or a toString throws, so does the conversion (failure is one outcome).
It is the language's conversion to a number: unary + is not FunctionalScript syntax, and where the EDAG has it the two differ, + throwing on a bigint (operators). It is the EDAG's own ['Number', exp], which graph outputs preserve and every executor answers; the front end folds nothing, Number("1") reaching the graph as a node, as ~1 does. The conversion is a value like any other: an operand, -Number("1") and Number(a) * 2; a base, Number(a).x; and a callee, however little calling a number is worth. The FunctionalScript writer spells it back as it is written, the operand an argument, Number(1+2).
Number is a reserved word, one of the names global names reserves, with Array, Object (built-in static functions) and String (string conversion) — fjs/js/keywords' reservedGlobals, a list beside the keywords rather than among them, since JavaScript has no such keyword. A module cannot bind it, as a const, a parameter or an import's local name (shared values), and it is no value: it stands as the callee of a call and as the namespace of the four predicates the language admits, Number.isInteger, Number.isSafeInteger, Number.isNaN and Number.isFinite, and the numeric constant Number.MAX_SAFE_INTEGER, read unguarded (built-in static functions), and is a compilation error (reserved word) anywhere else — bare, const f = Number;, or as the namespace of any other member, Number.parseFloat(x), each an admission of its own (built-ins). The constant is desugared to its value (static members). A key or a property name is not a reference, so { Number: 1 } and o.Number mean what they mean in JavaScript. Number() is 0, as in JavaScript, and reaches the graph as the literal, folded as -1 is. The conversion is an operator, not a function (operators and functions), so Number() and Number(x) are all the call shapes it has: every other is refused by design (Number takes one argument), Number(a, b) and Number(...a) among them. So is the guarded call Number?.(x), refused at the word (reserved word): a guard asks whether a value is nullish, and an operator is no value, so the expression asks nothing — JavaScript, whose Number is never nullish, reads it as Number(x).
The word is reserved for the guarantee undefined, NaN and Infinity already have (numbers): a global word means one thing wherever it stands. Number(x) is the conversion in every module, so a reader, an agent or a tool reads it without resolving a scope, where a JavaScript module that binds the word — const Number = x => x;, the shadowing JavaScript's linters flag — makes every Number(…) after it something else. And it keeps the language free to grow: a member of the namespace admitted later, Number.parseFloat say (built-ins), cannot change what an existing module means, since no module could have bound the word (global names). The one JavaScript spelling it costs, a module rebinding a standard global, is refused at compile time rather than read another way.
It is also the one key computed at run time, a[Number(i)] (property access).
The compiler demo offers the programs number in module.f.js, each with its real output.
The demo on this page runs sample.f.js, this page's example as a function of one number — Number(exp) of a string, a hexadecimal string, a bigint and the empty string — with every output fjs compile makes of it: the expression graph (EDAG), the Rust and the WebAssembly that Rust builds to. Its Run button calls the default export with 7 and 0.5, as JavaScript and as WebAssembly, and shows what each call returned or threw.
export default (...a) => [Number(String(a[0])), Number("0x10"), Number(1n), Number("") + a[0]];