Return null from Date.toJSON when the time is not finite

[phall1]

Date.prototype.toJSON had no lowering (SC2020). Node returns null when the time value is not finite, and otherwise the same string as toISOString. toISOString still throws RangeError.

JSON.stringify was also narrowing a string | null union to the checker's string before walking it, so the null result threw instead of printing null. A union that is already JSON-stringify-safe now keeps every arm. A union that is not JSON-safe still narrows.

const invalid = new Date(NaN);
JSON.stringify(invalid.toJSON()); // "null"
invalid.toJSON() === null; // true
invalid.toISOString(); // throws RangeError

A finite date prints the same ISO string from both methods:

const date = new Date("2024-01-01T12:00:00Z");
JSON.stringify([date.toJSON(), date.toISOString()]);
// ["2024-01-01T12:00:00.000Z","2024-01-01T12:00:00.000Z"]

Differential test: packages/compiler/test/date-tojson.test.ts. Native stdout, stderr, and exit match Node.

Before this change both cases failed to compile:

Date.prototype.toJSON is part of the standard library types but has no scriptc lowering yet (SC2020).

This showed up while compiling Effect 4. The public harness is https://github.com/phall1/effect-scriptc-compat. The change is the Date method, not an Effect workaround. Date values themselves are still not JSON-safe objects, and a binding the checker types as string can still reject the null result, because TypeScript types Date.toJSON() as string.

[vercel]

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