The `options` arg of `Temporal.ZonedDateTime.prototype.toLocaleString()` currently accepts "illegal" option `timeZone`

[Sector6759]

⚙ Compilation target

ESNext

⚙ Library

ESNext

Missing / Incorrect Definition

TypeScript/tsc/internal/bundled/libs/lib.esnext.temporal.d.ts

Line 320 in 0b832e1

toLocaleString(locales?: Intl.LocalesArgument, options?: Intl.DateTimeFormatOptions): string;

Sample Code

TypeScript allows the following code, when it should not.

Temporal.Now.zonedDateTimeISO("America/Chicago").toLocaleString(undefined, { timeZone: "Europe/Berlin" });

Result

Engine	Error
JavaScriptCore	TypeError: ZonedDateTime.toLocaleString does not accept a timeZone option; the ZonedDateTime's time zone is used
SpiderMonkey	TypeError: can't set option timeZone when Temporal.ZonedDateTime.toLocaleString is used
V8	TypeError: Invalid time zone specified: America/Chicago

Documentation Link

According to MDN:

The timeZone option must not be provided, as it is automatically set to be the date-time's timeZoneId.

The type definitions should be changed to

declare namespace Temporal {
  // ...
  interface ZonedDateTimeToLocaleStringOptions extends Omit<Intl.DateTimeFormatOptions, "timeZone"> {}
  // ...
  interface ZonedDateTime {
    // ...
    toLocaleString(locales?: Intl.LocalesArgument, options?: ZonedDateTimeToLocaleStringOptions): string;
  }
}

[RyanCavanaugh]

Temporal.ZonedDateTime.prototype.toLocaleString must reject a timeZone option because it always uses the receiver's time zone. TypeScript 6.0.3 and current native 7.1.0-dev.20260918.1 both accept the reported call with --lib esnext, while Edge throws TypeError for the same call.

The declaration should use an options type that excludes Intl.DateTimeFormatOptions["timeZone"], rather than accepting Intl.DateTimeFormatOptions directly. MDN documents this requirement: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/ZonedDateTime/toLocaleString#options