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JavaScript’s internationalization and location APIs make it easy to display dates, amounts, and positions—but their defaults and conventions can change how data is interpreted. Set locale, time zone, and currency explicitly for presentation, and document coordinate order, datum, and units at data boundaries.
1. Date formatting can inherit the runtime’s locale and time zone
If you omit options, Intl.DateTimeFormat uses defaults supplied by the runtime. The same code can therefore produce different output on a user’s device and on a server. When the intended audience or region matters, pass the locale and an IANA time-zone identifier explicitly. MDN documents identifiers including America/New_York and Australia/Sydney (MDN: Intl.DateTimeFormat).
const formatter = new Intl.DateTimeFormat("en-GB", {
dateStyle: "medium",
timeStyle: "short",
timeZone: "Australia/Sydney"
});
console.log(formatter.format(new Date()));
console.log(formatter.resolvedOptions());resolvedOptions() reports the effective locale, calendar, numbering system, and time zone selected by the runtime. It is useful for diagnosing unexpected output; setting options directly is the way to express the behavior your application requires.
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A JavaScript Date represents an instant. Formatting it in another time zone changes how that instant is displayed, not the instant itself. For example, the same instant can appear as different local clock times in New York and Sydney.
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A recurring appointment such as “9 a.m. every Monday in New York” has a different meaning from a one-time instant. As application design guidance, store the intended local date and time together with its governing time zone when that is what the user scheduled. An unqualified timestamp alone does not express that recurring local-time rule.
3. Locale and currency are separate choices
For currency display, Intl.NumberFormat needs a currency code such as USD, EUR, or JPY. The locale controls presentation conventions; the currency code identifies the money being represented. A locale does not reliably tell your application which currency to use. MDN describes the currency formatting options and their locale-sensitive behavior (MDN: Intl.NumberFormat).
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const amount = 1234.5;
const usDollars = new Intl.NumberFormat("en-US", {
style: "currency",
currency: "USD"
});
const japaneseYen = new Intl.NumberFormat("ja-JP", {
style: "currency",
currency: "JPY"
});
console.log(usDollars.format(amount));
console.log(japaneseYen.format(amount));The displayed conventions can differ in symbol placement, separators, and fraction digits. In particular, JPY formatting illustrates that minor-unit display conventions are not the same for every currency. Keep the currency code attached to the amount in your data model rather than inferring it from a display locale.
4. Localized strings are for display, not machine data
Do not use formatted dates or currency strings as stable serialized values or compare them against hardcoded display text. MDN notes that output can vary between implementations even for the same locale, and may contain non-breaking spaces or bidirectional control characters (MDN: NumberFormat.prototype.format).
Keep the underlying numeric amount and its currency code as structured data; format them when presenting them to a user. Apply the same separation to dates: retain the instant or scheduling fields your application needs, then format at the interface boundary. This avoids treating presentation details as an interchange format.
5. Geolocation and GeoJSON use different coordinate order
Browser geolocation exposes named properties such as latitude and longitude. GeoJSON geographic positions use an array with longitude first, then latitude: [longitude, latitude]. Copying two values without converting by name can silently swap the location.
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const position = await new Promise((resolve, reject) => {
navigator.geolocation.getCurrentPosition(resolve, reject);
});
const { latitude, longitude } = position.coords;
const geoJsonPosition = [longitude, latitude];RFC 7946 specifies the GeoJSON order and geographic coordinate conventions: “The first two elements are longitude and latitude, or easting and northing, precisely in that order and using decimal numbers.” (RFC 7946, section 3.1.1). Name the order in API contracts and convert explicitly between named browser properties and positional arrays. MDN documents the browser properties (MDN: GeolocationCoordinates).
6. Coordinates need a datum, units, and boundary rules
GeoJSON positions describe more than two numbers
RFC 7946 defines GeoJSON geographic positions as decimal degrees using the WGS 84 reference system. A third value, when present, is elevation in meters relative to the WGS 84 reference ellipsoid. The standard advises against positions with more than three elements because the meaning of additional values is ambiguous (RFC 7946, section 3.1.1).
Do not assume that every pair of numbers is latitude and longitude, or that a third value is an arbitrary height in an unspecified unit. A useful data contract identifies coordinate order, reference system, and units.
Antimeridian crossings can defeat ordinary min/max logic
GeoJSON bounding boxes that cross the antimeridian can have an east longitude numerically smaller than the west longitude. Code that assumes west is always the minimum longitude and east the maximum can therefore describe the wrong region. RFC 7946 includes antimeridian examples in its bounding-box section (RFC 7946, section 5.2); handle that case explicitly in bounds checks and map logic.
Geolocation results also carry uncertainty and time
A browser geolocation result includes more than coordinates: its coordinate data exposes an accuracy value, and the position result includes a measurement timestamp. Preserve and interpret those fields when the application needs to communicate how precise or how recent a location is. Latitude and longitude by themselves do not express either certainty or observation time (MDN: GeolocationCoordinates; MDN: GeolocationPosition).
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A practical boundary checklist
- For predictable date output, pass an explicit locale and time zone; inspect effective settings with
resolvedOptions(). - Distinguish an instant from a recurring local appointment, and retain a governing time zone for the latter.
- For money, keep the numeric value and currency code together; select the locale separately for display.
- Keep localized strings out of machine comparisons and serialization.
- Convert browser geolocation’s named latitude and longitude to GeoJSON’s longitude-first array order.
- Specify GeoJSON datum and units, preserve relevant accuracy and timestamp data, and account for antimeridian-crossing bounds.
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