54 lines
2.2 KiB
JavaScript
54 lines
2.2 KiB
JavaScript
const offsetFormatCache = {};
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const offsetCache = {};
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/**
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* The function returns UTC offset in minutes for the given date in specified
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* time zone.
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*
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* Unlike `Date.prototype.getTimezoneOffset`, this function returns the value
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* mirrored to the sign of the offset in the time zone. For Asia/Singapore
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* (UTC+8), `tzOffset` returns `480`, while `getTimezoneOffset` returns `-480`.
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*
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* It uses `Intl.DateTimeFormat` internally to access otherwise unavailable
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* runtime's time zone data, and falls back to basic manual parsing if Intl API
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* is not supported (e.g., older Node.js versions, React Native's Hermes, etc.).
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*
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* @param timeZone - Time zone name (IANA or UTC offset).
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* @param date - Date to check the offset for.
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*
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* @returns UTC offset in minutes (e.g., `480` for date in UTC+8).
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*/
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export function tzOffset(timeZone, date) {
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try {
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// Create `Intl.DateTimeFormat` with `"longOffset"` that gives us consistent
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// date strings like `"5/19/2026, GMT-08:00"`.
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const format = offsetFormatCache[timeZone] ||= new Intl.DateTimeFormat("en-US", {
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timeZone,
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timeZoneName: "longOffset"
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}).format;
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// Get `"-08:00"`.
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const offsetStr = format(date).split("GMT")[1];
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// Avoid parsing the same offset string.
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if (offsetStr in offsetCache) return offsetCache[offsetStr];
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// Calculate offset from `["-08", "00"]` and cache it.
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return calcOffset(offsetStr, offsetStr.split(":"));
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} catch {
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// Fallback to manual parsing if the runtime doesn't support
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// `±HH:MM/±HHMM/±HH`. See: https://github.com/nodejs/node/issues/53419.
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if (timeZone in offsetCache) return offsetCache[timeZone];
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const captures = timeZone?.match(offsetRe);
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if (captures) return calcOffset(timeZone, captures.slice(1));
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return NaN;
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}
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}
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const offsetRe = /([+-]\d\d):?(\d\d)?/;
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function calcOffset(cacheStr, values) {
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const hours = +(values[0] || 0);
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const minutes = +(values[1] || 0);
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// Convert seconds to minutes by dividing by 60 to keep the function return in minutes.
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const seconds = +(values[2] || 0) / 60;
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return offsetCache[cacheStr] = hours * 60 + minutes > 0 ? hours * 60 + minutes + seconds : hours * 60 - minutes - seconds;
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} |