timezone.js

import { MIN, MS } from "./constant.js";

const typeToPos = {
  year: 0,
  month: 1,
  day: 2,
  hour: 3,
  minute: 4,
  second: 5,
};

// Cache time-zone lookups from Intl.DateTimeFormat,
// as it is a *very* slow method.
const dtfCache = {};
const getDateTimeFormat = (timezone, options = {}) => {
  const timeZoneName = options.timeZoneName || "short";
  const key = `${timezone}|${timeZoneName}`;
  let dtf = dtfCache[key];
  if (!dtf) {
    dtf = new Intl.DateTimeFormat("en-US", {
      hour12: false,
      timeZone: timezone,
      year: "numeric",
      month: "2-digit",
      day: "2-digit",
      hour: "2-digit",
      minute: "2-digit",
      second: "2-digit",
      timeZoneName,
    });
    dtfCache[key] = dtf;
  }
  return dtf;
};

export default (o, c, d) => {
  let defaultTimezone;

  const makeFormatParts = (timestamp, timezone, options = {}) => {
    const date = new Date(timestamp);
    const dtf = getDateTimeFormat(timezone, options);
    return dtf.formatToParts(date);
  };

  const tzOffset = (timestamp, timezone) => {
    const formatResult = makeFormatParts(timestamp, timezone);
    const filled = [];
    for (let i = 0; i < formatResult.length; i += 1) {
      const { type, value } = formatResult[i];
      const pos = typeToPos[type];

      if (pos >= 0) {
        filled[pos] = parseInt(value, 10);
      }
    }
    const hour = filled[3];
    // Workaround for the same behavior in different node version
    // https://github.com/nodejs/node/issues/33027
    /* istanbul ignore next */
    const fixedHour = hour === 24 ? 0 : hour;
    const utcString = `${filled[0]}-${filled[1]}-${filled[2]} ${fixedHour}:${filled[4]}:${filled[5]}:000`;
    const utcTs = d.utc(utcString).valueOf();
    let asTS = +timestamp;
    const over = asTS % 1000;
    asTS -= over;
    return (utcTs - asTS) / (60 * 1000);
  };

  // find the right offset a given local time. The o input is our guess, which determines which
  // offset we'll pick in ambiguous cases (e.g. there are two 3 AMs b/c Fallback DST)
  // https://github.com/moment/luxon/blob/master/src/datetime.js#L76
  const fixOffset = (localTS, o0, tz) => {
    // Our UTC time is just a guess because our offset is just a guess
    let utcGuess = localTS - o0 * 60 * 1000;
    // Test whether the zone matches the offset for this ts
    const o2 = tzOffset(utcGuess, tz);
    // If so, offset didn't change and we're done
    if (o0 === o2) {
      return [utcGuess, o0];
    }
    // If not, change the ts by the difference in the offset
    utcGuess -= (o2 - o0) * 60 * 1000;
    // If that gives us the local time we want, we're done
    const o3 = tzOffset(utcGuess, tz);
    if (o2 === o3) {
      return [utcGuess, o2];
    }
    // If it's different, we're in a hole time.
    // The offset has changed, but the we don't adjust the time
    return [localTS - Math.min(o2, o3) * 60 * 1000, Math.max(o2, o3)];
  };

  const proto = c.prototype;

  proto.tz = function (timezone = defaultTimezone, keepLocalTime) {
    const oldOffset = this.utcOffset();
    const date = this.toDate();
    const target = date.toLocaleString("en-US", { timeZone: timezone });
    const diff = Math.round((date - new Date(target)) / 1000 / 60);
    let ins = d(target)
      .$set(MS, this.$ms)
      .utcOffset(-Math.round(date.getTimezoneOffset() / 15) * 15 - diff, true);
    if (keepLocalTime) {
      const newOffset = ins.utcOffset();
      ins = ins.add(oldOffset - newOffset, MIN);
    }
    ins.$x.$timezone = timezone;
    return ins;
  };

  proto.offsetName = function (type) {
    // type: short(default) / long
    const zone = this.$x?.$timezone || d.tz.guess();
    const result = makeFormatParts(this.valueOf(), zone, {
      timeZoneName: type,
    }).find((m) => m.type.toLowerCase() === "timezonename");
    return result && result.value;
  };

  const oldStartOf = proto.startOf;
  proto.startOf = function (units, startOf) {
    if (!this.$x || !this.$x.$timezone) {
      return oldStartOf.call(this, units, startOf);
    }

    const withoutTz = d(this.format("YYYY-MM-DD HH:mm:ss:SSS"));
    const startOfWithoutTz = oldStartOf.call(withoutTz, units, startOf);
    return startOfWithoutTz.tz(this.$x.$timezone, true);
  };

  d.tz = function (input, arg1, arg2) {
    const parseFormat = arg2 && arg1;
    const timezone = arg2 || arg1 || defaultTimezone;
    const previousOffset = tzOffset(+d(), timezone);
    if (typeof input !== "string") {
      // timestamp number || js Date || Day.js
      return d(input).tz(timezone);
    }
    const localTs = d.utc(input, parseFormat).valueOf();
    const [targetTs, targetOffset] = fixOffset(
      localTs,
      previousOffset,
      timezone
    );
    const ins = d(targetTs).utcOffset(targetOffset);
    ins.$x.$timezone = timezone;
    return ins;
  };

  d.tz.guess = function () {
    return Intl.DateTimeFormat().resolvedOptions().timeZone;
  };

  d.tz.setDefault = function (timezone) {
    defaultTimezone = timezone;
  };
};