[Node.js, Deno] Modify Date function when you can't use Temporal

modify-date.ts

// see https://deno.land/std/datetime/constants.ts?source
const MILLISECOND = 1;
const SECOND = MILLISECOND * 1e3;
const MINUTE = SECOND * 60;
const HOUR = MINUTE * 60;
const DAY = HOUR * 24;
const WEEK = DAY * 7;

type Unit =
  | "milliseconds"
  | "seconds"
  | "minutes"
  | "hours"
  | "days"
  | "weeks"
  | "months"
  | "quarters"
  | "years";

const unitToValue = {
  milliseconds: MILLISECOND,
  seconds: SECOND,
  minutes: MINUTE,
  hours: HOUR,
  days: DAY,
  weeks: WEEK,
} as const;

/**
 * Returns modified new Date object in the given count and unit.
 *
 * @example
 * ```ts
 * import { modifyDate } from "./modify-date.ts";
 *
 * const date = new Date("2023-08-10T10:00:00.000Z");
 *
 * modifyDate(date, 10, "seconds");
 * // output : new Date("2023-08-10T10:00:10.000Z");
 *
 * modifyDate(date, 60, "seconds");
 * // output : new Date("2023-08-10T10:01:00.000Z");
 *
 * modifyDate(date, -1, "hours");
 * // output : new Date("2023-08-10T09:00:00.000Z");
 *
 * modifyDate(date, 3, "months");
 * // output : new Date("2023-11-10T10:00:00.000Z");
 * ```
 */
export function modifyDate(date: Date, count: number, unit: Unit): Date {
  switch (unit) {
    case "months": {
      return modifyMonth(date, count);
    }
    case "quarters": {
      return modifyMonth(date, count * 4);
    }
    case "years": {
      const d = structuredClone(date);
      d.setFullYear(d.getFullYear() + count);
      return d;
    }
    default: {
      const v = unitToValue[unit];
      if (!v) throw new Error(`unexpected unit is given: ${unit}`);
      return new Date(date.getTime() + count * v);
    }
  }
}

function modifyMonth(date: Date, count: number) {
      const d = structuredClone(date);
      d.setMonth(d.getMonth() + count);
      return d;
}