@technobuddha/library/src/relative-time.ts

A large library of useful functions

import { addTime } from './add-time.ts';
import { secondsPerDay, secondsPerHour, secondsPerMinute, ticksPerSecond } from './date.ts';
import { floor } from './floor.ts';
import { formatDate } from './format-date.ts';
import { isSameDay } from './is-same-day.ts';
import { plural } from './plural.ts';
import { space } from './unicode.ts';

/**
 * Options for the {@link relativeTime} function
 * @group Time
 * @category Relative Time
 */
export type RelativeTimeOptions = {
  /** Describe the time difference as a time on a nearby day  */
  todayTomorrowYesterday?: boolean;
  /** Passed to {@link formatDate} to display a time */
  timeFormat?: string;
  /** Passed to {@link formatDate} to display a year, month and day */
  ymdFormat?: string;
  /** Passed to {@link formatDate} to display a month and day */
  mdFormat?: string;
};

/**
 * Describe the difference between two dates in a simple format
 * @param input - The date
 * @param relativeTo - The date to compare to
 * @param options - see {@link RelativeTimeOptions}
 * @returns string describing the time difference between the two dates
 * @group Time
 * @category Relative Time
 */
export function relativeTime(
  input: Date,
  relativeTo: Date,
  {
    todayTomorrowYesterday = false,
    timeFormat = 'H:mm TT',
    ymdFormat = 'MMMM D YYYY',
    mdFormat = 'MMMM D',
  }: RelativeTimeOptions = {},
): string {
  const text = [] as string[];

  if (todayTomorrowYesterday) {
    if (isSameDay(input, relativeTo)) {
      text.push(`today ${formatDate(input, timeFormat)} -`);
    } else if (isSameDay(input, addTime(relativeTo, { days: 1 }))) {
      text.push(`tomorrow ${formatDate(input, timeFormat)} -`);
    } else if (isSameDay(input, addTime(relativeTo, { days: -1 }))) {
      text.push(`yesterday ${formatDate(input, timeFormat)} -`);
    }
  }

  let diff = (input.getTime() - relativeTo.getTime()) / ticksPerSecond;
  let sign = 1;

  if (diff < 0) {
    sign = -1;
    diff = Math.abs(diff);
  }

  const d = floor(diff / secondsPerDay, { tolerance: 0.05 });
  const h = floor((diff - d * secondsPerDay) / secondsPerHour, { tolerance: 0.05 });
  const m = floor((diff - d * secondsPerDay - h * secondsPerHour) / secondsPerMinute, {
    tolerance: 0.05,
  });
  const s = floor(diff - d * secondsPerDay - h * secondsPerHour - m * secondsPerMinute, {
    tolerance: 0.05,
  });
  if (d > 90) {
    text.push(formatDate(input, ymdFormat));
    sign = 0;
  } else if (d > 30) {
    text.push(formatDate(input, mdFormat));
    sign = 0;
  } else if (d > 0) {
    text.push(plural('day', d, true));
    if (d < 4 && h > 1) {
      text.push(plural('hour', h, true));
    }
  } else if (h > 0) {
    text.push(plural('hour', h, true));
    if (h < 4 && m > 0) {
      text.push(plural('minute', m, true));
    }
  } else if (m > 0) {
    text.push(plural('minute', m, true));
    if (m < 4 && s > 0) {
      text.push(plural('second', s, true));
    }
  } else if (s > 0) {
    text.push(plural('second', s, true));
  }

  if (sign === -1) {
    text.push('ago');
  }
  if (sign === 1) {
    text.push(d || h || m || s ? 'from now' : 'now');
  }

  return text.join(space);
}