Simplified getRelativeTimeString

getRelativeTimeString.ts

// from https://twitter.com/Steve8708/status/1504131981444980739
// simplified to a function body of 8 tidy lines
// no loop needed, no 2d array of 3-tuples needed
// just 2 arrays, a findIndex call, and some indexing :)

export function getRelativeTimeString(
  date: Date | number,
  lang = "en"
): string {
  const timeMs = typeof date === "number" ? date : date.getTime();
  const deltaSeconds = Math.round((timeMs - Date.now()) / 1000);
  const cutoffs = [60, 3600, 86400, 86400 * 7, 86400 * 30, 86400 * 365, Infinity];
  const units: Intl.RelativeTimeFormatUnit[] = ["second", "minute", "hour", "day", "week", "month", "year"];
  const unitIndex = cutoffs.findIndex(cutoff => cutoff > Math.abs(deltaSeconds));
  const divisor = unitIndex ? cutoffs[unitIndex - 1] : 1;
  const rtf = new Intl.RelativeTimeFormat(lang, { numeric: "auto" });
  return rtf.format(Math.floor(deltaSeconds / divisor), units[unitIndex]);
}

// alternate: init the arrays + RTF only once, keep in closure
// worthwhile if doing lots of calls, RTF init isn't free
// this is probably how I would actually do this for prod shipped code I guess
export function makeRelativeTimeStringGetter(lang = "en") {
  const cutoffs = [60, 3600, 86400, 86400 * 7, 86400 * 30, 86400 * 365, Infinity];
  const units: Intl.RelativeTimeFormatUnit[] = ["second", "minute", "hour", "day", "week", "month", "year"];
  const rtf = new Intl.RelativeTimeFormat(lang, { numeric: "auto" });
  return function getRelativeTimeString(date: Date | number): string {
    const timeMs = typeof date === "number" ? date : date.getTime();
    const deltaSeconds = Math.round((timeMs - Date.now()) / 1000);
    const unitIndex = cutoffs.findIndex(cutoff => cutoff > Math.abs(deltaSeconds));
    const divisor = unitIndex ? cutoffs[unitIndex - 1] : 1;
    return rtf.format(Math.floor(deltaSeconds / divisor), units[unitIndex]);    
  }
}

console.log(getRelativeTimeString(Date.now() - 100)); // now
console.log(getRelativeTimeString(Date.now() - 10000)); // 10 seconds ago
console.log(getRelativeTimeString(Date.now() - 100000)); // 2 minutes ago
console.log(getRelativeTimeString(Date.now() - 10000000)); // 3 hours ago
console.log(getRelativeTimeString(Date.now() + 10000)); // in 10 seconds
console.log(getRelativeTimeString(Date.now() + 100000)); // in 1 minute
console.log(getRelativeTimeString(Date.now() + 10000000)); // in 2 hours
console.log(getRelativeTimeString(Date.now() + 100000000)); // tomorrow
console.log(getRelativeTimeString(Date.now() + 1000000000)); // next week
console.log(getRelativeTimeString(Date.now() + 10000000000)); // in 3 months
console.log(getRelativeTimeString(Date.now() + 100000000000)); // in 3 years

instead of Math.floor the function should use Math.round because it currently returns results that are off-by-one. For example:

const threeDaysAgo = new Date(Date.now() - 3 * 24 * 60 * 60 * 1000)
getRelativeTimeString(threeDaysAgo) // outputs "4 days ago"

@LewisJEllis

Hey, you have an issue, it can be unidentified

Thanks @vonovak -- variations of this gist are all over (google "Intl.RelativeTimeFormat do its magic") with the same error.