Intl.RelativeTimeFormat enables localized formatting of relative times without sacrificing performance.

We've all seen websites using Just now, an hour ago, a week ago etc. to describe when the post was created.
Almost all of these websites uses popular libraries like Moment.js, Globalize, date_fns and-all to use this feature.

Today we'd look at how we can do that with just Vanilla JavaScript!

A Quick Example

Just to show you how the Intl.RelativeTimeFormat constructor works

const when = new Intl.RelativeTimeFormat('en').format(-1, 'day');
console.log(when);
// "1 day ago"

Here we passed the Intl.RelativeTimeFormat an argument, a string with a BCP 47 language tag. This argument decides the output language.

Additionally, the Intl.RelativeTimeFormat constructor accepts an optional options argument, which gives us more control over the output. We'd talk about it later in this tutorial.

Then we told the Intl.RelativeTimeFormat constructor to format the relative time (the -1 value) in day unit.

Lets tweak this code a bit (copy this and check the code in your console)

const when = new Intl.RelativeTimeFormat('bn').format(-5, 'day');
console.log(when);
// "৫ দিন আগে" (translation: 5 days ago)

And of course we can set times in future as well

const when = new Intl.RelativeTimeFormat('en').format(2, 'day');
console.log(when);
// in 2 days

The options Object

The options object gives us more control over the output. It have three possible values:

• localeMatcher
• numeric
• style

Read about the options in details here

const options = {
  numeric: 'auto',
}

const when = new Intl.RelativeTimeFormat('en', options).format(-1, 'day')

console.log(when);
// yesterday

Possible Values for Unit

You can pass in the following values in Intl.RelativeTimeFormat(locale).format(relative time, **UNIT**):

• second
• minute
• hour
• day
• month
• quarter
• year

Logically Show When Something Was Created

Suppose your post was created at some date

const createdAt = new Date(2020, 11, 10); // December 10, 2020

And I, the user visited your post right now

const userVisited = new Date();

If we subtract createdAt from userVisited we'll get a number in milliseconds

const diff = userVisited - createdAt;
// some value in milliseconds

Now we need to convert this diff in days

const toSec = (diff / 1000);
// convert the milliseconds to seconds

const toMin = (toSec / 60);
// convert the seconds to minutes

const toHour = toMin / 60;
// convert the minutes to hours

const toDays = toHour / 24;
// convert the hours to days

// now we'll round the days up/down
const rounded = Math.round(toDays);

And finally we'll pass the rounded value to Intl.RelativeTimeFormat constructors format method

const when = new Intl.RelativeTimeFormat('en').format(-rounded, 'day');

// because your post was created in past
// we pass a minus sign before the rounded
// to get the output in past tense

That's it for today, obviously you can make this code a lot more dynamic to show different types of unit based on when the post was created. Try that out on your own.

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