In Javascript it's usually considered best practice to work with absolute numbers, as number is a defined type. However, when expressing these numbers in user interfaces, it's more likely we'll want to express them differently. For example, consider I have the following list of numbers. I want to still store them as numbers, but then add nd, st, rd, or th when showing them to the user:

let x = [ 1, 13, 22, 100, 1204, 133 ];

I could of course do this manually, and store each number of a specific prefix, or define custom rules, but this gets a little messy - and what if I need to support multiple languages. Although in english we write 3rd to represent 3rd place, this may not be the same in other languages.

Fortunately there is a solution in Javascript - the use of Intl.PluralRules. This will define plural rules based on locale:

let plurals = Intl.PluralRules();
let x = plurals.select(0) // Returns "other"
let y = plurals.select(1) // Returns "one"
let z = plurals.select(2) // Returns "other"

By default, PluralRules is configured as cardinal, which means anything above 1 is considered plural. Above, as you can see, PluralRules lets us differentiate whether a number is plural or not.

Things get more interesting when we set it to ordinal, which takes into consideration the ways we use numbers. Here, it will tell us language specific rules on how to handle each number - so we can do things like 2nd, 3rd, and 4th

const plurals = new Intl.PluralRules('en-US', { type: 'ordinal' });
let a = plurals.select(0); // Returns "other"
let b = plurals.select(1); // Returns "one"
let c = plurals.select(2); // Returns "two"
let d = plurals.select(3); // Returns "few"
let e = plurals.select(4); // Returns "other"

Now we can customize our outputs based on locale, so we don't run into weird issues. Here, we're using en-US, but any other valid locale will also work. To map our numbers to 1st, 2nd, 3rd, etc, we only have to create a mapping like this:

let x = [ 1, 13, 22, 100, 1204, 133 ];

const plurals = new Intl.PluralRules('en-US', { type: 'ordinal' });

let pluralMapping = {
    "one" : "st",
    "two" : "nd",
    "few" : "rd",
    "other" : "th"
}

let y = [];
x.forEach((item) => {
    let getPlural = plurals.select(item);
    let pluralEnding = pluralMapping[getPlural]
    y.push(`${item}${pluralEnding}`)
})

console.log(y); // ['1st', '13th', '22nd', '100th', '1204th', '133rd']