Using the Intl APIs in JavaScript

Using the Intl APIs in JavaScript

Learn how to use the new Intl object to format data into a specific locale

The Intl object is available in the global scope and is used for formatting strings, numbers, and date and time in the locale-specific format. It does the work of internationalizing information displayed to the user.

The JavaScript developers don’t have to ship KBs of locale-specific data in the main bundles. The Intl object has various constructors and methods that take in locale object as one of the parameters and format data as required. Here’s how the Intl global object looks like:

Intl in Global Scope

The Collator, DateTimeFormat, ListFormat, NumberFormat, PluralRules, RelativeTimeFormat are the constructors in the Intl namespace. They take in two arguments — locales and options. The locale argument must be a string in BCP 47 language tag or an array of such strings. If you’re interested in knowing more about the BCP 47 language tag, here’s an excerpt from MDN:

A BCP 47 language tag defines a language and minimally contains a primary language code. In its most common form it can contain, in order: a language code, a script code, and a country or region code, all separated by hyphens. While the tag is not case sensitive, it is recommended to use title case for script code, upper case for country and region codes and lower case for everything else.
The default value of the locale argument is set to the locale of the runtime. Some of the examples of the locale are — en (English), hi (Hindi), ta-in (Tamil-India). The options argument is optional and its structure varies for different constructors. It is basically used for providing additional information for formatting.

In this article, we’re going to look at some of the new APIs that are added in the Intl namespace. These new APIs were announced in Google I/O ‘19. Let’s get to these APIs in detail:


The RelativeTimeFormat constructor is used to generate human-friendly phrases for timestamps. It converts the hard-to-read date object into its relative time string. Let’s see this in action:

Intl.RelativeTimeFormat constructor with en locale

Notice the first argument to the Intl.RelativeTimeFormat constructor as en . The possible values for the numeric key in the options object are always and auto — always would format it in numbers as:

rtf.format(-1, 'month') would result in 1 month ago.

If you’d want to display the relative bits in Tamil, you can do so in no time as:

Intl.RelativeTimeFormat constructor with ta-in locale

The relative time strings yesterday, today, tomorrow, etc provide a better user experience! This is supported in Chrome 71 and FireFox 65.


The ListFormat constructor is used for joining strings using a disjunction or conjunction to form a meaningful phrase.

Intl.ListFormat constructor with en locale and default options object

The format method nicely concatenates the strings in the characters array using and. If not for the ListFormat API, we would have to write utility functions to place commas and and at appropriate places. The Intl.ListFormat API makes formatting strings a breeze!

We can customize the formatting experience by changing the default values in the options object as:

Intl.ListFormat constructor with en locale and disjunction type

The type disjunction joins the strings using the or literal. Let’s check out the possible values for various properties on the options object:

type: Possible values are conjunction, disjunction, and unit. The conjunction is the default value. The unit type is used for unit-based lists like:

Intl.ListFormat constructor with en locale and disjunction type

style: Possible values are long and short (or narrow)

The ListFormat API is available in Chrome 72.


JavaScript, unlike other high-level languages, provides flexibility with the data types of variables. An int, float, string or an object are all declared using var (or let and const for block-scoping).

But how big can a number be?

123456789123456789 * 11 // Prints 1358024680358024700

The unit place of the result of the above operation cannot be 0! Looks like, we need to do something for the bigger numbers. And the good news is, we already have BigInt for handling large numbers. Let’s do the above operation using BigInt:

123456789123456789n * 11n // Prints 1358024680358024679n

This now looks correct!

The bigger numbers are difficult to read in one go. Here comes the Intl.NumerFormat API to help! It is used to format numbers by adding locale-specific numeric separators. Let’s see how this works:

Intl.NumberFormat constructor

Notice how numbers are formatted as per the specified locale. The numbers are formatted with commas for the en locale and with spaces for the fr locale.

We can also pass in the options object to the constructor for customizing the formatting experience. Some of the properties available on the options object are:

style: Possible values are decimal, currency, and percent and the default value is decimal

currency: Currency code to be used for the currency style. Examples: USD, INR

currencyDisplay: Possible values are symbol and code and the default value is symbol

You can check all the properties in the options object here.

This is available in Chrome, FireFox, and Safari.

There’s one more exciting API in the list: Intl.DateTimeFormat


The Intl.DateTimeFormat API is used for locale-specific date and time formatting. The new method formatRange is now available on this API and it can be used as:

const df = new Intl.DateTimeFormat('en', {
    year: 'numeric',
    month: 'short',
    day: 'numeric'
const startDate = new Date('01-06-2019')
const endDate = new Date('10-06-2019')
df.formatRange(startDate, endDate) // Prints Jun 1-10 2019

Notice how the formatRange function groups together the dates of the same month. Please note: this function is available behind a flag in Chrome and will be shipped soon.

You can now remove that extra utility function from your bundle, which was just used for formatting date ranges!

There are many properties available in the options object such as timeZone, era, year, month, day, hour, minute, second and many more.


We saw some of the new additions to the Intl object:

  1. Intl.RelativeTimeFormat API can be used to generate relative and human-friendly phrases for timestamps.

  2. Intl.ListFormat API can be used to construct phrases from the array elements using the conjunction, disjunction, and unit types.

  3. Intl.NumberFormat can be used to format bigger numbers using locale-specific numeric literals.

  4. The formatRange method on the Intl.DateTimeFormat API can be used to generate range strings for timestamps.

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JavaScript developers should you be using Web Workers?

JavaScript developers should you be using Web Workers?

Do you think JavaScript developers should be making more use of Web Workers to shift execution off of the main thread?

Originally published by David Gilbertson at

So, Web Workers. Those wonderful little critters that allow us to execute JavaScript off the main thread.

Also known as “no, you’re thinking of Service Workers”.

Photo by Caleb Jones on Unsplash

Before I get into the meat of the article, please sit for a lesson in how computers work:

Understood? Good.

For the red/green colourblind, let me explain. While a CPU is doing one thing, it can’t be doing another thing, which means you can’t sort a big array while a user scrolls the screen.

This is bad, if you have a big array and users with fingers.

Enter, Web Workers. These split open the atomic concept of a ‘CPU’ and allow us to think in terms of threads. We can use one thread to handle user-facing work like touch events and rendering the UI, and different threads to carry out all other work.

Check that out, the main thread is green the whole way through, ready to receive and respond to the gentle caress of a user.

You’re excited (I can tell), if we only have UI code on the main thread and all other code can go in a worker, things are going to be amazing (said the way Oprah would say it).

But cool your jets for just a moment, because websites are mostly about the UI — it’s why we have screens. And a lot of a user’s interactions with your site will be tapping on the screen, waiting for a response, reading, tapping, looking, reading, and so on.

So we can’t just say “here’s some JS that takes 20ms to run, chuck it on a thread”, we must think about where that execution time exists in the user’s world of tap, read, look, read, tap…

I like to boil this down to one specific question:

Is the user waiting anyway?

Imagine we have created some sort of git-repository-hosting website that shows all sorts of things about a repository. We have a cool feature called ‘issues’. A user can even click an ‘issues’ tab in our website to see a list of all issues relating to the repository. Groundbreaking!

When our users click this issues tab, the site is going to fetch the issue data, process it in some way — perhaps sort, or format dates, or work out which icon to show — then render the UI.

Inside the user’s computer, that’ll look exactly like this.

Look at that processing stage, locking up the main thread even though it has nothing to do with the UI! That’s terrible, in theory.

But think about what the human is actually doing at this point. They’re waiting for the common trio of network/process/render; just sittin’ around with less to do than the Bolivian Navy.

Because we care about our users, we show a loading indicator to let them know we’ve received their request and are working on it — putting the human in a ‘waiting’ state. Let’s add that to the diagram.

Now that we have a human in the picture, we can mix in a Web Worker and think about the impact it will have on their life:


First thing to note is that we’re not doing anything in parallel. We need the data from the network before we process it, and we need to process the data before we can render the UI. The elapsed time doesn’t change.

(BTW, the time involved in moving data to a Web Worker and back is negligible: 1ms per 100 KB is a decent rule of thumb.)

So we can move work off the main thread and have a page that is responsive during that time, but to what end? If our user is sitting there looking at a spinner for 600ms, have we enriched their experience by having a responsive screen for the middle third?


I’ve fudged these diagrams a little bit to make them the gorgeous specimens of graphic design that they are, but they’re not really to scale.

When responding to a user request, you’ll find that the network and DOM-manipulating part of any given task take much, much longer than the pure-JS data processing part.

I saw an article recently making the case that updating a Redux store was a good candidate for Web Workers because it’s not UI work (and non-UI work doesn’t belong on the main thread).

Chucking the data processing over to a worker thread sounds sensible, but the idea struck me as a little, umm, academic.

First, let’s split instances of ‘updating a store’ into two categories:

  1. Updating a store in response to a user interaction, then updating the UI in response to the data change
  2. Not that first one

If the first scenario, a user taps a button on the screen — perhaps to change the sort order of a list. The store updates, and this results in a re-rendering of the DOM (since that’s the point of a store).

Let me just delete one thing from the previous diagram:

In my experience, it is rare that the store-updating step goes beyond a few dozen milliseconds, and is generally followed by ten times that in DOM updating, layout, and paint. If I’ve got a site that’s taking longer than this, I’d be asking questions about why I have so much data in the browser and so much DOM, rather than on which thread I should do my processing.

So the question we’re faced with is the same one from above: the user tapped something on the screen, we’re going to work on that request for hopefully less than a second, why would we want to make the screen responsive during that time?

OK what about the second scenario, where a store update isn’t in response to a user interaction? Performing an auto-save, for example — there’s nothing more annoying than an app becoming unresponsive doing something you didn’t ask it to do.

Actually there’s heaps of things more annoying than that. Teens, for example.

Anyhoo, if you’re doing an auto-save and taking 100ms to process data client-side before sending it off to a server, then you should absolutely use a Web Worker.

In fact, any ‘background’ task that the user hasn’t asked for, or isn’t waiting for, is a good candidate for moving to a Web Worker.

The matter of value

Complexity is expensive, and implementing Web Workers ain’t cheap.

If you’re using a bundler — and you are — you’ll have a lot of reading to do, and probably npm packages to install. If you’ve got a create-react-app app, prepare to eject (and put aside two days twice a year to update 30 different packages when the next version of Babel/Redux/React/ESLint comes out).

Also, if you want to share anything fancier than plain data between a worker and the main thread you’ve got some more reading to do (comlink is your friend).

What I’m getting at is this: if the benefit is real, but minimal, then you’ve gotta ask if there’s something else you could spend a day or two on with a greater benefit to your users.

This thinking is true of everything, of course, but I’ve found that Web Workers have a particularly poor benefit-to-effort ratio.

Hey David, why you hate Web Workers so bad?

Good question.

This is a doweling jig:

I own a doweling jig. I love my doweling jig. If I need to drill a hole into the end of a piece of wood and ensure that it’s perfectly perpendicular to the surface, I use my doweling jig.

But I don’t use it to eat breakfast. For that I use a spoon.

Four years ago I was working on some fancy animations. They looked slick on a fast device, but janky on a slow one. So I wrote fireball-js, which executes a rudimentary performance benchmark on the user’s device and returns a score, allowing me to run my animations only on devices that would render them smoothly.

Where’s the best spot to run some CPU intensive code that the user didn’t request? On a different thread, of course. A Web Worker was the correct tool for the job.

Fast forward to 2019 and you’ll find me writing a routing algorithm for a mapping application. This requires parsing a big fat GeoJSON map into a collection of nodes and edges, to be used when a user asks for directions. The processing isn’t in response to a user request and the user isn’t waiting on it. And so, a Web Worker is the correct tool for the job.

It was only when doing this that it dawned on me: in the intervening quartet of years, I have seen exactly zero other instances where Web Workers would have improved the user experience.

Contrast this with a recent resurgence in Web Worker wonderment, and combine that contrast with the fact that I couldn’t think of anything else to write about, then concatenate that combined contrast with my contrarian character and you’ve got yourself a blog post telling you that maybe Web Workers are a teeny-tiny bit overhyped.

Thanks for reading

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Further reading

An Introduction to Web Workers

JavaScript Web Workers: A Beginner’s Guide

Using Web Workers to Real-time Processing

How to use Web Workers in Angular app

Using Web Workers with Angular CLI

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