Remember how I promised you (or rather myself) two weeks ago that from now on I'd only write light, easy posts? Well… I broke that promise 😅 Last week I published a huge article about one of the least sexy topics imaginable: legacy applications. Somehow, despite that, the DEV editorial team liked it enough to include it in the Top 7 Posts of the Week ❤️

But today is finally the day for something a bit lighter. And I think it's a topic that deserves more attention.

These days we spend a lot of time talking about AI agents, and I absolutely understand why, because it's a fascinating field. But I sometimes feel we're forgetting that the programming languages we use every day are evolving too.

I spend most of my days writing JavaScript (well… TypeScript, obviously 😄). And yes, the ecosystem has matured a lot over the last few years. We no longer have endless React vs Angular wars. We're also mostly past those times when everyone loved Redux one month and six months later decided it was an overengineered monster and removed it from every application 😅.

But both the ecosystem and the ECMAScript standard keep evolving.

After the legendary ES6 release in 2015, which changed frontend development forever, the TC39 committee switched to an iterative approach and started shipping new features every year.

And the nice thing is that if you're using modern browsers or runtimes, you often get access to these features almost immediately.

So, what do we get in ECMAScript 2026?

Oh, and to keep things entertaining, all code examples in this article are based on the internal monologue of imaginary LinkedIn gurus with very strong opinions. Any resemblance to actual people is, naturally, purely coincidental 😇

Other articles from this series:

• Stop Installing Libraries: 10 Browser APIs That Already Solve Your Problems
• 9 Things You're Overengineering — The Browser Already Solved Them
• 16 Modern JavaScript Features That Might Blow Your Mind

1. Map.prototype.getOrInsert() / Upsert

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map/getOrInsert

This is one of those features that makes you wonder: "Wait… why didn't we have this ten years ago?"

Imagine we're collecting opinions from LinkedIn gurus. Until now, we had to do something like this:

const opinions = new Map();

function addOpinion(topic, author) {
  if (!opinions.has(topic)) {
    opinions.set(topic, []);
  }

  opinions.get(topic).push(author);
}

addOpinion("React is dead", "10x Engineer");
addOpinion("React is dead", "Principal AI Evangelist");

It works, but there's quite a bit of boilerplate.

With ES2026, things become much nicer:

const opinions = new Map();

opinions
  .getOrInsert("React is dead", [])
  .push("10x Engineer");

opinions
  .getOrInsert("React is dead", [])
  .push("Principal AI Evangelist");

console.log(opinions);

We also get a computed version:

const opinions = new Map();

opinions
  .getOrInsertComputed(
    "Nobody writes clean code anymore",
    () => []
  )
  .push("DDD Purist");

Come on, that's so much cleaner and easier to read. It's also perfect for caching! Much less code, much nicer API.

2. Iterator.concat()

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Iterator/concat

Before we talk about concat(), let's quickly answer a question:

What is an iterator?

An iterator is basically a cursor that lets us consume data one element at a time without storing everything in memory.

For example:

const posts = [
  "Angular is dead",
  "JavaScript was a mistake"
];

const iterator = posts.values();

console.log(iterator.next());
// { value: "Angular is dead", done: false }

console.log(iterator.next());
// { value: "JavaScript was a mistake", done: false }

console.log(iterator.next());
// { value: undefined, done: true }

And because iterators are lazy, we can even generate infinite sequences without blowing up memory:

function* endlessHotTakes() {
  while (true) {
    yield "Everything should be rewritten in Rust.";
  }
}

const iterator = endlessHotTakes();

console.log(iterator.next().value);
console.log(iterator.next().value);
console.log(iterator.next().value);

Last year we got Iterator Helpers, which introduced things like map(), filter(), and friends:

function* linkedinFeed() {
  yield "Use Rust for everything";
  yield "JavaScript was a mistake";
  yield "Clean code is dead";
  yield "Angular is dead";
}

const controversialTakes =
  linkedinFeed()
    .filter(post => post.includes("dead"))
    .map(post => `🔥 ${post}`);

console.log([...controversialTakes]);

And now we get another nice addition: Iterator.concat(). Suppose frontend gurus and backend gurus each have their own stream of wisdom:

function* frontendExperts() {
  yield "React is dead";
  yield "Nobody needs Redux";
}

function* backendExperts() {
  yield "Microservices solve everything";
  yield "Frontend developers don't understand architecture";
}

Now we can combine them elegantly:

const feed = Iterator.concat(
  frontendExperts(),
  backendExperts()
);

console.log([...feed]);

In my opinion, these iterator features don't get nearly as much attention as they deserve.

3. Array.fromAsync()

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fromAsync

This is basically the async equivalent of our beloved Array.from().

It's primarily designed for async iterators, although I suspect that most JavaScript developers don't use async iterators every day. For example, imagine comments arriving one by one from LinkedIn:

async function* angryComments() {
  yield "This could have been a plain HTML form.";
  yield "Angular is dead.";
  yield "React is dead.";
  yield "JavaScript was a mistake.";
}

const comments = await Array.fromAsync(
  angryComments()
);

console.log(comments); 
// This could have been a plain HTML form.
// Angular is dead.
// etc

But here's something many people don't realize: it also works with promises.

const opinions = [
  Promise.resolve("Nobody should use classes."),
  Promise.resolve("Signals change everything."),
  Promise.resolve("Microservices ruined software.")
];

const takes = await Array.fromAsync(opinions);

console.log(takes); 

And you can even map values immediately:

const comments = await Array.fromAsync(
  angryComments(),
  opinion => opinion.toUpperCase()
);

console.log(comments);

Pretty convenient.

4. Math.sumPrecise()

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/sumPrecise

This one solves a floating-point problem that's almost as old as the Internet itself 😄 As many of you probably know, in JavaScript:

console.log(0.1 + 0.2);
// 0.30000000000000004

I won't dive into why that happens today: we'd be here all night 😄

The important thing is that ES2026 introduces Math.sumPrecise(), which performs much more accurate summation and avoids the accumulation of rounding errors.

Suppose a LinkedIn guru is listing all the things that allegedly increase productivity:

const productivityBoosts = [
  0.1, // cold showers
  0.2, // AI agents
  0.3, // journaling
  0.4, // waking up at 4 AM
];

console.log(
  Math.sumPrecise(productivityBoosts)
);

Now, to be fair, most frontend developers will probably never need this. Plain old addition works perfectly fine for everyday use.

But if you're working in finance, statistics, simulations, or AI/ML, where tiny errors can accumulate over thousands or millions of operations, this becomes much more interesting.

5. Error.isError()

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/isError

This one is probably aimed more at library authors and advanced developers. Or maybe not? 😉 Until now, checking whether something was actually an error usually looked like this:

try {
  throw new Error(
    "Angular is dead."
  );
}
catch (e) {

  console.log(
    e instanceof Error
  );
}

And most of the time, that worked perfectly fine. Until it didn't.

If an error originated in another realm, for example an iframe, a Web Worker, or a VM context—instanceof Error could suddenly betray you:

const strangeThing =
  window.frames[0].eval(
    "new Error('JavaScript was a mistake.')"
  );

console.log(
  strangeThing instanceof Error
);

// false 

This problem has been annoying library authors for years, which is why many frameworks and libraries have their own helper functions for detecting errors. Now we finally get a built-in solution:

console.log(
  Error.isError(strangeThing)
);

// true

Simple and beautiful.

6. Base64 for Uint8Array

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toBase64

Quick reminder: Base64 is simply a way of converting binary data into plain text. Why would we do that? Because many protocols and formats are much happier dealing with text than raw bytes. For example, we might send an image as a Base64 string.

Now, someone might say: "Wait a minute, Base64 already existed in JavaScript!" And yes, it did. But it mostly worked with strings:

btoa("JavaScript was a mistake.");

When dealing with binary data, things quickly became ugly:

const bytes = new Uint8Array(buffer);

const base64 = btoa(
  String.fromCharCode(...bytes)
);

Not terrible, but definitely not something you'd call elegant.

Now we can simply write:

const screenshot =
  await fetch(
    "/proof/react-is-dead.png"
  );

const bytes =
  new Uint8Array(
    await screenshot.arrayBuffer()
  );

const base64 =
  bytes.toBase64();

console.log(base64);

Much cleaner. And, perhaps more importantly, much easier to understand six months later.

7. JSON.parse Source Text Access

MDN:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse

JSON.parse() is great. Until somebody sends you a gigantic number. Suppose a LinkedIn guru claims to have 999 trillion followers:

const json = `
{
  "followers":
  999999999999999999999999999999
}
`;

const data = JSON.parse(json);

console.log(data.followers);

Oops. JavaScript numbers have limits, and very large integers may lose precision.

With ES2026, the reviver callback gets access to the original source text, allowing us to recover huge values safely:

const data = JSON.parse(
  json,
  (key, value, context) => {

    if (key === "followers") {
      return BigInt(context.source);
    }

    return value;
  }
);

console.log(data.followers);

No more accidental corruption of giant numbers.

Most of us won't need this every day. But if you're dealing with IDs, financial data, or huge integer values, this can save you from some nasty bugs.

Bonus: The Great Absentees

I was really hoping these features would make it into ES2026, but unfortunately we'll have to wait another year.

Well, sort of. Modern browsers and runtimes have already started implementing them, but they are not officially part of the ECMAScript 2026 specification yet.

These are probably the two features I'm personally most excited about.

Temporal API

TC39 proposal:
https://github.com/tc39/proposal-temporal

If you've been doing frontend development long enough, chances are you've had at least one traumatic experience involving dates. Maybe something like this:

const date = new Date("2026-03-30");

console.log(date);

Depending on your timezone, congratulations! You've just entered the exciting world of: "Why is it yesterday?"

Or perhaps you've experienced everyone's favorite:

const today = new Date();

today.setMonth(today.getMonth() + 1);

And suddenly you're debugging why March plus one month somehow became May.

Or maybe you've been lucky enough to deal with time zones.

At which point you discover that time itself is apparently an illusion and that daylight saving time was invented specifically to ruin programmers' lives.

This is why many of us ended up using libraries like Moment.js. Ironically, even the Moment.js maintainers now recommend using other solutions. Then came date-fns, Luxon, and various other attempts to restore sanity.

They certainly help. Until somebody says: "By the way, users in Australia are reporting weird bugs." And that's usually where the fun begins.

Temporal aims to fix all of this by introducing proper date and time types.

Instead of:

const now = new Date();

we get things like:

const now = Temporal.Now.instant();

const birthday =
  Temporal.PlainDate.from(
    "1987-07-23"
  );

const meeting =
  Temporal.ZonedDateTime.from(
    "2026-11-03T10:00:00[Europe/Warsaw]"
  );

And suddenly everything becomes much more explicit.

By the way, I spent an embarrassing amount of time trying to figure out whether Temporal had actually made it into ES2026. Eventually, I found the TC39 repository, where it clearly says that although the proposal has already reached Stage 4, its expected publication year is 2027.

So we're close. Very close. And I can't wait ❤️

Explicit Resource Management (using)

TC39 proposal:
https://github.com/tc39/proposal-explicit-resource-management

This is one of those features that C# and Python developers have been quietly enjoying while the rest of us pretended that try/finally was perfectly fine 😄

Sometimes you create something that needs cleaning up afterward: file, database connection, stream, WebSocket, etc. Today, we usually write something like this:

const webinar =
  createAIAgentsWebinar();

try {

  webinar.start();

}
finally {

  webinar.close();

}

The finally block is often written with the same enthusiasm as unit tests. Worse, if we forget it entirely, we may end up with resource leaks and mysterious bugs.

With using, cleanup happens automatically when leaving the scope:

using webinar =
  createAIAgentsWebinar();

webinar.start();

Once we leave the scope, JavaScript automatically calls the cleanup logic. How cool is that?

It's one of those features that might not affect the average frontend developer every day, but for Node.js developers, library authors, and people working with streams and files, it can be incredibly useful.

Uff...

I have to admit, this article turned out to be way more work than I originally expected 😅 It was supposed to be short and easy. And somehow I ended up digging through TC39 proposals, MDN pages, browser implementations, and trying to figure out whether Temporal had actually made it into ES2026 or not.

But I hope it was worth it and that you enjoyed it ❤️

What about you? Which feature surprised you the most? And are you also eagerly waiting for Temporal to finally free us from all date-related suffering?

(Or at least that's what the LinkedIn gurus will probably tell us next week 😇)

If you enjoyed this article, feel free to connect with me on LinkedIn😊