Hello everyone! As we have with the last bunch of meetings, we're excited to tell you about all the new discussions taking place in TC39 meetings and how we try to contribute to them. However, this specific meeting has an even more special place in our hearts since Igalia had the privilege of organising it in our headquarters in A Coruña, Galicia. It was an absolute honor to host all the amazing delegates in our home city. We would like to thank everyone involved and look forward to hosting it again!
Let's delve together into some of the most exciting updates.
Array.from, which takes a synchronous iterable and dumps it into a new array, is one of Array's most frequently used built-in methods, especially for unit tests or CLI interfaces. However, there was no way to do the equivalent with an asynchronous iterator. Array.fromAsync solves this problem, being to Array.from as for await is to for. This proposal has now been shipping in all JS engines for at least a year (which means it's Baseline 2024), and it has been highly requested by developers.
From a bureaucratic point of view however, the proposal was never really stage 3. In September 2022 it advanced to stage 3 with the condition that all three of the ECMAScript spec editors signed off on the spec text; and the editors requested that a pull request was opened against the spec with the actual changes. However, this PR was not opened until recently. So in this TC39 meeting, the proposal advanced to stage 4, conditional on this editors actually reviewing it.
The Explicit Resource Management proposal introduces implicit cleanup callbacks for objects based on lexical scope. This is enabled through the new using x = declaration:
{ using myFile =open(fileURL); const someBytes = myFile.read();
// myFile will be automatically closed, and the // associated resources released, here at the // end of the block. }
The proposal is now shipped in Chrome, Node.js and Deno, and it's behind a flag in Firefox. As such, Ron Buckton asked for (and obtained!) consensus to approve it for Stage 4 during the meeting.
Similarly to Array.fromAsync, it's not quite Stage 4 yet, as there is still something missing before including it in the ECMAScript standard: test262 tests need to be merged, and the ECMAScript spec editors need to approve the proposed specification text.
The Error.isError(objectToCheck) method provides a reliable way to check whether a given value is a real instance of Error. This proposal was originally presented by Jordan Harband in 2015, to address concerns about it being impossible to detect whether a given JavaScript value is actually an error object or not (did you know that you can throw anything, including numbers and booleans!?). It finally became part of the ECMAScript standard during this meeting.
Intl.Locale objects represent Unicode Locale identifiers; i.e., a combination of language, script, region, and preferences for things like collation or calendar type.
For example, de-DE-1901-u-co-phonebk means "the German language as spoken in Germany with the traditional German orthography from 1901, using the phonebook collation". They are composed of a language optionally followed by:
a script (i.e. an alphabet)
a region
one or more variants (such as "the traditional German orthography from 1901")
a list of additional modifiers (such as collation)
Intl.Locale objects already had accessors for querying multiple properties about the underlying locale but was missing one for the variants due to an oversight, and the committee reached consensus on also exposing them in the same way.
The Intl.Locale Info Stage 3 proposal allows JavaScript applications to query some metadata specific to individual locales. For example, it's useful to answer the question: "what days are considered weekend in the ms-BN locale?".
The committee reached consensus on a change regarding information about text direction: in some locales text is written left-to-right, in others it's right-to-left, and for some of them it's unknown. The proposal now returns undefined for unknown directions, rather than falling back to left-to-right.
Our colleague Philip Chimento presented a regular status update on Temporal, the upcoming proposal for better date and time support in JS. The biggest news is that Temporal is now available in the latest Firefox release! The Ladybird, Graal, and Boa JS engines all have mostly-complete implementations. The committee agreed to make a minor change to the proposal, to the interpretation of the seconds (:00) component of UTC offsets in strings. (Did you know that there has been a time zone that shifted its UTC offset by just 20 seconds?)
The Immutable ArrayBuffer proposal allows creating ArrayBuffers in JS from read-only data, and in some cases allows zero-copy optimizations. After last time, the champions hoped they could get the tests ready for this plenary and ask for stage 3, but they did not manage to finish that on time. However, they did make a very robust testing plan, which should make this proposal "the most well-tested part of the standard library that we've seen thus far". The champions will ask to advance to stage 3 once all of the tests outlined in the plan have been written.
The iterator sequencing Stage 2.7 proposal introduces a new Iterator.concat method that takes a list of iterators and returns an iterator yielding all of their elements. It's the iterator equivalent of Array.prototype.concat, except that it's a static method.
Michael Ficarra, the proposal's champion, was originally planning to ask for consensus on advancing the proposal to Stage 3: test262 tests had been written, and on paper the proposal was ready. However, that was not possible because the committe discussed some changes about re-using "iterator result" objects that require some changes to the proposal itself (i.e. should Iterator.concat(x).next() return the same object as x.next(), or should it re-create it?).
The iterator chunking Stage 2 proposal introduces two new Iterator.prototype.* methods: chunks(size), which splits the iterator into non-overlapping chunks, and windows(size), which generates overlapping chunks offset by 1 element:
[1,2,3,4].values().chunks(2);// [1,2] and [3,4] [1,2,3,4].values().windows(2);// [1,2], [2,3] and [3,4]
The proposal champion was planning to ask for Stage 2.7, but that was not possible due to some changes about the .windows behaviour requested by the committee: what should happen when requesting windows of size n out of an iterator that has less than n elements? We considered multiple options:
Do not yield any array, as it's impossible to create a window of size n
Yield an array with some padding (undefined?) at the end to get it to the expected length
Yield an array with fewer than n elements
The committee concluded that there are valid use cases both for (1) and for (2). As such, the proposal will be updated to split .windows() into two separate methods.
AsyncContext is a proposal that allows having state persisted across async flows of control -- like thread-local storage, but for asynchronicity in JS. The champions of the proposal believe async flows of control should not only flow through await, but also through setTimeout and other web features, such as APIs (like xhr.send()) that asynchronously fire events. However, the proposal was stalled due to concerns from browser engineers about the implementation complexity of it.
In this TC39 session, we brainstormed about removing some of the integration points with web APIs: in particular, context propagation through events caused asynchronously. This would work fine for web frameworks, but not for tracing tools, which is the other main use case for AsyncContext in the web. It was pointed out that if the context isn't propagated implicitly through events, developers using tracing libraries might be forced to snapshot contexts even when they're not needed, which would lead to userland memory leaks. In general, the room seemed to agree that the context should be propagated through events, at the very least in the cases in which this is feasible to implement.
This TC39 discussion didn't do much move the proposal along, and we weren't expecting it to do so -- browser representatives in TC39 are mostly engineers working on the core JS engines (such as SpiderMonkey, or V8), while the concerns were coming from engineers working on web APIs. However, the week after this TC39 plenary, Igalia organized the Web Engines Hackfest, also in A Coruña, where we could resume this conversation with the relevant people in the room. As a result, we've had positive discussions with Mozilla engineers about a possible path forward for the proposal that did propagate the context through events, analyzing more in detail the complexity of some specific APIs where we expect the propagation to be more complex.
The Math.clamp proposal adds a method to clamp a numeric value between two endpoints of a range. This proposal reached stage 1 last February, and in this plenary we discussed and resolved some of the open issues it had:
One of them was whether the method should be a static method Math.clamp(min, value, max), or whether it should be a method on Number.prototype so you could do value.clamp(min, max). We opted for the latter, since in the former the order of the arguments might not be clear.
Another was whether the proposal should support BigInt as well. Since we're making clamp a method of Number, we opted to only support the JS number type. A follow-up proposal might add this on BigInt.prototype as well.
Finally, there was some discussion about whether clamp should throw an exception if min is not lower or equal to max; and in particular, how this should work with positive and negative zeros. The committee agreed that this can be decided during Stage 2.
With this, the Math.clamp (or rather, Number.prototype.clamp) proposal advanced to stage 2. The champion was originally hoping to get to Stage 2.7, but they ended up not proposing it due to the pending planned changes to the proposed specification text.
As it stands, JavaScript's built-in functionality for generating (pseudo-)random numbers does not accept a seed, a piece of data that anchors the generation of random numbers at a fixed place, ensuring that repeated calls to Math.random, for example, produce a fixed sequence of values. There are various use cases for such numbers, such as testing (how can I lock down the behavior of a function that calls Math.random for testing purposes if I don't know what it will produce?). This proposal seeks to add a new top-level Object, Random, that will permit seeding of random number generation. It was generally well received and advanced to stage 2.
Tab Atkins-Bittner, who presented the Seeded PRNG proposal, continued in a similar vein with "More random functions". The idea is to settle on a set of functions that frequently arise in all sorts of settings, such as shuffling an array, generating a random number in an interval, generating a random boolean, and so on. There are a lot of fun ideas that can be imagined here, and the committee was happy to advance this proposal to stage 1 for further exploration.
Eemeli Aro of Mozilla proposed a neat bugfix for two parts of JavaScript's internationalization API that handle numbers. At the moment, when a digit string, such as "123.456" is given to the Intl.PluralRules and Intl.NumberFormat APIs, the string is converted to a Number. This is generally fine, but what about digit strings that contain trailing zeroes, such as "123.4560"? At the moment, that trailing zero gets removed and cannot be recovered. Eemeli suggest that we keep such digits. They make a difference when formatting numbers and in using them for pluralizing words, such as "1.0 stars". This proposal advanced to stage 1, with the understanding that some work needs to be done to clarify how some some already-existing options in the NumberFormat and PluralRules APIs are to be understood when handling such strings. Eemeli's proposal is now at stage 1!
We shared the latest developments on the Decimal proposal and its potential integration with Intl, focusing on the concept of amounts. These are lightweight wrapper classes designed to pair a decimal number with an integer "precision", representing either the number of significant digits or the number of fractional digits, depending on context. The discussion was a natural follow-on to the earlier discussion of keeping trailing zeroes in Intl.NumberFormat and Intl.PluralRules. In discussions about decimal, we floated the idea of a string-based version of amounts, as opposed to one backed by a decimal, but this was a new, work-in-progress idea. It seems that the committee is generally happy with the underlying decimal proposal but not yet convinced about the need for a notion of an amount, at least as it was presented. Decimal stays at stage 1.
Many JS environments today provide some sort of assertion functions. (For example, console.assert, Node.js's node:assert module, the chai package on NPM.) The committee discussed a new proposal presented by Jacob Smith, Comparisons, which explores whether this kind of functionality should be part of the ECMAScript standard. The proposal reached stage 1, so the investigation and scoping will continue: should it cover rich equality comparisons, should there be some sort of test suite integration, should there be separate debug and production modes? These questions will be explored in future meetings.
If you look at the specifications for HTML, the DOM, and other web platform features, you can't miss the Web IDL snippets in there. This IDL is used to describe all of the interfaces available in web browser JS environments, and how each function argument is processed and validated.
IDL does not only apply to the specifications! The IDL code is also copied directly into web browsers' code bases, sometimes with slight modifications, and used to generate C++ code.
Tooru Fujisawa (Arai) from Mozilla brought this proposal back to the committee after a long hiatus, and presented a vision of how the same thing might be done in the ECMAScript specification, gradually. This would lower maintenance costs for any JS engine, not just web browsers. However, the way that function arguments are generally handled differs sufficiently between web platform APIs and the ECMAScript specification that it wouldn't be possible to just use the same Web IDL directly.
Tooru presented some possible paths to squaring this circle: adding new annotations to the existing Web IDL or defining new syntax to support the ECMAScript style of operations.
After the meeting on Thursday, we co-organized a community event with the help of our localtech communities. With an exciting agenda full of insightful and unique presentations and a lively networking session afterwards over some snacks, we hope to have started some interesting conversations in the communities and piqued the interest of JavaScript developers around them over these topics.
The May 2025 plenary was packed with exciting progress across the JavaScript language and internationalization features. It was also a special moment for us at Igalia as proud hosts of the meeting in our hometown of A Coruña.
We saw long-awaited proposals like Array.fromAsync, Error.isError, and Explicit Resource Management reach Stage 4, while others continued to evolve through thoughtful discussion and iteration.
We’ll continue sharing updates as the work evolves, until then, thanks for reading, and see you at the next meeting!
