ECMAScript Harmony

Brendan Eich brendan at
Wed Aug 13 14:26:56 PDT 2008

It's no secret that the JavaScript standards body, Ecma's Technical  
Committee 39, has been split for over a year, with some members  
favoring ES4, a major fourth edition to ECMA-262, and others  
advocating ES3.1 based on the existing ECMA-262 Edition 3 (ES3)  
specification. Now, I'm happy to report, the split is over.

The Ecma TC39 meeting in Oslo at the end of July was very productive,  
and if we keep working together, it will be seen as seminal when we  
look back in a couple of years. Before this meeting, I worked with  
John Neumann, TC39 chair, and ES3.1 and ES4 principals, especially  
Lars Hansen (Adobe), Mark Miller (Google), and Allen Wirfs-Brock  
(Microsoft), to unify the committee around shared values and a common  
roadmap. This message is my attempt to announce the main result of  
the meeting, which I've labeled "Harmony".

Executive Summary

The committee has resolved in favor of these tasks and conclusions:

1. Focus work on ES3.1 with full collaboration of all parties, and  
target two interoperable implementations by early next year.
2. Collaborate on the next step beyond ES3.1, which will include  
syntactic extensions but which will be more modest than ES4 in both  
semantic and syntactic innovation.
3. Some ES4 proposals have been deemed unsound for the Web, and are  
off the table for good: packages, namespaces and early binding. This  
conclusion is key to Harmony.
4. Other goals and ideas from ES4 are being rephrased to keep  
consensus in the committee; these include a notion of classes based  
on existing ES3 concepts combined with proposed ES3.1 extensions.

Detailed Statement

A split committee is good for no one and nothing, least of all any  
language specs that might come out of it. Harmony was my proposal  
based on this premise, but it also required (at least on the part of  
key ES4 folks) intentionally dropping namespaces.

This is good news for everyone, both those who favor smaller changes  
to the language and those who advocate ongoing evolution that  
requires new syntax if not new semantics. It does mean that some of  
the ideas going back to the first ES4 proposals in 1999, implemented  
variously in JScript.NET and ActionScript, won't make it into any ES  
standard. But the benefit is collaboration on unified successor  
specifications to follow ES3, starting with ES3.1 and continuing  
after it with larger changes and improved specification techniques.

One of the use-cases for namespaces in ES4 was early binding (use  
namespace intrinsic), both for performance and for programmer  
comprehension -- no chance of runtime name binding disagreeing with  
any earlier binding. But early binding in any dynamic code loading  
scenario like the web requires a prioritization or reservation  
mechanism to avoid early versus late binding conflicts.

Plus, as some JS implementors have noted with concern, multiple open  
namespaces impose runtime cost unless an implementation works  
significantly harder.

For these reasons, namespaces and early binding (like packages before  
them, this past April) must go. This is final, they are not even a  
future possibility. To achieve harmony, we have to focus not only on  
nearer term improvements -- on "what's in" or what could be in --  we  
must also strive to agree on what's out.

Once namespaces and early binding are out, classes can desugar to  
lambda-coding + Object.freeze and friends from ES3.1. There's no need  
for new runtime semantics to model what we talked about in Oslo as a  
harmonized class proposal (I will publish wiki pages shortly to show  
what was discussed).

We talked about desugaring classes in some detail in Oslo. During  
these exchanges, we discussed several separable issues, including  
classes, inheritance, like patterns, and type annotations. I'll avoid  
writing more here, except to note that there were clear axes of  
disagreement and agreement, grounds for hope that the committee could  
reach consensus on some of these ideas, and general preference for  
starting with the simplest proposals and keeping consensus as we go.

We may add runtime helpers if lambda-coding is too obscure for the  
main audience of the spec, namely implementors who aim to achieve  
interoperation, but who may not be lambda-coding gurus. But we will  
try to avoid extending the runtime semantic model of the 3.1 spec, as  
a discipline to guard against complexity.

One possible semantic addition to fill a notorious gap in the  
language, which I sketched with able help from Mark Miller: a way to  
generate new Name objects that do not equate as property identifiers  
to any string. I also showed some sugar, but that is secondary at  
this point. Many were in favor of this new Name object idea.

There remain challenges, in particular getting off of the untestable  
and increasingly unwieldy ES1-3.x spec formalism. I heard some  
generally agree, and no one demur, about the ES4 approach of using an  
SML + self-hosted built-ins reference implementation (RI).

We are going to look into stripping the RI of namespaces and early  
binding (which it uses to ensure normative self-hosted behavior, not  
susceptible to "user code" modifying the meaning of built-ins),  
simplifying it to implement ES3.1plus or minus (self-hosted built-ins  
may require a bit more magic). More on that effort soon.

ES3.1 standardizes getters and setters that were first implemented at  
Mozilla and copied by Apple and Opera. More such de-facto  
standardization is on the table for a successor edition in the  
harmonized committee.

I heard good agreement on low-hanging "de-facto standard" fruit,  
particularly let as the new var, to match block-scoped const as still  
proposed (IIRC) in 3.1. Also some favorable comments about simple  
desugarings such as expression closures and destructuring assignment,  
and other changes in JS1.7 and 1.8 that do not require new runtime  
semantic models.

Obviously, these require new syntax, which is appropriate for a major  
post-3.1 "ES-harmony" edition. Syntax is user interface, there's no  
reason to avoid improving it. What's more, the intersection semantics  
of extended ES3 implementations conflict and choke off backward- 
compatible *semantics* for syntax that may even parse in all top four  
browsers (e.g., functions in blocks).

Both the appropriateness of new syntax, and the need to make  
incompatible (with ES3 extensions) semantic changes, motivate opt-in  
versioning of harmonized successor edition. I believe that past  
concerns about opt-in versioning requiring server file suffix to MIME  
type mapping maintenance were assuaged (browsers in practice, and  
HTML5 + RFC 4329, do not consider server-sent Content-Type -- the web  
page author can write version parameters directly in script tag type  

Some expressed interest in an in-language pragma to select version;  
this would require immediate version change during parsing. It's a  
topic for future discussions.

The main point, as important as cutting namespaces in my view, is  
that the committee has a vision for extending the language  
syntactically, not trying to fit new semantics entirely within some  
combination of existing "three of four top browsers" syntax and  
standard library extensions.

As Waldemar Horwat (Google) said on the final day, the meeting was  
seminal, and one of the most productive in a long while. Much work  
remains on 3.1 and Harmony, but we are now on a good footing to make  
progress as a single committee.


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