ECMAScript Harmony

ECMAScript Harmony

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 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.

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 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.

John Resig - ECMAScript Harmony

The ECMAScript 4 specification development was very ad-hoc in nature (primarily tackled by Adobe, Mozilla, Opera, and Google): Implementors agreed upon a set of features that they wished to implement and a specification was molded out of the remaining consensus. Building a specification tailored by implementation is a very pragmatic means of reaching a reasonable result.

However there was a fundamental split related to how much of the specification should be implemented. Enter ECMAScript 3.1. This working group (lead by Microsoft and Yahoo) set out to implement some minor changes and bug fixes to ECMAScript 3 while remaining as a subset of full ECMAScript 4 functionality.

This means a couple things: First, you can forget a lot of what you learned about ECMAScript 4, previously. Many of the complicated concepts contained in the language have been tossed. Instead there is a considerable amount of effort going in to making sure that new features will be easily duplicable through other means.

For example, ECMAScript 3.1 provides a new method - called Object.freeze() - which allows you to pass in an object and "freeze" it, preventing it from being modified any further. This is a subset of the functionality that is needed to implement classes in ECMAScript 4 (classes are immutable).

This means that classes will be a part of the new language but they will be defined as simply being syntactic sugar for a series of plain methods or conventions (such as using closures and Object.freeze to create a Class-like experience). Making sure that complex concepts break down into a simplified form will serve users well - giving them a considerable amount of leverage in using the language.

Probably the most important development within all this is the codification of existing de-facto standards. For example, the concept of JavaScript getters and setters (implemented by Mozilla, Apple, and Opera) are going to be quickly fast-tracked into the specification (in the case of getters and setters they already have been). Seeing real-world code quickly make a bee-line for standardization is truly heartwarming.

Major Web Standard Updated: ECMA-262 5th Edition has been approved

This revision of ECMA-262 will be known as ECMAScript, Fifth Edition. It was previously developed under the working name ECMAScript 3.1, which will no longer be used.

The Fifth Edition codifies de facto interpretations of the language specification that have become common among browser implementations and adds support for new features that have emerged since the publication of the Third Edition. Such features include accessor properties, reflective creation and inspection of objects, program control of property attributes, additional array manipulation functions, support for the JSON object encoding format, and a strict mode that provides enhanced error checking and program security.

The last major revision of the ECMAScript standard was the Third Edition, published in 1999. After completion of the Third Edition, significant work was done to develop a Fourth Edition. Although development of a Fourth Edition was not completed, that work influenced ECMAScript, Fifth Edition and is continuing to influence the ongoing development of ECMAScript. Work on future ECMAScript editions continues as part of the previously announced ECMAScript Harmony project.