Yehuda Katz is a member of the Ember.js, Ruby on Rails and jQuery Core Teams; his 9-to-5 home is at the startup he founded, Tilde Inc.. There he works on Skylight, the smart profiler for Rails, and does Ember.js consulting. He is best known for his open source work, which also includes Thor and Handlebars. He travels the world doing open source evangelism and web standards work.
A Proposal for ES.next Proposals
September 13th, 2011
Over the past few years, I have occasionally expressed frustration (in public and private) about the process for approving new features to the next edition of ECMAScript. In short, the process is extremely academic in nature, and is peppered with inside baseball terms that make it nearly impossible for lay developers to provide feedback about proposed new features. In general, this frustration was met with a general assumption that the current process works the way it does for good reason, and that academic descriptions of the new features was the correct (and only) way to properly discuss them.
I have nothing against new features being described in the language of implementors, but I would like to propose some additions to the current process that would make it significantly easier for language consumers (especially framework and library implementors) to provide timely feedback about the proposal in the hope of making an impact before it’s too late.
I would like proposals for new features to have the following elements, in addition to whatever elements are already normally included (such as BNF for any new syntax).
What Problem Are We Solving?
Provide Non-Trivial Example Code Showing the Problem
If the proposal is solving a problem that exists in the wild, it should be possible to identify non-trivial examples of the problem rearing its head. At the very least, the process of identifying or synthesizing these examples will help language users understand what real-world problems the proposal is attempting to solve. At best, finding real-world examples will help refine the proposal at an early stage.
Show How the Example Code is Improved With the Proposal
After identifying or synthesizing example code to illustrate the problem, show how the problem would be improved if the proposal was accepted. In some cases, the problem is as simple as “needing a large library to perform this operation” and the solution is “building common functionality into the language”. In an example from a related field, the DOM library, the problem addressed by
querySelectorAll was “many incompatible implementations of a CSS3 selector library”. The solution was to build the functionality into the browser.
In this case, a mistake in the
querySelectorAll specification, which was resolved by the addition of
queryScopedSelectorAll in the Selectors API Level 2 could have been addressed ahead of time by evaluating real-life code using selector engine libraries. Of course, the DOM library is not the same as the language specification, so the example is merely an analogy.
What are Alternate ES3 (or ES5) Solutions to the Same Problem?
This will allow language users to provide feedback about how the solution on offer stacks up compared to existing pure-JS solutions.
Are there any restrictions that do not exist in original pure-JS solutions?
Are there any restrictions in the proposal that limit its utility as a solution to the problem in question, especially if those restrictions do not apply to solutions currently used by language users.
If the Proposal is For New Syntax, What if Anything Does it Desugar To?
Also, if the proposal desugars, why choose this particular desugaring as opposed to alternatives?
If New Syntax, Can it Be Compiled to ES3 (or ES5)?
If the proposal can desugar to an older version of the specification, can a source-to-source translator be written? If so, is there a reference implementation of a source-to-source translator written in that version?
By writing such a source-to-source translator, existing language users can experiment with the new syntax easily in a browser environment without requiring a separate compilation pass. This also allows users to build an in-browser translation UI (similar to try CoffeeScript), which can improve general understanding of the new syntax and produce important feedback.
To be more specific, what I would like to see here is a general-purpose source-to-source translation engine written in ES3 with a mechanism for plugging in translation passes for specific features. If new features come with translation passes, it would be trivial for language users to try out new features incrementally in production applications (with a nice development-time workflow). This would provide usability feedback at an early enough stage for it to be useful.
If a New Standard Library, Can it Be Polyfilled to ES3 (or ES5)?
If the proposal is for a new library whose syntax is valid in an earlier version of the specification, can it be implemented in terms of the existing primitives available in that version. If necessary, primitives not defined by the language, but provided historically by web browsers, can be used instead. The goal is to provide shims for older browsers so that a much broader group of people can experiment with the APIs and provide feedback.
In general, new libraries that parse in older versions of the specifications should also come with a well-defined mechanism to detect whether the feature is present, to make it easy for library and framework vendors, as well as the general public, to opt their users into the new features where appropriate.
Even if a fully backwards-compatible shim cannot be provided, it is still useful to provide a partial implementation together with a feature detection mechanism. At the very least, error-checking shims can be useful, so language users can easily understand the interface to the proposed library.
Does the Proposal Change any Existing Semantics?
In some cases, the proposal unavoidably changes existing semantics. For example, ES5 changed the semantics of an indirect call to
eval (a call to an alias to eval, such as
x = eval; x('some code') to use the global environment for the evaluated code. In ES3, indirect calls to eval behaved the same as direct calls to eval.
These cases are rare, and in most cases, require an explicit opt-in (such as the directive
When such changes are made, especially when they do not require an opt-in, they should be explicitly called out in the proposal to gather feedback about their likely impact on existing code. Even when they require an opt-in, information about the frequency of their use could be useful to assess the difficulty of opting in.
Since these opt-ins often enable new features as well as changing existing semantics, understanding the impact of the opt-in on existing code would help language users assess their overall utility and timeframe for adoption. This information could also help drive these decisions.