Graceful syntax errors in js


There's currently no way of introducing anything new into the language that breaks syntax. I think that point has been made very clearly with harmony/es6. Can we introduce a way to make these transitions easier in the future?

This proposal is a cross-post copy from my email to the es-discuss mailing list. I wanted to post it here too because not everybody reads es-discuss and I think the proposal is also an interesting blog post even if the list disagrees completely shoots the proposal down. Of course, that could never happen.. *cough*

CSS has a very simple way of gracefully ignore rules it doesn't know. In CSS you can specify rules and features and whatever the engine doesn't understand, it will ignore gracefully and continue. While this obviously won't work out of the box for js. But I think we can work around it. In CSS this is scoped to individual rules. In js there's no notion of rules (and "lines" won't work either) but you could talk about functions or modules (or blocks?) in the same way. I think modules are a bit too big scoped for this but I'd like to discuss the generic idea first. We can always bikeshed over the syntax later. I'll work with function-like syntax for now.

Say a browser wanted to support a new keyword. Right now, that's impossible to even experiment with unless you write an entire engine in js and have a duplicate source code for fallback. You simply can't do something like

x ||= y;

without breaking the code permanently and without exception. We can only add stuff that fits perfectly within the existing grammar. There's also no way for a vendor to implement this as an experiment because it will still break in older versions of the browser or other vendors. So what if we could do something like this?

function foo(x,y){
"if es7";
return x ||= y;
} // the "/directive" serves as a delimiter because otherwise you'd never know where to stop parsing when you don't support it
if (!foo) {
var foo = function(){
x = x || y;
return x;

So now you have a way to experiment with the new extension and a simple way of extending the language permanently, while still having backwards compat (at least up to the point that we introduce this system), graceful failures and a proper way of figuring out a fallback. If a vendor did not support the feature asked for it could declare foo as null or something allowing easy support detection.

The declared function should work within the existing js language of course and should be callable from within js with the regular mechanics. This includes stuff like call, apply, and bind. Whatever the function itself does with the arguments or the context is up to the implementation. Whatever it returns should again be a proper js value. This allows you to easily fallback to an "es5" function because the fingerprint would be the same (and besides, I have no idea how else this could work otherwise anyways).

This also could allow introduction of vendor prefixes. However, for now I'd like to focus on the general principle though and leave the prefix discussion for later.

Some syntax possibilities:

function foo(){ "es7"; x ||= y; }
function foo(){ "es7"; x ||= y; "/es7"; }
condFunction foo(){ "es7"; x ||= y; }
condFunction "es7" foo(){ x ||= y; }
condFunction "es7" foo(){ x ||= y; } "es7";

Personally, I'd prefer a function-scoped capability over a module-scoped capability because I think I'd like to apply this to smaller snippets rather than an entire module. Or maybe both.

Open things for me:
- How to efficiently specify the (scope of) things you want to opt-in to?
- How to limit the problem with interoperability this would undoubtedly cause (even with prefixes).
- Proper keyword/syntax for this, would still like to keep this js-ish
- Is nesting desirable? Reasonably feasible? Especially wrt the delimiter
- This system would also allow for non-js extensions (like coffeescript or even ruby/etc), is that a problem?

If we think such a system is viable and we can find a simple way of introducing it to the language, we should do it sooner than later. The sooner it's in, the less "stuck" we will be in the future.