Assessing stability of new JavaScript features

Gregory Szorc gps at
Thu Jun 20 20:26:22 UTC 2013

The JavaScript language and SpiderMonkey are rapidly evolving. As new 
features are added to the language and SM, I keep finding myself asking 
"is this feature stable enough to be used in production code 
(Firefox/Toolkit/etc)?" (To be clear, I'm talking about 
Firefox/chrome-privileged JS - not JS on the web.)

AFAIK, the best way to currently answer that question is 1) ask a 
SpiderMonkey dev 2) look through the code and see what others are doing. 
You could arguably add 3) look at docs on MDN. But the "experimental" 
warning is on features that have been used in Firefox for over a year or 
skew towards suitability for the feature on the web, so I don't trust it.

These options aren't satisfactory. #1 is dependent on asking the right 
person and hoping there isn't bias towards "using." #2 relies on #1 
first being followed and second being accurate and thus is not robust.

Since using "alpha" JS features can undermine the quality and stability 
of Firefox, can increase development time (refactoring code as the 
feature implementation changes from under you), and since people are 
generally in the dark over the "stability" of new JS features, I think 
something needs to be done.

What I'm personally looking for is an authoritative document that says 
things like:

* JavaScript feature X has been blessed for stable [internal Firefox 
feature] usage as of Firefox N.
* JavaScript feature Y is available in Firefox X but only for 
experimental, non-critical usage - it's API will likely change.
* JavaScript feature Z should not be used for new development after 
Firefox M: use X instead.

I want this both as someone who writes a lot of JavaScript (I want to 
make my life easier by using new, more powerful language constructs) and 
as a reviewer (I don't want to allow unstable features to leak into the 

Where this would live, I don't know. It sounds a lot like code style 
guidelines to me. It would need periodic input from the JS team to vet 
the stability assurances. And, maybe we want module owners to perform a 
secondary sign-off on new feature usage.

For reference, some of the new JavaScript features that still have a 
cloud over (in my mind at least) are generators, fat arrow functions, 
set, map, ParallelArray, Proxy, ... From my casual following of JS 
language development, this list will only grow longer over time, so this 
problem is only going to get worse.

If others feel the same, what can we do about this?


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