how about a v8 engine option?

@jonathonong: Yeah, I discussed that in paragraph 4, but it does have user experience issues. Do you have thoughts on those? That also won't provide a solution to other kinds of iojs incompatibility that may not be coming this month, but could happen.

I think that there is potential for io.js to introduce new apis not directly tied to V8 that may also lead to this situation. Ultimately, all new features introduces by io.js not available under node would likely require a feature-xxx declaration on the package. Correct me if I'm wrong, but I think generally npm ecosystems could help solve this problem in a simple way (e.g. limit search to "packages that leverage ES7 await"). In the end, it would be first, up to developer to decide whether to give that package a try and, secondly, to feature detect and polyfill when necessary. The same is true for node right now with packages like native-or-promise or async-listener for example.

There is some overlapse with the engines definition, but with time, versions will have less meaning, just like user agents, and become a bigger burden to developers to maintain.

I'm -1 on using engines for this.

Best practice is for module authors to feature detect when possible, this would encourage them not to do that.

There actually should be a "cost" to using new features prior to them being widely adopted but I wouldn't rule out the possibility of people creating tooling for older versions of node that check for modules using new features and compile them down to ES5.

Agreed. Several packages targeted at node 0.11 (especially for generators stuff) are recommending users to rely on transpilers such as regenerator for node 0.10 compatibility. And then there's always Traceur and 6to5 runtimes for those who want to ES6 features in node and do not want make the switch to io.js just yet.

v8 engine option makes the most sense imho.

I agree that testing v8 versions doesn't really give you any more information since there now will be combinations of what flags io.js (or node.js) is started with.

@mikeal how about exposing those features through package.json somehow? That at least give package to package feature dependencies over doing it within the modules (I can see scenarios where it wouldn't make sense to test for certain features since you'd probably wouldn't even install it in the first place without the feature in place). These entries could probably even be generated.

It is possible to automatically find out version of V8 (and any other parameters that affect compatibility) from specified engine and version, and use it, when later there will be another version compatible with specified.

For example, if specified { "engines": { "iojs": ">=1.0.0" } } and node 0.15+ will be compatible with iojs 1.0, when you will try to install on old node version, error message will say that you need iojs >= 1.0.0 OR node >= 0.15, which is compatible.

And when you will install this package on node >= 0.15 it will just warn, that current engine is not same, that was specified, but is compatible.

@sunflowerdeath npm could certainly keep a table of iojs and node versions and their associated v8 versions, and so translate iojs >= 1.0.0 into v8 >= x.x.x. I don't think we currently can see other parameters that effect compatibility though. But yeah, io.js could expose those, but for that to work node.js would also have to be convinced to expose them.

I'd love to see both - a consistent "engines" notation, which indicates what it's been /tested/ on, and a parallel "features" notation (I do not care about the name, bikeshed all you want), which indicates which JS features it depends on. Then it's left up to the npm cli, or the transpiler, or the saas platform, or whatever (or some npm module) to determine dynamically what is required to make the module work, or whether it's possible at all. Thoughts?

Currently, all my packages that use generators have a foo.es6.js with the actual source, a foo.es5.js with transpiled code that is generated by the prepublish script, and a foo.js that feature-detects and requires the appropriate file.
No need for magic in package.json, and works across all engines. It might be nice to have a more convenient wrapper around this kind of feature detection / transpilation.

I'm not thinking "magic" as much as "documentation" - your approach is great, but how am I to know that's the format you've used? As described, I'd have to duck type it, whereas with a package.json notation, you'd explicitly call out what you supported.

adding more metadata to package.json about language features used, or v8 versions tested against/supported:

1. complicates package.json
2. easily causes package.json to become out of date
3. easily causes package.json to contain incorrect information

These are all fine, so long as you are vigilant about updating your package.json, but if we're honest, most people aren't.

automating detection of features used:

1. makes assumptions about how you're going to use the module (transpiling? different version of node/iojs? loading a particular build shipped with the module?)
2. causes headaches when using or developing modules

This seems like a really, really bad thing to bake into npm, and probably not a very good thing to do in an individual project either (although it's obviously up to you to make the call on how you ship your project)

Currently, all my packages that use generators have a foo.es6.js with the actual source, a foo.es5.js with transpiled code that is generated by the prepublish script, and a foo.js that feature-detects and requires the appropriate file.

Yes, oh 1000 times yes. That being said it wouldn't be bad practice (although not required) for something in package.json for batch scripts doing analytics. Not everything can be polyfilled via a transpiler. For those things I wouldn't want npm to warn me, but it's also a like riding a Ferrari to the grocery store to need static analysis to figure this out from a machine perspective.

👍 for using package.json main point to compiled ES5 source. 🍻 🐴

...all my packages that use generators have a foo.es6.js with the actual source, a foo.es5.js with transpiled code ... prepublish script ...

I'd prefer if index.js was the canonical, as-written es6 code, while es5/index.js gets built by a prepub script. Module builders who are still in es5-land can require('foo/es5') while everyone else can just require('foo').

How about something similar to an npm preinstall script? That script will be written in ES5 and does ES6/V8 feature detection. If any error is thrown, then npm install fails and notify the user to upgrade?

That way feature detection can be as complicated as anyone likes, and npm packages can make sure certain features exist before being installed.

I would prefer having the extra metadata in package.json for non-standard features. At least as a console warning so the consumer knows to fetch the appropriate polyfill or upgrade their platform version.

@greim Anything where the main require isn't commonJS is just not going to get adopted by the community, imo. The vast majority of module builders are going to be in ES5-land for a long time.

@ljharb Is require('foo/es5') not CJS? It doesn't seem too cumbersome to me.

(BTW I'm not suggesting npm should auto-transpile things to es5 for everyone; only suggesting a convention module authors could follow.)

@greim It is, but the main require - the default if i require the module by name with no slashes.

@ljharb please don't do preinstall scripts, it's bad enough that we can't take them out of npm.
Transpilation should be done at prepublish, and feature tests should occur at runtime.
We can probably put together some npm packages to ease the pain of prepublish transpiling and feature-detecting.

I definitely don't advocate preinstall scripts. I fully agree with your comment.

Related: #253 (comment). It suggest that the solution is { "engines": { "node": ">=1.0.0" } }.

I've mentioned a like idea in the tail of #90, and opened a quickly-shut follow-up ticket specifically on this issue npm/npm#90.

@nathan7 @indexzero @mikeal what kills me about the "be a smart author" route- such as the .es5.js/.es6.js variant- is that it leaves great responsibility on each package, and from that a whole bunch of decisions get packages into each app that may or may not conflict with a consumer's desired use-case.

As someone bundling es6+es5 apps and transpiling them myself, minifying them, and otherwise preparing them for deployment, I really don't want that extra baggage to be dealt with at all by the library- I'd rather have a library that just targets ES6, know that it targets ES6, and work that into my toolchain, rather than have a library "help" me by using transpiler X to do the job.

The preferred default state for a package is as simple as possible. Anything any author does to make their package fit for consumption is going to make it harder for me the consumer to overwrite the baked in assumptions & impose the choices I'm making in my distribution. I don't want to deal with your source maps, I don't want to deal with your transpiling- I just want javascript, and my build-chain will handle these things as I feel appropraite.

I see the "put more complexity on the package" versus "simple packages" as a AMD 2.0 debate: I'd way rather make authorship dumb/simple, and leave questions of processing to as late as possible, at the responsibility of the consumer & their chosen build-chain. Otherwise too many assumptions get baked into each package.

@rektide we can bundle all of this complexity into a package of its own that you simply include. the AMD approach isn't the npm approach.

Example feature-detection package: has-generators

// foo.js
module.exports = require('has-generators') ? require('./foo.es6') : require('./foo.es5')

If your stuff requires multiple ES6 things, (require('has-foo') && require('has-bar')) ? … : ….
We can create these tiny little packages with ease for all the ES6 features that are available in io.js today.