Support for npm@4's new prepublish behavior

[Jessidhia]

npm@4 was released a few minutes ago, and it deprecates the prepublish verb, renaming it to prepare, and adding a prepublishOnly (which runs only on publish).

In yarn's case, the only changes should be also running prepare wherever prepublish ran and prepublishOnly on publish (if yarn can publish?). prepublish should still be run, but probably should get a deprecation warning.

npm@5 is expected to stop running prepublish entirely on npm install and npm pack (i.e. run only on npm publish), and then an npm@>=6 should deprecate prepublishOnly by renaming it to prepublish.

[samhh]

I'm happy to take this one.

What's Yarn's approach to compatibility with older versions of npm (or is that a non-issue regards how Yarn works?)? I ask because if we change the publish script to prepublishOnly then you lose support for npm <4. If we keep prepublish, then there's no point in adding prepublishOnly as it will be renamed back to prepublish eventually as you said, and presumably in the meantime for npm 4+5 prepublish as an alias for prepare will still do whatever prepublishOnly does.

So for the time being all I've done is add prepare after prepublish in the install phase, the only other place where prepublish appears. And I think that's all that needed doing?

Do correct me if I'm wrong, first contribution to Yarn. :-)

[timoxley]

@samhh you may want to investigate #1499

[Jessidhia]

Oh, yeah, feel free to take over that PR/branch. I've been kinda out of time to update it 😢

[timoxley]

@Kovensky @samhh appears they're asking for an RFC https://github.com/yarnpkg/rfcs

[samhh]

Just started a new job, not convinced I'll have the time to do this anymore. If anyone else does have the time do grab it :-)

[bestander]

Done in #3004