Add preflight check to guard against wrong versions of webpack/eslint/jest higher up the tree

[gaearon]

It looks like this:

I thought about extracting it to gist but I kinda like that we can substitute the dep name right into the message. We might also want to do something similar for duplicate React.

There is an opt-out mechanism mentioned at the very end (cc @jlongster):

SKIP_PREFLIGHT_CHECK=true

in .env file.

"Fixes" #1795.

[Timer]

Assuming the package is found and the version matches, do we need to continue looking up the tree?
In this case, we could splice it out of the array.

Our dependencies should never be hoisted out of our project [except in the case of Yarn Workspaces] and this fails spectacularly currently.

---

I'm fine with this if we want to be cautious and support Yarn Workspaces in the future

[Timer]

It's a shame we have to look multiple levels, because if we only needed the first occurrence we could be sly and use require (simplifying this logic):

depsToCheck.map(name=>{try{const{ version }=require(`${name}/package.json`)// ...}catch(e){// ignored}})

[gaearon]

We want to support YW so I think we shouldn’t stop.

What fails?

[gaearon]

Also just one level doesn’t help. In my example we’ll still find “our own” webpack but the project above might have its copy.

[Timer]

What fails?

react-scripts when used with Yarn Workspaces, not this PR (what #3435 is attempting to solve).

[Timer]

Also just one level doesn’t help. In my example we’ll still find “our own” webpack but the project above might have its copy.

You're right. This LGTM.

[Timer]

Should we mention valid cases?

When not using YW or Lerna+Hoist, it's valid to nest like so:

/node_modules
/eslint@3.x
/client
/node_modules
/eslint@4.x
/react-scripts@2.x
/eslint@4.x (hoisted)

[gaearon]

Open to other wordings!