"Unable to fetch the package" error thrown when used:

node -v: v14.14.0
npm -v: 7.6.3

Hello

My initial attempt was to try npx is-esm yargs. It was my first reflex because I knew that this library had made the transition to ESM as detailed in their blog post from 2020: https://yargs.js.org/2020/09/07/yargs-16.html

Unfortunately it failed with a Unable to detect the module format. As the caught error is silenced, I don't know yet what's happening.

By inspecting the package.json of yargs, we can get a few clues that ESM is supported because there's an exports object with definition pointing to .mjs file. https://github.com/yargs/yargs/blob/main/package.json

So maybe is-esm could fallback on this kind of heuristics in case the rollup method does not work as intended?

Thanks!

It looks like at the moment, npm v7 returns "No" in any case.

This is an test run, first on node 12 / npm 6; then node 12 / npm 7, and then back on node 12 / npm 6:

➜  nvm use 12.20.0
Now using node v12.20.0 (npm v6.14.13)
➜  npx is-esm @angular/core
npx: installed 58 in 2.737s
✔ Yes


➜  npm install -g npm@7
…
+ [email protected]
added 62 packages from 25 contributors, removed 242 packages and updated 193 packages in 4.412s
➜  nvm use 12.20.0
Now using node v12.20.0 (npm v7.18.1)
➜  npx is-esm @angular/core
⚠ No


➜  npm install -g npm@6

removed 62 packages, changed 99 packages, and audited 436 packages in 2s

3 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities
➜  nvm use 12
Now using node v12.20.0 (npm v6.14.13)
➜  npx is-esm @angular/core
npx: installed 58 in 2.618s
✔ Yes

I tried to debug it, but so far found that running installCommand doesn't not create node_modules in the temporary directory.

With npm@6 the stdout of installModules() is:

added 2 packages from 2 contributors and audited 2 packages in 0.866s
found 0 vulnerabilities

But npm@7 it is:

up to date, audited 7 packages in 925ms

found 0 vulnerabilities

I think it's related to the --prefix, as running the same command in the temp dir without it works fine.