TypeError: Class extends value undefined is not a constructor or null at Object.<anonymous> - PostCSS < v8 + Stylelint v14 compat issue about stylelint-config-sass-guidelines HOT 2 CLOSED

There are two options that come to mind to mitigate this, set up a peerDependecy on postcss in this npm package similar to the same version that postcss-scss uses.

Alternatively, I can set up postcss as a dependency in this config and set at the same version that postcss-scss lists as a peerDependency.

Typically I lean towards setting it up as a peerDependency similar to how stylelint is set up in this package currently:
https://github.com/bjankord/stylelint-config-sass-guidelines/blob/main/package.json#L37-L38

On the flip side, I know people often ignore peerDependency warnings, if a package has a peerDependency and its not in your dependency tree, (node_modules), yarn and npm (v4-v6) will warn about it I believe and I think in npm 7 will auto install peer dependencies. Setting postcss as a dependency in this project should help people running into this error.

For now, I'm planning to set postcss as a peerDependency in this config to the same version postcss-scss sets the peerDependency on postcss and pass the responsibility to projects consuming this config to install the needed version of postcss.

If more people run into this issue, I'll likely move to switch postcss from a peerDependency in this package to a dependency as I know people often overlook the peerDependency warnings and will likely be confused by this error message:

TypeError: Class extends value undefined is not a constructor or null at Object.<anonymous>

from stylelint-config-sass-guidelines.

Found some related issues:

• stylelint-scss/stylelint-config-standard-scss#2 (comment)
• stylelint/stylelint#5632 (comment)

It seems if you can not upgrade the root version of postcss to v8 in your projects, you will not be able to use stylelint scss based configs.

from stylelint-config-sass-guidelines.