fix: default context should be undefined instead of null

[manuelbieh]

loaderUtils.interpolateName breaks if it passes null to path.relative

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

When css-loader 3.0.0 is used with react-dev-utils/getCSSModuleLocalIdent the build fails because getLocalIdent passes context to loaderUtils.interpolateName. interpolateName then uses this context to get a relative path using path.relative(). While undefined (the value in ^2.1.1) is ... undefined and thus simply ignored by path.relative(), null is an object and causes the error below (shortened):

TypeError [ERR_INVALID_ARG_TYPE]: The "from" argument must be of type string. Received type object
    at C:\htdocs\test\css-loader-issue\test.module.css:1:1
    at validateString (internal/validators.js:107:11)
    at Object.relative (path.js:434:5)
    at Object.interpolateName (C:\htdocs\test\css-loader-issue\node_modules\loader-utils\lib\interpolateName.js:71:10)
    at Object.getLocalIdent (C:\htdocs\test\css-loader-issue\webpack.config.js:22:37)
    at generateScopedName (C:\htdocs\_git\css-loader\dist\utils.js:142:39)
    [...]

This PR removes the context default option completely, implicitly setting its value back to undefined.

Breaking Changes

Existing tests are still passing so I assume there should be no breaking changes. I didn't add any additional tests though.

Additional Info

You should be able to reproduce this behavior by cloning my demo repo and run npm run build.

[alexander-akait]

strange, we use options.context || '' so it should be empty lint and no problems

[manuelbieh]

Yes, you do, but only in your internal getLocalIdent() function in utils.js. If a custom getLocalIdent function is passed as option:

modules: { 
  getLocalIdent: (loaderContext, localIdentName, localName, options) => {
    // this should be undefined:
    options.context === null
  }
}

… it's null instead of undefined. So if third-party getLocalIdent functions want to pass context to path.relative() they must take care of checking it for null || '' themselves.

[manuelbieh]

… you can check it by adding a console.log(options) to node_modules/loader-utils/lib/interpolateName.js:39. It is:

{ context: null, hashPrefix: '', regExp: null }

in 3.0.0 and

{ regExp: undefined, hashPrefix: '', context: undefined }

in 2.1.1.

context being null instead of undefined causes the interpolateName function to fail because of this line (70):

      directory = path
        .relative(context, resourcePath + '_')

[alexander-akait]

hm, looks regression, let's add test for this case

[manuelbieh]

Just added a test. Please have a look!

[alexander-akait]

Good job, thanks!