extract-loader and css-loader sourceMap is undefined

[DavidKk]

Issue

i use extract-loader and css-loader to generate source map, but the sourceURL always prepend aundefined

body{color:red}body a{color:#fff}body.open{background-color:#fef}

/*# sourceURL=undefined/src/index.scss */
...

Reason

I found that sourceRoot does not necessarily exist, sometimes it equals undefined.

I look for some codes, and found that https://github.com/peerigon/extract-loader/blob/master/src/extractLoader.js#L141, it uses extractExports function to transform, (Function toString()) sandbox.module.exports; And the css-loader (webpack-contrib/css-loader#674 (comment)) use cssWithMappingToString and the variables cssMapping.sourceRoot to concat string, but https://github.com/webpack-contrib/css-loader/blob/master/src/index.js#L76 has been remove the sourceRoot, so that extract-loader can not found sourceRoot anymore, and sourceRoot always to undefined

Suggest

I suggest extract-loader can compatibles it, when the sourceRoot is undefined

Environment

My package.jsonand webpack config below:

// package.json
{
    "extract-loader": "^3.1.0",
    "css-loader": "^3.4.0",
}

// webpack config
{
  loader: 'extract-loader'
},
{
  loader: 'css-loader',
  options: {
    sourceMap: true
  }
},
{
  loader: 'sass-loader',
  options: {
    sourceMap: true,
    sassOptions: {
      sourceMapRoot: '/'
    }
  }
}

[DavidKk]

Report wrong place, i have been forward to css-loader... webpack-contrib/css-loader#1034