More than a single candidate module for core-js/object/assign

webpack-isomorphic-tools/plugin] [error]  More than a single candidate module was found in webpack stats for require()d path "[object Object]"
[webpack-isomorphic-tools/plugin]   {
  "id": 304,
  "identifier": "/__redacted__/node_modules/babel-runtime/core-js/object/assign.js",
  "name": "./~/babel-runtime/core-js/object/assign.js",
  "index": 525,
  "index2": 525,
  "size": 94,
  "cacheable": true,
  "built": true,
  "optional": false,
  "prefetched": false,
  "chunks": [
    0
  ],
  "assets": [],
  "issuer": "/__redacted__/node_modules/babel-runtime/helpers/extends.js",
  "failed": false,
  "errors": 0,
  "warnings": 0,
  "reasons": [
    {
      "moduleId": 3,
      "moduleIdentifier": "/__redacted__/node_modules/babel-runtime/helpers/extends.js",
      "module": "./~/babel-runtime/helpers/extends.js",
      "moduleName": "./~/babel-runtime/helpers/extends.js",
      "type": "cjs require",
      "userRequest": "babel-runtime/core-js/object/assign",
      "loc": "3:21-67"
    },
    {
      "moduleId": 29,
      "moduleIdentifier": "/__redacted__/node_modules/react-bootstrap/lib/styleMaps.js",
      "module": "./~/react-bootstrap/lib/styleMaps.js",
      "moduleName": "./~/react-bootstrap/lib/styleMaps.js",
      "type": "cjs require",
      "userRequest": "babel-runtime/core-js/object/assign",
      "loc": "3:21-67"
    }
  ],
  "source": "module.exports = { \"default\": require(\"core-js/library/fn/object/assign\"), __esModule: true };"
}
[webpack-isomorphic-tools/plugin]   {
  "id": 838,
  "identifier": "/__redacted__/node_modules/isomorphic-style-loader/node_modules/babel-runtime/core-js/object/assign.js",
  "name": "./~/isomorphic-style-loader/~/babel-runtime/core-js/object/assign.js",
  "index": 734,
  "index2": 727,
  "size": 94,
  "cacheable": true,
  "built": true,
  "optional": false,
  "prefetched": false,
  "chunks": [
    0
  ],
  "assets": [],
  "issuer": "/__redacted__/node_modules/isomorphic-style-loader/lib/insertCss.js",
  "failed": false,
  "errors": 0,
  "warnings": 0,
  "reasons": [
    {
      "moduleId": 10,
      "moduleIdentifier": "/__redacted__/node_modules/isomorphic-style-loader/lib/insertCss.js",
      "module": "./~/isomorphic-style-loader/lib/insertCss.js",
      "moduleName": "./~/isomorphic-style-loader/lib/insertCss.js",
      "type": "cjs require",
      "userRequest": "babel-runtime/core-js/object/assign",
      "loc": "3:14-60"
    }
  ],
  "source": "module.exports = { \"default\": require(\"core-js/library/fn/object/assign\"), __esModule: true };"
}
/__redacted__/node_modules/webpack-isomorphic-tools/babel-transpiled-modules/plugin/write assets.js:369
            throw new Error('More than a single candidate module was found in webpack stats');
            ^

Error: More than a single candidate module was found in webpack stats
    at /__redacted__/node_modules/webpack-isomorphic-tools/babel-transpiled-modules/plugin/write assets.js:369:10
    at resolver (/__redacted__/node_modules/webpack-isomorphic-tools/node_modules/require-hacker/babel-transpiled-modules/require hacker.js:131:17)
    at Function._module3.default._findPath (/__redacted__/node_modules/webpack-isomorphic-tools/node_modules/require-hacker/babel-transpiled-modules/require hacker.js:436:24)
    at Function.Module._resolveFilename (module.js:335:25)
    at Function._module3.default._resolveFilename (/__redacted__/node_modules/webpack-isomorphic-tools/node_modules/require-hacker/babel-transpiled-modules/require hacker.js:403:34)
    at Function.Module._load (module.js:287:25)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at Object.<anonymous> (!./../../node_modules/isomorphic-style-loader/lib/insertCss.js.webpack-module:4:15)
    at Module._compile (module.js:435:26)
    at Object._module3.default._extensions.(anonymous function) [as .webpack-module] (/__redacted__/node_modules/webpack-isomorphic-tools/node_modules/require-hacker/babel-transpiled-modules/require hacker.js:264:11)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at Object.<anonymous> (/__redacted__/src/sass/LoginSignupForm.scss.webpack-module:4:21)
    at Module._compile (module.js:435:26)
    at Object._module3.default._extensions.(anonymous function) [as .webpack-module] (/__redacted__/node_modules/webpack-isomorphic-tools/node_modules/require-hacker/babel-transpiled-modules/require hacker.js:264:11)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:311:12)
    at Module.require (module.js:366:17)
    at require (module.js:385:17)
    at populate_assets (/__redacted__/node_modules/webpack-isomorphic-tools/babel-transpiled-modules/plugin/write assets.js:395:31)
    at Object.write_assets [as default] (/__redacted__/node_modules/webpack-isomorphic-tools/babel-transpiled-modules/plugin/write assets.js:63:2)
    at Compiler.<anonymous> (/__redacted__/node_modules/webpack-isomorphic-tools/babel-transpiled-modules/plugin/plugin.js:175:27)
    at Compiler.applyPlugins (/__redacted__/node_modules/webpack/node_modules/tapable/lib/Tapable.js:26:37)
    at Compiler.<anonymous> (/__redacted__/node_modules/webpack/lib/Compiler.js:193:12)
    at Compiler.emitRecords (/__redacted__/node_modules/webpack/lib/Compiler.js:282:37)
    at Compiler.<anonymous> (/__redacted__/node_modules/webpack/lib/Compiler.js:187:11)
    at /__redacted__/node_modules/webpack/lib/Compiler.js:275:11

[sompylasar]

I tried to use target: 'node' and failed miserably, so I had to revert to webpack-isomorphic-tools.

Below are my results after having resolved the issue and successfully integrated with https://github.com/kriasoft/isomorphic-style-loader

@halt-hammerzeit you might be interested in adding a manual for isomorphic-style-loader next to the one for style-loader.

// ---------
// The `withStyles` decorator is the main benefit of `isomorphic-style-loader`
// compared to `style-loader` -- it enables critical-path style extraction and
// partial loading of styles.
// We use `webpack-isomorphic-tools` that does not build using `webpack` on the
// server-side, so we can't use the `isomorphic-style-loader` on the server-side
// as it is designed.
// We could just use the `withStyles` architecture plus `style-loader` for style insertion,
// but `style-loader` does not expose a `removeCss` function that would rollback the `insertCss`,
// and that function is required to make `withStyles` complete -- thus, we don't.
// -- sompylasar
//

// webpack-isomorphic-tools config, the part responsible for styles (server-side)

    style_modules: {
      // ---------
      // The CSS Modules on the server-side should export objects with classnames and the CSS text.
      // The `extensions` defines the regular expression to match the module names (it is constructed under the hood).
      // The `filter` function finds the modules that are CSS Modules.
      // The `path` function converts module full names into asset paths.
      // The `parser` function transforms the module source code to export
      // objects that are expected from `require('*.scss')` on the server-side.
      // -- sompylasar
      //
      extensions: [
        'less',
        'scss',
      ],

      filter: function (module, regex, options, log) {
        // ---------
        // The `isomorphic-style-loader`, like `style-loader`, is a pitching loader, and during the `pitch`
        // it changes the module so that the remaining part of the module request starts with `css-loader` (the next one to the right).
        // @see https://github.com/kriasoft/isomorphic-style-loader/blob/5e4b0627fcde383a20b53f66d8f3c47d05062722/src/index.js#L14
        // @see https://github.com/kriasoft/isomorphic-style-loader/blob/5e4b0627fcde383a20b53f66d8f3c47d05062722/src/index.js#L21
        // @see https://webpack.github.io/docs/loaders.html#pitching-loader
        // @see https://github.com/webpack/webpack/issues/360#issuecomment-49028914
        // -- sompylasar
        //
        var ret = WebpackIsomorphicToolsPlugin.style_loader_filter(module, regex, options, log);
        if (ret) {
          log.debug('style_modules.filter ' + module.name + ' -> ' + ret);
        }
        return ret;
      },
      path: function (module, options, log) {
        // ---------
        // `isomorphic-style-loader` is similar to `style-loader`,
        // see detailed comment in `filter` above.
        // Here we strip the loaders to return the asset path.
        // The asset path value will become the key in the `webpack-assets.json`.
        // -- sompylasar
        //
        var ret = WebpackIsomorphicToolsPlugin.style_loader_path_extractor(module, options, log);
        log.debug('style_modules.name ' + module.name + ' -> ' + ret);
        return ret;
      },
      parser: function (module, options, log) {
        // ---------
        // `webpack-isomorphic-tools` does not use `webpack` loaders,
        // it uses its own way to convert assets to JS, called parsers.
        // The parser results are collected and cached in the `webpack-assets.json` file.
        // Usually, we would imitate behavior of loaders for known asset types
        // to provide proper `exports` objects on the server-side.
        // But `isomorphic-style-loader`'s `exports` object exports
        // functions `_getCss` and `_insertCss` which for some reason cannot be
        // serialized into `webpack-assets.json` although in `require-hacker`
        // which is responsible for serialization the CommonJS modules are handled specifically.
        // @see https://github.com/halt-hammerzeit/require-hacker/blob/ccd584d68fb8fee8a117b7ddfe0b2bcaeed309e8/source/require%20hacker.js#L280-L297
        // @see https://github.com/halt-hammerzeit/require-hacker/blob/ccd584d68fb8fee8a117b7ddfe0b2bcaeed309e8/source/require%20hacker.js#L451-L456
        // So we reuse the CSS Modules converter from `webpack-isomorphic-tools`
        // that builds a module that exports the CSS Modules classnames
        // and an additional `_style` string property with raw CSS text.
        // This structure gets serialized properly.
        // @see https://github.com/halt-hammerzeit/webpack-isomorphic-tools/blob/5e468729fe0a59eaed4f8a0f9da5ba92a4a7e3c5/source/plugin/plugin.js#L194-L199
        // -- sompylasar
        //
        var ret = WebpackIsomorphicToolsPlugin.css_modules_loader_parser(module, options, log);
        log.debug('style_modules.parser \n' + module.source + '\n\n -----------> \n\n' + ret + '\n');
        return ret;
      }
    }

// webpack config, the part responsible for style loaders (browser-side)

  const cssLoaderQuery = {
    modules: true,
    importLoaders: 2,  // `postcss-loader` + ( `sass-loader` or `less-loader` ).
    sourceMap: SOURCE_MAP_ENABLED,
    localIdentName: (__DEVELOPMENT__ ? '[local]___[hash:base64:5]' : undefined),
  };
  const lessSassLoaderQuery = {
    outputStyle: 'expanded',
    sourceMap: SOURCE_MAP_ENABLED,
  };
  const styleLoaders = [
    'isomorphic-style-loader',
    'css-loader?' + JSON.stringify(cssLoaderQuery),
    'postcss-loader',
  ];
  const styleLoadersWithTest = [
    {
      test: /\.less$/,
      loaders: [
        ...styleLoaders,
        'less-loader?' + JSON.stringify(lessSassLoaderQuery),
      ],
    },
    {
      test: /\.scss$/,
      loaders: [
        ...styleLoaders,
        'sass-loader?' + JSON.stringify(lessSassLoaderQuery),
      ],
    },
  ];

// collect styles on the rendering critical path (server-side)

        // CSS for all rendered React components.
        const cssTexts = [];
        function insertCss(styles) {
          // ---------
          // Although `styles` is expected to have `_getCss()` function on the server-side,
          // it won't, because on the server-side we use `webpack-isomorphic-tools`
          // to convert the modules to JS, not `webpack` loaders.
          // It has `_style` string which is generated by the
          // `WebpackIsomorphicToolsPlugin.css_modules_loader_parser`.
          // See `webpackIsomorphicToolsConfig` for more details.
          // -- sompylasar
          //
          cssTexts.push( styles._style );
        }

// collect styles on the rendering critical path (browser-side)

function insertCss(styles) {
  // ---------
  // On the browser-side we use `webpack` loaders to convert the modules to JS,
  // so here we have `_insertCss()` function generated by `isomorphic-style-loader`.
  // -- sompylasar
  //
  styles._insertCss();
}

/**
 * Provides `insertCss` function via React context to the `withStyles`-decorated components.
 *
 * Usage: <InsertCssProvider insertCss={insertCss}>{contentDOM}</InsertCssProvider>
 */
export default class InsertCssProvider extends Component {
  static propTypes = {
    insertCss: PropTypes.func.isRequired,
    children: PropTypes.node,
  }

  static childContextTypes = {
    insertCss: PropTypes.func,
  }

  getChildContext() {
    return {
      insertCss: (styles) => {
        const { insertCss } = this.props;
        // We may get an array of styles here if `withStyles` is used more than once.
        // TODO(sompylasar): Get to know when exactly this array happens.
        if (Array.isArray(styles)) {
          styles.forEach(insertCss);
        }
        else {
          insertCss(styles);
        }
      },
    };
  }

  render() {
    const { children } = this.props;
    return React.Children.only(children);
  }
}

[sergeylukin]

Hi,

I think I was getting that error (More than a single candidate module was found) too because I had 2 components with same structure:

componentA/
componentA/index.js
componentA/styles.css

and

componentB/
componentB/index.js
componentB/styles.css

where both components required styles.css relatively (i.e. import styles from './style.css').

I think I fixed that by renaming each styles.css to their corresponding component name respectively. My setup was pretty full of stuff so I will test that again with minimal setup and will let you know once I have more info.