Плагин написан с нуля и протестирован

Author: shkarupa-alex

.eslintrc.json

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+{
+  "env": {
+    "es6": true,
+    "mocha": true,
+    "node": true
+  },
+  "extends": ["eslint:recommended", "google"],
+  "plugins": [
+    "standard",
+    "promise"
+  ]
+}

.gitignore

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+# Logs
+logs
+*.log
+npm-debug.log*
+
+# Runtime data
+pids
+*.pid
+*.seed
+
+# Directory for instrumented libs generated by jscoverage/JSCover
+lib-cov
+
+# Coverage directory used by tools like istanbul
+coverage
+
+# nyc test coverage
+.nyc_output
+
+# Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
+.grunt
+
+# node-waf configuration
+.lock-wscript
+
+# Compiled binary addons (http://nodejs.org/api/addons.html)
+build/Release
+
+# Dependency directories
+node_modules
+jspm_packages
+
+# Optional npm cache directory
+.npm
+
+# Optional REPL history
+.node_repl_history
+
+# IDE
+.idea
+
+# Test artefacts
+test/cases/**/produced*

.travis.yml

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+language: node_js
+node_js:
+  - "6"
+  - "7"

LICENSE

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+MIT License
+
+Copyright (c) 2017 INTERVOLGA.RU
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

README.md

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+# optimize-cssnano-plugin [![Build Status](https://travis-ci.org/intervolga/optimize-cssnano-plugin.svg?branch=master)](https://travis-ci.org/intervolga/optimize-cssnano-plugin)

index.js

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+const cssnano = require('cssnano');
+
+/**
+ * Optimize cssnano plugin
+ *
+ * @param {Object} options
+ */
+function OptimizeCssnanoPlugin(options) {
+  this.options = Object.assign({
+    sourceMap: false,
+    cssnanoOptions: {
+      preset: 'default',
+      map: false,
+    },
+  }, options);
+
+  if (this.options.sourceMap) {
+    this.options.cssnanoOptions.map = Object.assign(
+      {inline: false},
+      this.options.cssnanoOptions.map || {});
+  }
+}
+
+OptimizeCssnanoPlugin.prototype.apply = function(compiler) {
+  const self = this;
+
+  compiler.plugin('emit', function(compilation, callback) {
+    // Search for CSS assets
+    const assetsNames = Object.keys(compilation.assets)
+      .filter((assetName) => {
+        return /\.css$/i.test(assetName);
+      });
+
+    let hasErrors = false;
+    const promises = [];
+
+    // Generate promises for each minification
+    assetsNames.forEach((assetName) => {
+      // Original CSS
+      const asset = compilation.assets[assetName];
+      const originalCss = asset.source();
+      // .replace(/\/\*# sourceMappingURL.+\*\//, '');
+
+      // Options for particalar cssnano call
+      const options = JSON.parse(JSON.stringify(self.options.cssnanoOptions));
+      options.to = assetName;
+
+      // Extract or remove previous map
+      const mapName = assetName + '.map';
+      if (options.map) {
+        // Use previous map if exist...
+        if (compilation.assets[mapName]) {
+          const mapObject = JSON.parse(compilation.assets[mapName].source());
+
+          // ... and not empty
+          if (mapObject.sources.length > 0 || mapObject.mappings.length > 0) {
+            options.map.prev = compilation.assets[mapName].source();
+          }
+        }
+      } else {
+        delete compilation.assets[mapName];
+      }
+
+      // Run minification
+      const promise = cssnano.process(originalCss, options)
+        .then((result) => {
+            if (hasErrors) {
+              return;
+            }
+
+            // Extract CSS back to assets
+            const processedCss = result.css;
+            compilation.assets[assetName] = {
+              source: function() {
+                return processedCss;
+              },
+              size: function() {
+                return processedCss.length;
+              },
+            };
+
+            // Extract map back to assets
+            if (result.map) {
+              const processedMap = result.map.toString();
+
+              compilation.assets[mapName] = {
+                source: function() {
+                  return processedMap;
+                },
+                size: function() {
+                  return processedMap.length;
+                },
+              };
+              // processedCss += `\n/*# sourceMappingURL=${assetName + '.map'} */`;
+            }
+          }
+        ).catch(function(err) {
+            hasErrors = true;
+            throw new Error('CSS minification error: ' + err.message +
+              '. File: ' + assetName);
+          }
+        );
+      promises.push(promise);
+    });
+
+    Promise.all(promises)
+      .then(function() {
+        callback();
+      })
+      .catch(callback);
+  });
+};
+
+module.exports = OptimizeCssnanoPlugin;