fix: don't crash when no extracted comments (#387)

src/index.js

@@ -306,7 +306,11 @@ class UglifyJsPlugin {
           }
 
           // Write extracted comments to commentsFile
-          if (commentsFile && extractedComments.length > 0) {
+          if (
+            commentsFile &&
+            extractedComments &&
+            extractedComments.length > 0
+          ) {
             if (commentsFile in compilation.assets) {
               const commentsFileSource = compilation.assets[
                 commentsFile

test/__snapshots__/minify-option.test.js.snap

@@ -183,6 +183,12 @@ exports[`when applied with \`minify\` option matches snapshot for \`terser\` min
 
 exports[`when applied with \`minify\` option matches snapshot for \`terser\` minifier: warnings 1`] = `Array []`;
 
+exports[`when applied with \`minify\` option matches snapshot for \`uglify-js\` minifier while extracting comments: errors 1`] = `Array []`;
+
+exports[`when applied with \`minify\` option matches snapshot for \`uglify-js\` minifier while extracting comments: main.js 1`] = `"!function(e){var t={};function n(r){if(t[r])return t[r].exports;var o=t[r]={i:r,l:!1,exports:{}};return e[r].call(o.exports,o,o.exports,n),o.l=!0,o.exports}n.m=e,n.c=t,n.d=function(e,t,r){n.o(e,t)||Object.defineProperty(e,t,{enumerable:!0,get:r})},n.r=function(e){\\"undefined\\"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:\\"Module\\"}),Object.defineProperty(e,\\"__esModule\\",{value:!0})},n.t=function(e,t){if(1&t&&(e=n(e)),8&t)return e;if(4&t&&\\"object\\"==typeof e&&e&&e.__esModule)return e;var r=Object.create(null);if(n.r(r),Object.defineProperty(r,\\"default\\",{enumerable:!0,value:e}),2&t&&\\"string\\"!=typeof e)for(var o in e)n.d(r,o,function(t){return e[t]}.bind(null,o));return r},n.n=function(e){var t=e&&e.__esModule?function(){return e.default}:function(){return e};return n.d(t,\\"a\\",t),t},n.o=function(e,t){return Object.prototype.hasOwnProperty.call(e,t)},n.p=\\"\\",n(n.s=0)}([function(e,t){e.exports=function(){var baz=document.getElementById(\\"root\\").innerHTML;document.getElementById(\\"demo\\").innerHTML=\\"Paragraph changed.\\"+baz}}]);"`;
+
+exports[`when applied with \`minify\` option matches snapshot for \`uglify-js\` minifier while extracting comments: warnings 1`] = `Array []`;
+
 exports[`when applied with \`minify\` option matches snapshot for errors into \`minify\` option and \`parallel\` is \`true\`: errors 1`] = `
 Array [
   "Error: main.js from UglifyJs

test/minify-option.test.js

@@ -212,4 +212,43 @@ describe('when applied with `minify` option', () => {
       }
     });
   });
+
+  it('matches snapshot for `uglify-js` minifier while extracting comments', () => {
+    const compiler = createCompiler({
+      entry: `${__dirname}/fixtures/minify/es5.js`,
+      output: {
+        path: `${__dirname}/dist-uglify-js`,
+        filename: '[name].js',
+        chunkFilename: '[id].[name].js',
+      },
+    });
+
+    new UglifyJsPlugin({
+      extractComments: true,
+      minify(file) {
+        // eslint-disable-next-line global-require
+        return require('terser').minify(file, {
+          mangle: {
+            reserved: ['baz'],
+          },
+        });
+      },
+    }).apply(compiler);
+
+    return compile(compiler).then((stats) => {
+      const errors = stats.compilation.errors.map(cleanErrorStack);
+      const warnings = stats.compilation.warnings.map(cleanErrorStack);
+
+      expect(errors).toMatchSnapshot('errors');
+      expect(warnings).toMatchSnapshot('warnings');
+
+      for (const file in stats.compilation.assets) {
+        if (
+          Object.prototype.hasOwnProperty.call(stats.compilation.assets, file)
+        ) {
+          expect(stats.compilation.assets[file].source()).toMatchSnapshot(file);
+        }
+      }
+    });
+  });
 });