karma.conf.js

const path = require('path');
const widthCoverage = process.env.COVERAGE === 'true';

let coverageReporters = [];

if (widthCoverage) {
    coverageReporters.push('coverage-istanbul');
}

module.exports = function karmaConfig(config) {
    config.set({
                   browsers:                 ['ChromeHeadless'],
                   singleRun:                true,
                   frameworks:               ['mocha'],
                   files:                    ['./test.js'],
                   preprocessors:            {'./test.js': 'webpack'},
                   reporters:                ['mocha'].concat(coverageReporters),
                   webpack:                  {
                       mode:        "development",
                       target:      "web",
                       performance: {hints: false},
                       resolve:     {
                           alias: {
                               'react-scrollbars-custom': path.resolve(__dirname, './src'),
                           },
                       },
                       module:      {
                           rules: [
                               {
                                   test:    /\.js$/,
                                   exclude: /(node_modules)/,
                                   use:     {
                                       loader:  'babel-loader',
                                       options: {
                                           sourceMaps:     false,
                                           comments:       true,
                                           cacheDirectory: false,
                                           presets:        [
                                               "@babel/preset-react",
                                               [
                                                   "@babel/preset-env",
                                                   {
                                                       "targets": {
                                                           "chrome": 58,
                                                       },
                                                   },
                                               ],
                                           ],
                                           plugins:        [
                                               [
                                                   "istanbul",
                                                   {
                                                       exclude: /node_modules|\.spec\.js$/,
                                                   },
                                               ],
                                               "@babel/plugin-proposal-class-properties",
                                           ],
                                       },
                                   },
                               },
                           ],
                       },
                   },
                   coverageIstanbulReporter: {
                       dir:                   path.join(__dirname, 'coverage'),
                       reports:               ['lcov', 'clover', 'html'],
                       combineBrowserReports: true,
                       fixWebpackSourcePaths: true,
                       'report-config':       {
                           html: {
                               subdir: 'html',
                           },
                       },
                   },
               });
};