karma.conf.js

module.exports = function(config) {
    const options = {

        // base path that will be used to resolve all patterns (eg. files, exclude)
        basePath: "",

        // available frameworks: https://npmjs.org/browse/keyword/karma-adapter
        frameworks: [
            "mocha"
        ],

        // list of files / patterns to load in the browser
        files: [
            "https://cdnjs.cloudflare.com/ajax/libs/babel-polyfill/6.23.0/polyfill.min.js",
            "http://chaijs.com/chai.js",
            "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js",
            "**/__tests__/*.js",
            // "**/__tests__/*.jsx"
        ],

        // list of files to exclude
        exclude: [
            // "./node_modules/**/*.*"
        ],

        // preprocess matching files before serving them to the browser
        // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
        preprocessors: {
            "**/*.js" : [ "webpack" ],
            "**/*.jsx": [ "webpack" ]
        },

        webpack: {
            // karma watches the test entry points
            // (you don't need to specify the entry option)
            // webpack watches dependencies

            // webpack configuration
            module: {
                rules: [
                    {
                        test  : /\.jsx?$/,
                        loader: "babel-loader"
                    },
                    {
                        test   : /\.less$/,
                        use: [
                            "style-loader?singleton",
                            "css-loader",
                            "postcss-loader",
                            "less-loader"
                        ]
                    },
                    {
                        test: /\.json5$/,
                        loader: 'json5-loader'
                    }
                ]
            },

            resolve : {
                extensions : [ ".js", ".jsx", ".json5" ]
            }
        },

        // webpack-dev-middleware configuration
        webpackMiddleware: {
            noInfo: true
        },

        // test results reporter to use
        // possible values: 'dots', 'progress'
        // available reporters: https://npmjs.org/browse/keyword/karma-reporter
        reporters: [
            "mocha"
        ],

        // web server port
        port: 9876,

        // enable / disable colors in the output (reporters and logs)
        colors: true,

        // level of logging
        // possible values: config.LOG_DISABLE || config.LOG_ERROR || config.LOG_WARN || config.LOG_INFO || config.LOG_DEBUG
        logLevel: config.LOG_INFO,

        // enable / disable watching file and executing tests whenever any file changes
        autoWatch: true,

        // start these browsers
        // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
        browsers: [
            "FirefoxHeadless"
        ],

        // Continuous Integration mode
        // if true, Karma captures browsers, runs the tests and exits
        singleRun: false,

        // Concurrency level
        // how many browser should be started simultaneous
        concurrency: Infinity,

        customLaunchers: {
            FirefoxHeadless: {
                base: 'Firefox',
                flags: [
                    '-headless',
                ],
            }
        },

        // PhantomJS custom settings
        phantomjsLauncher: {

            // Have phantomjs exit if a ResourceError is encountered (useful if
            // karma exits without killing phantom)
            exitOnResourceError: true
        },

        mochaReporter: {
            showDiff: true
        }
    };

    if (process.env.CIRCLECI) {
        options.browsers = [
            "PhantomJS"
        ];
    };

    config.set(options);
}