I'm trying to use Karma + Mocha for the first time - phantomjs

Question

I'm trying to use Karma + Mocha for the first time.
The main condition is to run the unit tests in a Docker container without any GUI. I don't have any browser, and my only choice is to use PhantomJS.
I have configured all necessary plugins, configs etc., but I received the following error:
ReferenceError: Can't find variable: exports
Before the test run, I built the JavaScript source code with Babel to the 'build' directory and then started Karma from there.
What could be causing this problem?
Here is my karma.conf.js:
// Karma configuration
// Generated on Mon Apr 25 2016 13:04:28 GMT+0300 (RTZ 2 (зима))
const path = require('path');
const webpack = require('webpack');
module.exports = function(config) {
var confBuilder = require('./webpack.config');
confBuilder.module.loaders.push({
test: /\.jsx$/,
loader: 'isparta',
include: path.resolve('src'),
});
confBuilder.devtool = 'inline-source-map';
config.set({
// base path that will be used to resolve all patterns (eg. files, exclude)
basePath: '',
// frameworks to use
// available frameworks: https://npmjs.org/browse/keyword/karma-adapter
frameworks: ['mocha', 'chai'/*'jasmine' /*, 'requirejs'*/],
// list of files / patterns to load in the browser
files: [
{pattern: 'build/utils/*.js', load: false}
],
// preprocess matching files before serving them to the browser
// available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
preprocessors: {
'tests.webpack.js': ['webpack', 'sourcemap'],
'build/utils/*.js': ['babel']
},
// test results reporter to use
// possible values: 'dots', 'progress'
// available reporters: https://npmjs.org/browse/keyword/karma-reporter
reporters: ['mocha', 'coverage', 'jenkins'],
plugins: [
require('karma-webpack'),
require('karma-mocha'),
require('karma-chai'),
require('karma-mocha-reporter'),
require('karma-jenkins-reporter'),
require('karma-chrome-launcher'),
require('karma-sourcemap-loader'),
require('karma-coverage'),
require('karma-phantomjs-launcher'),
require('karma-requirejs'),
require('karma-babel-preprocessor')
],
webpack: confBuilder,
webpackServer: {
noInfo: true,
},
coverageReporter: {
check: {
global: {
statements: 86,
branches: 80,
functions: 95,
lines: 40,
},
},
},
// start these browsers
// available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
browsers: ['PhantomJS'],
// Continuous Integration mode
// if true, Karma captures browsers, runs the tests and exits
singleRun: true,
autoWatch: true,
})
}