Webpack2 + inject-loader2.0.1 (blog post)

.babelrc

{
    "env": {
        "testing": {
            "presets": [
                "es2015"
            ]
        }
    },
    "presets": [
        ["es2015", { "modules": false }]
    ]
}

entry.js

import { getSpecialValue } from "filea";

export function getValue() {
    return getSpecialValue() * 2;
}

entry.spec.js

describe("entry", function() {

    beforeEach(function() {
        let fileInjector = require("inject-loader!entry");
        this.filea = {
            getSpecialValue: function() {
                return 1;
            }
        };
        this.entry = fileInjector({
            "filea": this.filea
        });
        sinon.spy(this.filea, "getSpecialValue");
    });

    it("works without override", function() {
        expect(require("entry.js").getValue()).to.equal(20);
    });

    it("overrides", function() {
        expect(this.entry.getValue()).to.equal(2);
    });

    it("gets a special value from filea", function() {
        this.entry.getValue();
        expect(this.filea.getSpecialValue.calledOnce).to.be.true;
    });

});

filea.js

export function getSpecialValue() {
    return 10;
}

filea.spec.js

import { getSpecialValue } from "filea";

describe("filea", function() {

    describe("getSpecialValue", function() {

        it("returns a special value", function() {
            expect(getSpecialValue()).to.equal(10);
        });

    });

});

karma.config.js

module.exports = function(config) {
  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', 'sinon'],
    // list of files / patterns to load in the browser
    files: [
      './*.spec.js'
    ],
    // list of files to exclude
    exclude: [
    ],
    // preprocess matching files before serving them to the browser
    // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
    preprocessors: {
        "./*.spec.js": ["webpack"]
    },
    // webpack configuration
    webpack: require("./webpack.config.js"),
    // middleware for webpack
    webpackMiddleware: {
        stats: "errors-only"
    },
    reporters: ['progress'],
    // 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: ['ChromeWithoutSecurity'],
    customLaunchers: {
        ChromeWithoutSecurity: {
            base: 'Chrome',
            flags: ['--disable-web-security']
        }
    },
    // 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
  })
}

package.json

{
  "name": "my-app",
  "version": "1.0.0",
  "main": "entry.js",
  "scripts": {
    "build": "webpack --progress",
    "test": "NODE_ENV=testing karma start --single-run",
    "test:watch": "NODE_ENV=testing karma start"
  },
  "dependencies": {
    "babel-core": "^6.18.2",
    "babel-loader": "^6.2.8",
    "babel-preset-es2015": "^6.18.0",
    "inject-loader": "2.0.1",
    "webpack": "2.1.0-beta.27"
  },
  "devDependencies": {
    "chai": "^3.5.0",
    "karma": "^1.3.0",
    "karma-chai": "^0.1.0",
    "karma-chrome-launcher": "^2.0.0",
    "karma-mocha": "^1.3.0",
    "karma-sinon": "^1.0.5",
    "karma-webpack": "^1.8.0",
    "mocha": "^3.2.0",
    "sinon": "^1.17.6"
  }
}

webpack.config.js

const path = require("path");
const webpack = require("webpack");

module.exports = {

    entry: {
        script: path.resolve(__dirname, "./entry.js")
    },

    module: {
        rules: [
            {
                test: /\.js$/,
                use: "babel-loader",
                exclude: /(\/node_modules\/|test\.js|\.spec\.js$)/
            }
        ]
    },

    output: {
        path: "./dist",
        filename: "script.js",
        pathinfo: true
    },

    resolve: {
        extensions: [".js"],
        modules: [
            __dirname,
            path.resolve(__dirname, "./node_modules")
        ]
    }

};