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Testing

Karma setup for Typescript

Webpacker does not setup Karma by default, so you've to manually install it along with its dependencies as per your need. Following things marked as optional can be used to fancify the test results (Recommended).

// package.json
"scripts": {
  "test": "NODE_ENV=test karma start"
},
"dependencies": {
  "typescript": "^2.5.2"
},
"devDependencies": {
  "karma": "^1.7.1",
  "karma-webpack": "^2.0.4",
  "karma-chrome-launcher": "^2.2.0",
  "karma-jquery": "^0.2.2",
  "karma-jasmine": "^1.1.0",
  "karma-jasmine-jquery": "^0.1.1",
  "jasmine-core": "^2.8.0",
  [optional] "karma-coverage": "^1.1.1",
  [optional] "karma-coverage-istanbul-reporter": "^1.3.0",
  [optional] "karma-spec-reporter": "0.0.31",
  [optional] "istanbul-instrumenter-loader": "^3.0.0",
}

It is beneficial to use the same webpack configuration file (generated by webpacker) in Karma configuration to avoid redundancy. Following line tells Karma not to write transpiled source files onto filesystem while testing to avoid Error: EACCES: permission denied, mkdir '/_karma_webpack_' error. Then inject a new rule a.k.a. loader in the existing ones (needed only if you have installed istanbul-instrumenter-loader) to generate a coverage report under /coverage directory.

// config/webpack/test.js
const environment = require('./environment')
environment.plugins.get('Manifest').options.writeToFileEmit = process.env.NODE_ENV !== 'test'
environment.loaders.append('istanbul-instrumenter', {
  test: /\.ts$/,
  enforce: "post",
  loader: "istanbul-instrumenter-loader",
  query: {
    esModules: true
  },
  exclude: ["node_modules", /\.test\.ts$/]
}) /* optional */
module.exports = environment.toWebpackConfig()

Finally, update karma.conf.js to read the same test.js file mentioned above. Rest of the things are mandatory (few marked as optional wherever appropriate).

// karma.conf.js
const webpackConfig = require('./config/webpack/test.js')
module.exports = function(config) {
  config.set({
    basePath: "",
    frameworks: ["jquery-3.2.1", "jasmine-jquery", "jasmine"],
    plugins: [
      "karma-jquery",
      "karma-jasmine-jquery",
      "karma-jasmine",
      "karma-webpack",
      "karma-chrome-launcher",
      "karma-coverage-istanbul-reporter" /* optional */,
      "karma-spec-reporter" /* optional */
    ],
    files: [ "/* add spec files */" ],
    exclude: [],
    webpack: webpackConfig,
    preprocessors: {"/* add spec files */" : ["webpack"]},
    mime: { "text/x-typescript": ["ts"] },
    reporters: ["progress", "coverage-istanbul" /* optional */],
    coverageIstanbulReporter: {
      reports: [ 'html', 'lcovonly', 'text-summary' ],
      fixWebpackSourcePaths: true
    } /* optional */,
    port: 9876,
    colors: true,
    logLevel: config.LOG_INFO,
    autoWatch: true,
    browsers: ["Chrome"],
    singleRun: true
  });
};

Lazy compilation

Webpacker lazily compiles assets in test env so you can write your tests without any extra setup and everything will just work out of the box.

Here is a sample system test case with hello_react example component:

// Example React component

import React from 'react'
import ReactDOM from 'react-dom'
import PropTypes from 'prop-types'

const Hello = props => (
  <div>Hello David</div>
)

document.addEventListener('DOMContentLoaded', () => {
  ReactDOM.render(
    <Hello />,
    document.body.appendChild(document.createElement('div')),
  )
})
<%# views/pages/home.html.erb %>

<%= javascript_pack_tag "hello_react" %>
# Tests example React component
require "application_system_test_case"
class HomesTest < ApplicationSystemTestCase
  test "can see the hello message" do
    visit root_url
    assert_selector "h5", text: "Hello! David"
  end
end

Capybara setup for Rails

Make sure you configure Rails to serve static files from the public directory in the test environment.

# config/environments/test.rb
# Configure public file server for tests with Cache-Control for performance.
config.public_file_server.enabled = true

Webpacker Configuration and Rails Tests

Webpacker ships with three javascript configuration files: test.js, development.js, and production.js. The NODE_ENV environment variable determines which config will be used. NODE_ENV is independent of RAILS_ENV and is set to development by default. This means that rails test or rspec will use development.js by default, not test.js.

For more information see Why doesn't Webpacker use my test config when I run Rails tests?