How can I run tests with a headless browser?

Question

As a more complete answer based on William Hampshire‘s one, Cuga‘s comment and my personal additions.

Short answer: using ChromeHeadless

You can just use Headless Chrome:

ng test --browsers ChromeHeadless

You need to have Chrome 59+.

But if you need PhantomJS (and/or chaning the default ng test behaviour with no arguments) read the following.

Longer answer: using PhantomJS

EDIT: Be aware that PhantomJS project has been archived, see this thread.

Setup

In order to be able to (optionally) run your tests without a browser, using PhantomJS, you should:

1) Install some dependencies:

npm install --save-dev karma-phantomjs-launcher
npm install --save intl

2) Add PhantomJS to the Karma’s plugin list

Open karma.conf.js and add require('karma-phantomjs-launcher') to the plugins array, for example:

module.exports = function (config) {
  config.set({
    basePath: '',
    frameworks: ['jasmine', '@angular/cli'],
    plugins: [
      require('karma-jasmine'),
      require('karma-chrome-launcher'),
      require('karma-phantomjs-launcher'),
      // ...
    ],

3) Enable polyfills

Open your src/polyfills.ts file and uncomment the following lines:

BROWSER POLYFILLS

import 'core-js/es6/symbol';
import 'core-js/es6/object';
import 'core-js/es6/function';
import 'core-js/es6/parse-int';
import 'core-js/es6/parse-float';
import 'core-js/es6/number';
import 'core-js/es6/math';
import 'core-js/es6/string';
import 'core-js/es6/date';
import 'core-js/es6/array';
import 'core-js/es6/regexp';
import 'core-js/es6/map';
import 'core-js/es6/weak-map';
import 'core-js/es6/set';

APPLICATION IMPORTS

import 'intl';
import 'intl/locale-data/jsonp/en';

How to run the tests

Specifying the browsers when running the command

No you can either run the test using Chrome (the angular-cli default):

ng test --browsers Chrome

Or PhantomJS (headless):

ng test --browsers PhantomJS

Changing the default behaviour of just `ng test`

It is possible to change the default behaviour of ng test (so when no --browsers argument is provided) by changing the value of the browsers array in karma.conf.js.

It can now be set to just use Chrome (default angular-cli setup):

browsers: ['Chrome'],

or PhantomJS:

browsers: ['PhantomJS'],

or even both:

browsers: ['Chrome', 'PhantomJS'],