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Karma Config

Karma configuration builder with sensible defaults to minimize boilerplate

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Karma can be configured to do pretty much anything you can imagine, but you'll probably find yourself repeating the same boilerplate configuration in every project.

Karma Config eliminates the redundancy and boilerplate by applying sensible defaults so you only need to specify the one-off settings that are particular to your project. It can reduce a typical karma.config.js file from dozens of lines to just two or three lines.


  • Applies sensible defaults, but will never override your settings
  • Uses the appropraite browsers for the platform (e.g. Safari on Mac, Edge on Windows)
  • Uses headless browsers when running in CI/CD
  • Produces code-coverage reports if KARMA_COVERAGE env var is set, or if you add --coverage when running Karma

Related Projects

  • karma-host-environment
    Provides host environment info (user agent, Node.js version, environment variables, etc.)


Use npm to install Karma Config as a dev dependency. Be sure to install karma and karma-cli too.

npm install --save-dev karma karma-cli @jsdevtools/karma-config


Using Default values

For typical projects, the defaults may be all you need, in which case your config can be one line:


module.exports = require("@jsdevtools/karma-config")();

Depending on the operating system and whether its running in a CI/CD environment, the above code will produce this config:

  // Defaults to the Karma-Verbose-Reporter
  // See
  reporters: ["verbose"],

  // The browsers will vary depending on the OS.
  // In CI/CD environments, FirefoxHeadless and ChromeHeadless are used instead.
  browsers: ["Firefox", "Chrome"],

  frameworks: [
    // Defaults to the Mocha test framework.

    // This makes it easy to detect which browser your tests are running in.
    // Also provides access to environment variables.
    // See

  files: [
    // Assumes your tests are under the "test" folder and are named *.spec.js
    // or *.test.js.  (.mjs and .jsx file extensions are also supported)

    // Allows your tests to dynamically access any file in the "test" folder.
    // Useful for loading test data from CSV or JSON files.
    { pattern: "test/**/*", included: false, served: true }

  preprocessors: {
    // Uses Webpack to bundle your tests and their dependencies
    "test/**/*.+(spec|test).+(js|jsx|mjs)": ["webpack"]

  webpack: {
    // Webpack development mode it easier to debug failing tests
    mode: "development",

    // Inlne source maps ensure proper stack traces in errors,
    // and allow you to debug your original source code rather than bundled code.
    devtool: "inline-source-map",

Adding or Overriding Settings

You can explicitly specify any Karma settings, and Karma Config will honor them. Your settings will be merged with the settings that Karma Config generates. This allows you to add additional settings that aren't normally set by Karma Config. It also allows you to override anything that Karma Config would normally generate.


module.exports = require("@jsdevtools/karma-config")({
  config: {
    port: 12345,                                    // Set Karma's port number
    browserNoActivityTimeout: 5000,                 // Set Karma's inactivity timeout
    browsers: ["Opera", "Safari"]                   // Always use these browsers, regardless of OS
    webpack: {
      resolve: {
        extensions: [".js", ".jsx", ".ts", ".tsx"]  // Configure Webpack to resolve TypeScript file
      mode: "production",                           // Override the default Webpack mode
      module: {
        rules: [
          { test: /\.ts$/, use: "ts-loader" }       // Configure Webpack to support TypeScript

Another option is to use Karma Config's buildConfig function, which returns the generated config object. You can then make whatever modifications you want to it before passing it to Karma.


const { buildConfig } = require("@jsdevtools/karma-config");

module.exports = (karma) => {
  // Let Karma Config generate its config, as usual
  let config = buildConfig();

  // Modify the config object however you want
  config.port = 12345;
  config.browserNoActivityTimeout = 5000;
  config.webpack.mode = "production";
  config.webpack.module.rules.push({ test: /\.ts$/, use: "ts-loader" });

  // Pass the final config to Karma


Karma Config exposes a few options that allow you to control the config that it generates. Here's an example that demonstrates setting a few options:


module.exports = require("@jsdevtools/karma-config")({
  sourceDir: "my/source/dir",             // Override the default sourceDir ("src")
  testDir: "my/test/dir",                 // Override the default testDir ("test")
  coverage: true,                         // Always produce a code-coverage report
  browsers: {
    firefox: false,                       // Never test in Firefox
    ie: true,                             // Always test in Internet Explorer

All Options

Option Type Default Value Description
browsers object This object allows you to specify which browsers to test on boolean true for Linux
false on other platforms
Whether to test on Chrome.
browsers.firefox boolean true for Linux
false on other platforms
Whether to test on Firefox.
browsers.safari boolean true for Mac
false on other platforms
Whether to test on Safari.
Can use SauceLabs if configured.
browsers.edge boolean true for Windows
false on other platforms
Whether to test on Edge (the EdgeHTML engine, not Chromium).
Can use SauceLabs if configured. boolean false Whether to test on Internet Explorer.
Can use SauceLabs if configured.
sourceDir string src The relative path of the source directory.
testDir string test The relative path of the test directory.
tests string string[] object object[] ${testDir}/**/*.spec.js ${testDir}/**/*.test.js ${testDir}/**/*.spec.jsx ${testDir}/**/*.test.jsx ${testDir}/**/*.spec.mjs ${testDir}/**/*.test.mjs One or more file patterns that specify your test files. These are the files that will be bundled by Webpack and run by Karma.
fixtures string string[] object object[] none One or more file patterns that specify your test fixtures. Test fixtures will be run before any of your test files are loaded, which gives you an opportunity to setup the runtime environment, load polyfills, etc.
serve string string[] object object[] ${testDir}/**/* One or more file patterns that Karma will allow to be served. This is useful for loading test data from CSV or JSON files.
transpile boolean false* Indicates whether your source code should be transpiled to ES5 syntax to support older web browsers. If set to true, then Webpack will be configured to use Babel.

* If is enabled, then this option defaults to true when running on Windows. You can explicitly set it to false if desired.
coverage boolean false Indicates whether code coverage analysis should be performed. If set to true, then Webpack will be configured to inject code-coverage instrumentation and write code-coverage reports in the ./coverage/ directory.

This option can also be enabled by setting the KARMA_COVERAGE environment variable, or by using the --coverage command-line flag when running Karma.
platform string process.platform The operating system platform (e.g. "linux", "win32", "darwin", etc.). This determines which browsers will be launched by Karma.
CI boolean auto-detected Indicates whether Karma is running in a CI environment. If set to true, then Karma will be configured to run headless browsers where possible.

Karma Config will auto-detect most CI environments. This option can also be enabled by setting the CI or KARMA_CI environment variables.
config object {} Explicit Karma configuration settings. This is useful for adding additional settings that aren't normally set by Karma Config, or for overriding Karma Config's settings.


Safari, Edge, and Internet Explorer aren't supported on some CI/CD services. Karma Config allows you to use SauceLabs to test on these browsers.

Karma Config uses the Sauce Connect proxy to open a secure channel to SauceLabs. Sauce Connect is only supported on Linux and requires the SAUCE_USERNAME and SAUCE_ACCESS_KEY environment variables to be set.

module.exports = require("@jsdevtools/karma-config")({
  browsers: {
    // No need to explicitly set these, since they are tested by default on Linux
    // firefox: true,
    // chrome: true,

    // Test these browsers using SauceLabs
    ie: true,
    edge: true,
    safari: true,


Contributions, enhancements, and bug-fixes are welcome! Open an issue on GitHub and submit a pull request.


To build/test the project locally on your computer:

  1. Clone this repo
    git clone h

  2. Install dependencies
    npm install

  3. Build the code
    npm run build

  4. Run the tests
    npm test


Karma Config is 100% free and open-source, under the MIT license. Use it however you want.

This package is Treeware. If you use it in production, then we ask that you buy the world a tree to thank us for our work. By contributing to the Treeware forest you’ll be creating employment for local families and restoring wildlife habitats.

Big Thanks To

Thanks to these awesome companies for their support of Open Source developers ❤

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