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Grunt plugin to run Mocha tests in a (nearly) headless Mozilla Firefox browser-engine via SlimerJS

Build Status NPM version Dependency Status devDependency Status

Run your Mocha browser tests in SlimerJS's npm wrapper with all the regular reporters.

Some internal parts sourced from grunt-mocha by @kmiyashiro.

⚠️ Very early version, user beware.

Known issues

  • no error messages in async errors
  • stack traces missing
  • needs a lot more real-world use
  • needs write-to-file support for some reporters

Getting Started

This plugin requires Grunt ~0.4.1

If you haven't used Grunt before, be sure to check out the Getting Started guide, as it explains how to create a Gruntfile as well as install and use Grunt plugins. Once you're familiar with that process, you may install this plugin with this command:

$ npm install grunt-mocha-slimer --save-dev

Once the plugin has been installed, it may be enabled inside your Gruntfile with this line of JavaScript:


The "grunt-mocha-slimer" task

For an example html see the ./test folder.

Basic options

    mocha_slimerjs: {
        all: {
            options: {
                ui: 'bdd'
                reporter: 'Spec'
                run: true,
            src: ['test/index.html']

Advanced options

    mocha_slimerjs: {
        all: {
            options: {
                ui: 'bdd'
                reporter: 'Spec'
                grep: 'some keyword'
                // SlimerJS timeout 
                timeout: 10000,
                // set to false and call it later for async tests (AMD etc) 
                run: false,
                // run SlimerJS via 'xvfb-run': for true headless testing 
                // must be true on Travis-CI, use: (process.env.TRAVIS === 'true') 
                xvfb: true,
                // pass http urls (use grunt-contrib-connect etc) 
                urls: ['http://localhost:8080/test/index.html']

Better docs at a later date.

Why not a PhantomJS based grunt + mocha?

  • SlimerJS uses Firefox/XulRunner (Gecko) so it's something different PhantomJS' Webkit (V8).
  • PhantomJS uses an ancient Webkit version that is missing some modern features, like Uint8ClampedArray, Float64Array etc. You need this to test interesting canvas libraries on a generic node.js based CI like Travis.
  • SlimerJS could enable supports addons and user-profiles (let me know if there is a use-case).
  • grunt-mocha-slimer seems slightly faster then grunt-mocha as all tests are in one process (not sure why grunt-mocha doesn't do that, we'll see).
  • This modules streams test- and log data over SlimerJS' stdout for snappy feedback.

Why the flashing windows?

This is the nature of SlimerJS and the engine it runs, as . On Linux and OSX SlimerJS has support for xvfb if you have it installed. For more info see the SlimerJS documentantion.

You can enable this by setting xvfb: true (you need this for Travis-CI, see below).

Is this ready to use?

Wisdom says no, adventure and necessity say.. give it a try. It is the simplest way to do all the mocha-flavoured things you cannot do in PhantomJS's crappy archaic Webkit.

I needed it for some OSS projects and it seems to work (well, at least it properly fails if assertions fail).



  • loot some more features from grunt-mocha
    • growl support
    • output-to-file (for XUnit etc)
  • support more SlimerJS commandline parameters?
    • user profile handling?
    • proxy support?
  • extract mocha/slimer runner to own module


  • expose screenshot feature?
  • expose a file-io stream (to dump debug data to disk)?
  • auto-inject mocha JS & CSS?
  • support direct JS tests without html?
  • auto-enable xvfb on known CI's?


  • WebDriver support (use CasperJS)


  • 0.0.x - Early releases.


Contributions are very welcome, please create an Issue before doing something major.

In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using Grunt.


Copyright (c) 2014 Bart van der Schoor

Licensed under the MIT license.

Various snippets copy/hacked from grunt-mocha by Kelly Miyashiro.