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n. Gerbils: Inquisitive, friendly animals that rarely bite, TDD for the rest of us

Gerbil attemps to be an uber simple and minimalistic testing framework for javascript.

Now with npm

$ npm install gerbil

Or just include the .js and run tests within a normal browser.

You can now execute the tests with node without to depend on the browser

var scenario = require('gerbil').scenario;

scenario("Testing with node", {
  "should work in a terminal": function(g){

// Or if you want to access some global Gerbil stuff

var Gerbil = require('gerbil');
var scenario = Gerbil.scenario;

What it's included?


Good ol' assert, checks boolean.


Just like assert but checks types AND value.


Asserts the throw of an exception.


Asserts the type of the object to evaluate.


Mark the test as pending.


Run the test within a time.


Run async code. Eg. callbacks, timers.

Example output

Console Errors

Console Errors


// Name the scenario you want to test and pass an object with your tests.
scenario("Some useful stuff that needs to work", {
  // Reserved names are 'setup', 'before', 'after' and 'cleanup'. They define
  // the steps to be executed.
  // Every test gets one parameter, this is the test suite itself.
  // Modifying 'this' will affect the context in the tests, it's useful when
  // using 'setup' to initialize some state.
  'setup': function(g) {
    this.validName = 'Gerbil';
  // Within the test 'this' gets the config defined in 'setup'
  'should get the correct name': function(g) {
    g.assertEqual(this.validName, 'Gerbil');

  // Test in the feature, useful to test future events or timers.
  'in the future': function(g) {
    this.time = new Date().getTime();

    g.setTimeout(function() {
      g.assert(new Date().getTime() > this.time);
    }, 1000);

  // Test async code.
  // Using the async function you can control the status of the test. This is
  // really useful when you are testing callbacks.
  // But remember, it's your responsability to end() the test.
  'should be able to test asyncronous code': function(g) {
    var asyncStuff = function() {
      this.callback = null;

    asyncStuff.prototype = {
      eventually: function(fn) {
        this.callback = fn;

      exec: function() {
        setTimeout(function(c) {
        }, 500, this);

    g.async(function() {
      var async = new asyncStuff;
      async.eventually(function() {
        // end() will end the current scenario and trigger a summary



scenario("This is my scenario", {
  "setup":  function() {
    // When scenario starts
    this.someThing = new Thing;
  "before": function() {
    // Before every test
  "after":  function() {
    // After every test
  "cleanup": function() {
    // When the scenario ends
    this.someThing = false;

  "MagicThing should have a length": function(g) {
    g.assertEqual(this.someThing.length, 1);

  "MagicThing should be valid": function(g) {

Scenario config and global config.

var myCoolFormatter = {
  // Passing tests
  "ok": function(msg) {},

  // Failing tests
  "fail": function(msg) {},

  // Pending tests
  "pending": function(msg) {},

  // The start of a scenario
  "scenario": function(msg) {},

  // Report at the end of a scenario
  "summary": function(msg) {}

scenario("Fancy scenario", {
  "config": function(c) {
    c.formatter = myCoolFormatter;
  "somewhere over the rainbow": function(g) {

// Or if you want to affect every gerbil scenario

Gerbil.globalConfig = {
  formatter: myCoolFormatter


Withing the config object you can add two types of callbacks, 'start' and 'finish'. This can help you to trigger events after the scenario finished or a nice sand clock when it starts.

scenario('configuration', {
  'config': function(c) {
    c.start = function(object) {};
    c.finish = function(results) {};

// Of course you can define then globally:

Gerbil.globalConfig = {
  start: function(object) {},
  finish: function(results) {}

What's the catch?

The results are only shown in the console, the one from console.log if you use it in a browser. Run it with an open inspector or define a custom formatter if you want prettier results. And in the bottom you will find the summary

Browser tests


  1. Get a gerbil as a pet


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