This testing guide is based on Mocha BDD interface (describe / it).

Main principles

Tests must start with a clean state

This means prefer beforeEach to before. Re-instantiate objects before running each it blocks. Create every file required by a test in a beforeEach (or commit them in fixtures/). Reset any side effects done on the test environment after each test.

Tests must be runnable in isolation

Each test must pass if they’re run alone. You can run a single test by using mocha test.js --grep 'test name'.

Stub most performance heavy operation

When possible, always stub networks or other long operations.

We use sinon.js for most stubbing needs.

Naming convention

describe blocks should cover three types of information: Object to be tested, method/property, circumstantial group (basically, “when this”).

it blocks cover assertions. They should use as few lines of code as possible. There should be as many it block as there is assertion on a method effect.

Instance methods and properties should be prefixed by a bang sign (#find()). Static methods and properties should be prefixed by a dot (.exclude()).

// Given this object
function Class() {
  this.args = nopt();
};
Class.exclude = function () {};
Class.name = 'Yeoman';
Class.prototype.find = function () {};

// We'd have this test structure
describe('Class', function () {
  describe('.exclude()', function () {});
  describe('.name', function () {});
  describe('#find()', function () {});
  describe('#args', function () {});
});

Methods should end using parentheses, but should not include expected parameters (parameters should be covered in documentation comments).

it blocks should be declarative.

// BAD
it('should find generators');

// GOOD
it('find generators');

Assertion

Don’t add message to assertions unless the error thrown makes it unclear what failed.

If you must add a message, then describe the expected outcome and why it failed. For example:

// BAD
assert(Generator.appname, 'Generator has an `appname` property');

// GOOD
assert(Generator.appname, 'Expected Generator to have an `appname` property');

Remember that these message are the error message thrown with the failure. Let those be useful in these occasions.

Stylistic preferences

.bind() throwing assertions

When testing that a method throws with invalid parameters, prefer binding the parameters to creating an anonymous function.

// BAD
assert.throws(function () {
  class.method('Invalid param');
});

// GOOD
assert.throws(class.method.bind(class, 'Invalid param'));