Better Javascript testing through ScrewUnit

May 12, 2008 Pivotal Labs

The following is an excerpt of ScrewUnit’s new copious documentation:

Writing Good Tests

A great test maximizes these features:

  • it provides documentation, explaining the intended functioning of the system as well as how the source code works;
  • it supports ongoing development, as you bit-by-bit write a failing test and make it pass;
  • it supports later refactorings and prevents regression as you add other features;
  • and it requires little modification as the implementation of the system changes, especially changes to unrelated code.

This section focuses principally on tests as documentation. To provide documentation, as well as support future modification, a test should be readable and well organized. Here are some recommendations on how to do just that.

Use Nested Describes to Express Context

Often, when you test a system (a function, an object), it behaves differently in different contexts. Use nested describes liberally to express the context under which you make an assertion.

describe("Caller#prioritize", function() {
  describe("when there are two callers in the queue", function() {
    describe("and one caller has been waiting longer than another", function() {
      ...
    });
  });
});

In addition to using nested describes to express context, use them to organize tests by the structural properties of your source code and programming language. In Javascript this is typically prototype and function. A parent describe for a prototype contains nested describes for each of its methods. If you have cross-cutting concerns (e.g., related behavior that spans across methods or prototypes), use a describe to group them conceptually.

describe("Car", function() {
  describe("#start", function() {
  });

  describe("#stop", function() {
  });

  describe("callbacks", function() {
    describe("after_purchase", function() {
    });
  });

  describe("logging", function() {
  });
});

In this example, one parent describe is used for all Car behavior. There is a describe for each method. Finally, cross-cutting concerns like callbacks and logging are grouped because of their conceptual affinity.

Test Size

Individual tests should be short and sweet. It is sometimes recommended to make only one assertion per test:

it("chooses the caller who has been waiting the longest", function() {
  expect(Caller.prioritize()).to(equal, caller_waiting_the_longest);
});

According to some, the ideal test is one line of code. In practice, it may be excessive to divide your tests to be this small. At ten lines of code (or more), a test is difficult to read quickly. Be pragmatic, bearing in mind the aims of testing.

Although one assertion per test is a good rule of thumb, feel free to violate the rule if equal clarity and better terseness is achievable:

it("returns the string representation of the boolean", function() {
  expect($.print(true)).to(equal, 'true');
  expect($.print(false)).to(equal, 'false');
});

Two tests would be overkill in this example.

Variable Naming

Name variables descriptively, especially ones that will become expected values in assertions. caller_waiting_the_longest is better than c1.

Dividing code between tests and befores

If there is only one line of setup and it is used in only one test, it may be better to include the setup in the test itself:

it("decrements the man's luck by 5", function() {
  var man = new Man({luck: 5});

  cat.cross_path(man);
  expect(man.luck()).to(equal, 0);
});

But in general, it’s nice to keep setup code in before blocks, especially if the setup can be shared across tests.

describe('Man', function() {
  var man;
  before(function() {
    man = new Man({luck: 5});
  });

  describe('#decrement_luck', function() {
    it("decrements the luck field by the given amount", function() {
      man.decrement_luck(3);
      expect(man.luck()).to(equal, 2)
    });
  });
  ...

});

Preconditions

It is ideal, if there is any chance that your preconditions are non-obvious, to make precondition asserts in your test. The last example, were it more complicated, might be better written:

it("decrements the luck field by the given amount", function() {
  expect(man.luck()).to(equal, 5);

  man.decrement_luck(3);
  expect(man.luck()).to(equal, 2)
});

Whitespace, as seen here, can be helpful in distinguishing setup and preconditions from the system under test (SUT) and its assertions. It is nice to be consistent in your use of whitespace (e.g., “always follow a group of preconditions by a newline”). But it is better to use whitespace as makes the most sense given the context. As with everything in life, do it consciously and deliberately, but change your mind frequently.

Behavioral Testing

Behavioral testing, that is, asserting that certain functions are called rather than certain values returned, is best done with closures. The dynamic nature of JavaScript makes mocking frameworks mostly unnecessary.

it("invokes #decrement_luck", function() {
  var decrement_luck_was_called = false;
  man.decrement_luck = function(amount) {
    decrement_luck_was_called = true;
  });

  cat.cross_path(man);
  expect(decrement_luck_was_called).to(equal, true);
});

Extensive Documentation for ScrewUnit is now available. Download it here:

http://github.com/nkallen/screw-unit/tree/master

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