jasmine-given

Adds a Given-When-Then DSL to jasmine as an alternative style for specs

npm install jasmine-given
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Build Status

2.x series

We just released a version 2.0.0, thanks to the contributions of @ronen, to bring jasmine-given closer to parity with rspec-given. In particular, jasmine-given will now:

  • ensure that Given statements will always execute before any When statements. This is counter-intuitive at first, but can really help you DRY up specs that require variable setup.
  • allow users to use And in place of multiple Then statements; when using And following a Then, the set-up will only be executed for the first Then, which could be a significant speed-up depending on the test while looking cleaner than chaining Then statements with parentheses.

Keep in mind that the former will be a breaking change for many test suites that currently use jasmine-given, so be sure to allot yourself some time to address any test failures that occur because a Given was incidentally placed after a When in a way that doesn't agree with the new execution order.

jasmine-given

jasmine-given is a Jasmine helper that encourages leaner, meaner specs using Given, When, and Then. It is a shameless tribute to Jim Weirich's terrific rspec-given gem.

Download the latest version here.

The basic idea behind the "*-given" meme is a humble acknowledgement of given-when-then as the best English language analogue we have to arrange-act-assert. With rspec and jasmine, we often approximate "given-when-then" with "let-beforeEach-it" (noting that jasmine lacks let).

The big idea is "why approximate given-when-then, when we could actually just use them?"

The small idea is "if we couldn't write English along with our it blocks then we'd be encouraged to write cleaner, clearer matchers to articulate our expectations."

The subtle idea is that all "given"s should be evaluated before the "when"s. This can DRY up your specs: you don't need to repeat a series of "when"s in order to test the final result with different initial "given"s.

All ideas are pretty cool. Thanks, Jim!

Example (CoffeeScript)

Oh, and jasmine-given looks much nicer in CoffeeScript, so I'll show that example first:

describe "assigning stuff to this", ->
  Given -> @number = 24
  Given -> @number++
  When -> @number *= 2
  Then -> @number == 50
  # or
  Then -> expect(@number).toBe(50)

describe "assigning stuff to variables", ->
  subject=null
  Given -> subject = []
  When -> subject.push('foo')
  Then -> subject.length == 1
  # or
  Then -> expect(subject.length).toBe(1)

As you might infer from the above, Then will trigger a spec failure when the function passed to it returns false. As shown above, traditional expectations can still be used, but using simple booleans can make for significantly easier-to-read expectations when you're asserting something as obvious as equality.

Example (JavaScript)

Of course, jasmine-given also works fine in JavaScript; but as you can see, it's exceptionally clunky in comparison:

describe("assigning stuff to this", function() {
  Given(function() { this.number = 24; });
  Given(function() { this.number++; });
  When(function() { this.number *= 2; });
  Then(function() { return this.number === 50; });
  // or
  Then(function() { expect(this.number).toBe(50) });
});

describe("assigning stuff to variables", function() {
  var subject;
  Given(function() { subject = []; });
  When(function() { subject.push('foo'); });
  Then(function() { return subject.length === 1; });
  // or
  Then(function() { expect(subject.length).toBe(1); });
});

Execution order: Givens then Whens then Thens

The execution order for executing a Then is to execute all preceding Given blocks from the outside in, and next all the preceding When blocks from the outside in, and then the Then. This means that a later Given can affect an earlier When! While this may seem odd at first glance, it can DRY up your specs, especially if you are testing a series of When steps whose final outcome depends on an initial condition. For example:

    Given -> user
    When -> login user

    describe "clicking create", ->

        When -> createButton.click()
        Then -> expect(ajax).toHaveBeenCalled()

        describe "creation succeeds", ->
            When -> ajax.success()
            Then -> object_is_shown()

            describe "reports success message", ->
                Then -> feedback_message.hasContents "created"

            describe "novice gets congratulations message", ->
                Given -> user.isNovice = true
                Then -> feedback_message.hasContents "congratulations!"

            describe "expert gets no feedback", ->
                Given -> user.isExpert = true
                Then -> feedback_message.isEmpty()

For the final three Thens, the execution order is:

       Given -> user
       When -> login user
       When -> createButton.click()
       When -> ajax.success()
       Then -> feedback_message.hasContents "created"

       Given -> user
       Given -> user.isNovice = true
       When -> login user
       When -> createButton.click()
       When -> ajax.success()
       Then -> feedback_message.hasContents "congratulations!"

       Given -> user
       Given -> user.isExpert = true
       When -> login user
       When -> createButton.click()
       When -> ajax.success()
       Then -> feedback_message.isEmpty()

Without this Given/When execution order, the only straightforward way to get the above behavior would be to duplicate then Whens for each user case.

Supporting Idempotent "Then" statements

Jim mentioned to me that Then blocks ought to be idempotent (that is, since they're assertions they should not have any affect on the state of the subject being specified). As a result, one improvement he made to rspec-given 2.x was the And method, which—by following a Then—would be like invoked n Then expectations without executing each Then's depended-on Given and When blocks n times.

Take this example from jasmine-given's spec:

describe "eliminating redundant test execution", ->
  describe "a traditional spec with numerous Then statements", ->
    timesGivenWasInvoked = timesWhenWasInvoked = 0
    Given -> timesGivenWasInvoked++
    When -> timesWhenWasInvoked++
    Then -> timesGivenWasInvoked == 1
    Then -> timesWhenWasInvoked == 2
    Then -> timesGivenWasInvoked == 3
    Then -> timesWhenWasInvoked == 4

Because there are four Then statements, the Given and When are each executed four times. That's because it would be unreasonable for Jasmine to expect each it function to be idempotent.

However, spec authors can leverage idempotence safely when writing in a given-when-then format. You opt-in with jasmine-given by using And blocks, as shown below:

  describe "chaining Then statements", ->
    timesGivenWasInvoked = timesWhenWasInvoked = 0
    Given -> timesGivenWasInvoked++
    When -> timesWhenWasInvoked++

    Then -> timesGivenWasInvoked == 1
    And -> timesWhenWasInvoked == 1
    And -> timesGivenWasInvoked == 1
    And -> timesWhenWasInvoked == 1

    Then -> timesWhenWasInvoked == 2

In this example, Given and When are only invoked one time each for the first Then, because jasmine-given rolled all of those Then & And statements up into a single it in Jasmine. Note that the label of the it is taken from the Then only.

Leveraging this feature is likely to have the effect of speeding up your specs, especially if your specs are otherwise slow (integration specs or DOM-heavy).

The above spec can also be expressed in JavaScript:


describe("eliminating redundant test execution", function() {
  describe("a traditional spec with numerous Then statements", function() {
    var timesGivenWasInvoked = 0,
        timesWhenWasInvoked = 0;
    Given(function() { timesGivenWasInvoked++; });
    When(function() { timesWhenWasInvoked++; });
    Then(function() { return timesGivenWasInvoked == 1; });
    Then(function() { return timesWhenWasInvoked == 2; });
    Then(function() { return timesGivenWasInvoked == 3; });
    Then(function() { return timesWhenWasInvoked == 4; });
  });

  describe("chaining Then statements", function() {
    var timesGivenWasInvoked = 0,
        timesWhenWasInvoked = 0;
    Given(function() { timesGivenWasInvoked++; });
    When(function() { timesWhenWasInvoked++; });
    Then(function() { return timesGivenWasInvoked == 1; })
    And(function() { return timesWhenWasInvoked == 1; })
    And(function() { return timesGivenWasInvoked == 1; })
    And(function() { return timesWhenWasInvoked == 1; })
  });
});

Invariants

Rspec-given also introduced the notion of "Invariants". An Invariant lets you specify a condition which should always be true within the current scope. For example:


    Given -> @stack = new MyStack @initialContents

    Invariant -> @stack.empty? == (@stack.depth == 0)

    describe "With some initial contents", ->
        Given -> @initialContents = ["a", "b", "c"]
        Then -> @stack.depth == 3

        describe "Pop one", ->
           When -> @result = @stack.pop
           Then -> @stack.depth == 2

        describe "Clear all", ->
           When -> @stack.clear()
           Then -> @stack.depth == 0

    describe "With no contents", ->
      Then -> @stack.depth == 2

    …etc…

The Invariant will be checked before each Then block. Note that invariants do not appear as their own tests; if an invariant fails it will be reported as a failure within the Then block. Effectively, an Invariant defines an implicit And which gets prepended to each Then within the current scope. Thus the above example is a DRY version of:


    Given -> @stack = new MyStack @initialContents

    describe "With some initial contents", ->
        Given -> @initialContents = ["a", "b", "c"]
        Then -> @stack.depth == 3
        And -> @stack.empty? == false

        describe "Pop one", ->
           When -> @result = @stack.pop
           Then -> @stack.depth == 2
         And -> @stack.empty? == false

        describe "Clear all", ->
           When -> @stack.clear()
           Then -> @stack.depth == 0
           And -> @stack.empty? == true

    describe "With no contents", ->
      Then -> @stack.depth == 0
      And -> @stack.empty? == true

    …etc…

except that the Invariant is tested before each Then rather than after.

"it"-style test labels

Jasmine-given labels your underlying it blocks with the source expression itself, encouraging writing cleaner, clearer matchers -- and more DRY than saying the same thing twice, once in code and once in English. But there are times when we're using third-party libraries or matchers that just don't read cleanly as English, even when they're expressing a simple concept.

Or, perhaps you are using a collection of Then and And statements to express a single specification. So, when needed, you may use a label for your Then statements:

    Then "makes AJAX POST request to create item", -> expect(@ajax_spy).toHaveBeenCalled()
    And -> @ajax_spy.mostRecentCall.args[0].type = 'POST'
    And -> @ajax_spy.mostRecentCall.args[0].url == "/items"
    And -> @ajax_spy.mostRecentCall.args[0].data.item.user_id == userID
    And -> @ajax_spy.mostRecentCall.args[0].data.item.name == itemName

Using with Node.js

To use this helper with Jasmine under Node.js, simply add it to your package.json with

$ npm install jasmine-given --save-dev

And then from your spec (or in a spec helper), require('jasmine-given'). Be sure that it's loaded after jasmine itself is added to the global object, or else it will load minijasminenode which will, in turn, load jasmine into global for you (which you may not be intending).

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