ptg 12.3 Creating an XMLHttpRequest Object 253 Microsoft.XMLHTTP will do, as Msxml2.XMLHTTP.3.0 (again, ships with IE6) includes the Microsoft.XMLHTTP alias for backwards compatibility. 12.3.3 Implementing tddjs.ajax.create With knowledgeof the different objects available, we can take a shot at implementing ajax.create, as seen in Listing 12.6. Listing 12.6 Creating an XMLHttpRequest object tddjs.namespace("ajax").create = function () { var options = [ function () { return new ActiveXObject("Microsoft.XMLHTTP"); }, function () { return new XMLHttpRequest(); } ]; for (var i = 0, l = options.length; i < l; i++) { try { return options[i](); } catch (e) {} } return null; }; Running the tests confirms that our implementation is sufficient. First test green! Before we hasten on to the next test, we should look for possible duplication and other areas that could be improved through refactoring. Although there is no obvi- ous duplication in code, there is already duplication in execution—the try/catch to find a suitable object is executed every time an object is created. This is wasteful, and we can improve the method by figuring out which object is available before defining it. This has two benefits: The call time overhead is eliminated, and fea- ture detection becomes built-in. If there is no matching object to create, then there will be no tddjs.ajax.create, which means that client code can simply test for its existence to determine if XMLHttpRequest is supported by the browser. Listing 12.7 improves the method. From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 254 Abstracting Browser Differences: Ajax Listing 12.7 Checking for support upfront (function () { var xhr; var ajax = tddjs.namespace("ajax"); var options = [/* */]; // Same as before for (var i = 0, l = options.length; i < l; i++) { try { xhr = options[i](); ajax.create = options[i]; break; } catch (e) {} } }()); With this implementation in place, the try/catch will only run at load time. If successfully created, ajax.create will call the correct function directly. The test still runs green, so we can focus on the next requirement. 12.3.4 Stronger Feature Detection The test we just wrote is bound to work as long as it is run with the basic JsTestDriver setup (seeing as JsTestDriver requires the XMLHttpRequest object or equivalent). However, the checks we did in Listing 12.3 are really feature tests that verify the capa- bilities of the returned object. Because we have a mechanism for verifying the object only once, it would be nice to make the verification as strong as possible. For this reason, Listing 12.8 performs the same tests in the initial execution, making us more confident that a usable object is returned. It requires the tddjs.isHostMethod method from Chapter 10, Feature Detection, in lib/tdd.js. Listing 12.8 Adding stronger feature detection /* */ try { xhr = options[i](); if (typeof xhr.readyState == "number" && tddjs.isHostMethod(xhr, "open") && tddjs.isHostMethod(xhr, "send") && tddjs.isHostMethod(xhr, "setRequestHeader")) { ajax.create = options[i]; break; } } catch (e) {} From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 12.4 Making Get Requests 255 12.4 Making Get Requests We will start working on the request API by describing our ultimate goal: a simple interface to make requests to the server using a URL, an HTTP verb, and possi- bly success and failure callbacks. We’ll start with the GET request, as shown in Listing 12.9; save it in test/request _ test.js. Listing 12.9 Test for tddjs.ajax.get TestCase("GetRequestTest", { "test should define get method": function () { assertFunction(tddjs.ajax.get); } }); Taking baby steps, we start by checking for the existence of the get method. As expected, it fails because the method does not exist. Listing 12.10 defines the method. Save it in src/request.js Listing 12.10 Defining tddjs.ajax.get tddjs.namespace("ajax").get = function () {}; 12.4.1 Requiring a URL The get method needs to accept a URL. In fact, it needs to require a URL. Listing 12.11 has the scoop. Listing 12.11 Testing for a required URL "test should throw error without url": function () { assertException(function () { tddjs.ajax.get(); }, "TypeError"); } Our code does not yet throw any exceptions at all, so we expect this method to fail because of it. Luckily it does, so we move on to Listing 12.12. Listing 12.12 Throwing exception if URL is not a string tddjs.namespace("ajax").get = function (url) { if (typeof url != "string") { throw new TypeError("URL should be string"); } }; From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 256 Abstracting Browser Differences: Ajax Tests pass. Now, is there any duplication to remove? That full namespace is al- ready starting to stick out as slightly annoying. By wrapping the test in an anonymous closure, we can “import” the ajax namespace into the local scope by assigning it to a variable. It’ll save us four keystrokes for each reference, so we go for it, as seen in Listing 12.13. Listing 12.13 “Importing” the ajax namespace in the test (function () { var ajax = tddjs.ajax; TestCase("GetRequestTest", { "test should define get method": function () { assertFunction(ajax.get); }, "test should throw error without url": function () { assertException(function () { ajax.get(); }, "TypeError"); } }); }()); We can apply the same trick to the source file as well. While we’re at it, we can utilize the scope gained by the anonymous closure to use a named function as well, as seen in Listing 12.14. The function declaration avoids troublesome Internet Explorer behavior with named function expressions, as explained in Chapter 5, Functions. Listing 12.14 “Importing” the ajax namespace in the source (function () { var ajax = tddjs.namespace("ajax"); function get(url) { if (typeof url != "string") { throw new TypeError("URL should be string"); } } ajax.get = get; }()); From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 12.4 Making Get Requests 257 12.4.2 Stubbing the XMLHttpRequest Object In order for the get method to do anything at all, it needs to create an XML- HttpRequest object. We simply expect it to create one using ajax.create. Note that this does introduce a somewhat tight coupling between the request API and the create API. A better idea would probably be to inject the transport object. However, we will keep things simple for now. Later when we see the big picture clearer, we can always refactor to improve. In order to verify that an object is created, or rather, that a method is called, we need to somehow fake the original implementation. Stubbing and mocking are two ways to create objects that mimic real objects in tests. Along with fakes and dummies, they are often collectively referred to as test doubles. 12.4.2.1 Manual Stubbing Test doubles are usually introduced in tests either when original implementations are awkward to use or when we need to isolate an interface from its dependencies. In the case of XMLHttpRequest, we want to avoid the real thing for both reasons. Rather than creating an actual object, Listing 12.15 is going to stub out the ajax.create method, make a call to ajax.get, and then assert that ajax.create was called. Listing 12.15 Manually stubbing the create method "test should obtain an XMLHttpRequest object": function () { var originalCreate = ajax.create; ajax.create = function () { ajax.create.called = true; }; ajax.get("/url"); assert(ajax.create.called); ajax.create = originalCreate; } The test stores a reference to the original method and overwrites it with a func- tion that, when called, sets a flag that the test can assert on. Finally, the original method is restored. There are a couple of problems with this solution. First of all, if this test fails, the original method will not be restored. Asserts throw an Assert- Error exception when they fail, meaning that the last line won’t be executed unless From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 258 Abstracting Browser Differences: Ajax the test succeeds. To fix this we can move the reference and restoring of the original method to the setUp and tearDown methods respectively. Listing 12.16 shows the updated test case. Listing 12.16 Stubbing and restoring ajax.create safely TestCase("GetRequestTest", { setUp: function () { this.ajaxCreate = ajax.create; }, tearDown: function () { ajax.create = this.ajaxCreate; }, /* */ "test should obtain an XMLHttpRequest object": function () { ajax.create = function () { ajax.create.called = true; }; ajax.get("/url"); assert(ajax.create.called); } }); Before we fix the next problem, we need to implement the method in question. All we have to do is add a single line inside ajax.get, as in Listing 12.17. Listing 12.17 Creating the object function get(url) { /* */ var transport = tddjs.ajax.create(); } With this single line in place the tests go green again. 12.4.2.2 Automating Stubbing The next issue with the stubbing solution is that it’s fairly verbose. We can mitigate this by extracting a helper method that creates a function that sets a flag when called, From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 12.4 Making Get Requests 259 and allows access to this flag. Listing 12.18 shows one such possible method. Save it in lib/stub.js. Listing 12.18 Extracting a function stubbing helper function stubFn() { var fn = function () { fn.called = true; }; fn.called = false; return fn; } Listing 12.19 shows the updated test. Listing 12.19 Using the stub helper "test should obtain an XMLHttpRequest object": function () { ajax.create = stubFn(); ajax.get("/url"); assert(ajax.create.called); } Now that we know that ajax.get obtains an XMLHttpRequest object we need to make sure it uses it correctly. The first thing it should do is call its open method. This means that the stub helper needs to be able to return an object. Listing 12.20 shows the updated helper and the new test expecting open to be called with the right arguments. Listing 12.20 Test that the open method is used correctly function stubFn(returnValue) { var fn = function () { fn.called = true; return returnValue; }; fn.called = false; return fn; } From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 260 Abstracting Browser Differences: Ajax TestCase("GetRequestTest", { /* */ "test should call open with method, url, async flag": function () { var actual; ajax.create = stubFn({ open: function () { actual = arguments; } }); var url = "/url"; ajax.get(url); assertEquals(["GET", url, true], actual); } }); We expect this test to fail because the open method isn’t currently being called from our implementation, implying that actual should be undefined. This is exactly what happens and so we can write the implementation, as in Listing 12.21. Listing 12.21 Calling open function get(url) { /* */ transport.open("GET", url, true); } Now a few interesting things happen. First, we hardcoded both the HTTP verb and the asynchronous flag. Remember, one step at a time; we can make those configurable later. Running the tests shows that whereas the current test succeeds, the previous test now fails. It fails because the stub in that test did not return an object, so our production code is attempting to call undefined.open, which obviously won’t work. The second test uses the stubFn function to create one stub, while manually creating a stub open method in order to inspect its received arguments. To fix these problems, we will improve stubFn and share the fake XMLHttpRequest object between tests. From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 12.4 Making Get Requests 261 12.4.2.3 Improved Stubbing To kill the manual stub open method, Listing 12.22 improves the stubFn func- tion by having it record the arguments it receives and making them available for verification in tests. Listing 12.22 Improving the stub helper function stubFn(returnValue) { var fn = function () { fn.called = true; fn.args = arguments; return returnValue; }; fn.called = false; return fn; } Using the improved stubFn cleans up the second test considerably, as seen in Listing 12.23. Listing 12.23 Using the improved stub function "test should call open with method, url, async flag": function () { var openStub = stubFn(); ajax.create = stubFn({ open: openStub }); var url = "/url"; ajax.get(url); assertEquals(["GET", url, true], openStub.args); } We now generate a stub for ajax.create that is instructed to return an object with one property: a stubbed open method. To verify the test we assert that open was called with the correct arguments. The second problem was that adding the call to transport.open caused the first test, which didn’t return an object from the stubbed ajax.create method, to fail. To fix this we will extract a fake XMLHttpRequest object, which can be shared between tests by stubbing ajax.create to return it. The stub can be conveniently created in the test case’s setUp. We will start with the fakeXMLHttpRequest object, which can be seen in Listing 12.24. Save it in lib/fake _ xhr.js. From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 262 Abstracting Browser Differences: Ajax Listing 12.24 Extracting fakeXMLHttpRequest var fakeXMLHttpRequest = { open: stubFn() }; Because the fake object relies on stubFn, which is defined in lib/stub.js, we need to update jsTestDriver.conf to make sure the helper is loaded before the fake object. Listing 12.25 shows the updated configuration file. Listing 12.25 Updating jsTestDriver.conf to load files in correct order server: http://localhost:4224 load: - lib/stub.js - lib/*.js - src/*.js - test/*.js Next up, we update the test case by elevating the ajax.create stub to setUp. To create the fakeXMLHttpRequest object we will use Object. create from Chapter 7, Objects and Prototypal Inheritance, so place this func- tion in lib/object.js. Listing 12.26 shows the updated test case. Listing 12.26 Automate stubbing of ajax.create and XMLHttpRequest TestCase("GetRequestTest", { setUp: function () { this.ajaxCreate = ajax.create; this.xhr = Object.create(fakeXMLHttpRequest); ajax.create = stubFn(this.xhr); }, /* */ "test should obtain an XMLHttpRequest object": function () { ajax.get("/url"); assert(ajax.create.called); }, "test should call open with method, url, async flag": function () { From the Library of WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... content="text/html; charset=utf-8"> Ajax Test Ajax Test ... it actually runs in the real world 12.5.1 The Integration Test To use the API we need an HTML page to host the test The test page will make a simple request for another HTML page and add the results to the DOM The test page can be viewed in Listing 12.37 with the test script, successful_get _test. js, following in Listing 12.38 Listing 12.37 Test HTML document Listing 12.38 The integration test script function startSuccessfulGetTest() { var output = document.getElementById("output"); if (!output) { return; } Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark From the Library of WoweBook.Com 270 Abstracting Browser Differences: Ajax function log(text)... e.message); } } As you can see from the test script’s log function, I intend to run the tests in some ancient browsers The fragment being requested can be seen in Listing 12.39 Listing 12.39 HTML fragment to be loaded asynchronously Remote page Hello, I am an HTML fragment and I using XMLHttpRequest was fetched 12.5.2 Test Results Running the tests is mostly a pleasurable experience... reaction as TDD-ers is to capture it in a test Capturing the bug by verifying that our code handles the exception is all fine, but does not help Firefox . request, as shown in Listing 12.9; save it in test/ request _ test. js. Listing 12.9 Test for tddjs.ajax.get TestCase("GetRequestTest", { " ;test should define get method": function. namespace in the test (function () { var ajax = tddjs.ajax; TestCase("GetRequestTest", { " ;test should define get method": function () { assertFunction(ajax.get); }, " ;test should. WoweBook.Com Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. ptg 260 Abstracting Browser Differences: Ajax TestCase("GetRequestTest", { /* */ " ;test should call