Abstracting Browser Differences: Ajax - Test-Driven JavaScript Development - page 263

var url = "/url";
ajax.get(url);
assertEquals(["GET", url, true], this.xhr.open.args);
}
});
Much better. Re-running the tests confirm that they now all pass. Moving for-
ward, we can add stubs to the fakeXMLHttpRequest object as we see fit, which
will make testing ajax.get significantly simpler.
12.4.2.4 Feature Detection and ajax.create
ajax.get now relies on the ajax.create method, which is not available in the
case that the browser does not support the XMLHttpRequest object. To make sure
we don't provide an ajax.get method that has no way of retrieving a transport,
we will define this method conditionally as well. Listing 12.27 shows the required
test.
Listing 12.27 Bailing out if ajax.create is not available
(function () {
var ajax = tddjs.namespace("ajax");
if (!ajax.create) {
return;
}
function get(url) {
/* ... */
}
ajax.get = get;
}());
With this test in place, clients using the ajax.get method can add a similar
test to check for its existence before using it. Layering feature detection this way
makes it manageable to decide what features are available in a given environment.
12.4.3 Handling State Changes
Next up, the XMLHttpRequest object needs to have its onreadystatechange
handler set to a function, as Listing 12.28 shows.