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Listing 13.5 Expecting start to throw an exception on missing URL
"test start should throw exception for missing URL":
function () {
var poller = Object.create(ajax.poller);
assertException(function () {
}, "TypeError");
As usual, we run the test before implementing it. The first run coughs up an error
stating that there is no Object.create method. To fix this we fetch it from Chap-
ter 7, Objects and Prototypal Inheritance, and stick it in tdd.js . What happens next
is interesting; the test passes. Somehow a TypeError is thrown, yet we haven't done
anything other than defining the object. To see what's happening, we edit the test and
remove the assertException call, simply calling poller.start() directly
in the test. JsTestDriver should pick up the exception and tell us what's going on.
As you might have guessed, the missing start method triggers a TypeError
of its own. This indicates that the test isn't good enough. To improve the situation we
add another test stating that there should be a start method, as seen in Listing 13.6.
Listing 13.6 Expecting the poller to define a start method
"test should define a start method":
function () {
With this test in place, we now get a failure stating that start was expected to
be a function, but rather was undefined . The previous test still passes. We will
fix the newly added test by simply adding a start method, as in Listing 13.7.
Listing 13.7 Adding the start method
(function () {
var ajax = tddjs.namespace("ajax");
function start() {
ajax.poller = {
start: start
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