Phát triển Javascript - part 4 docx

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Automated Testing
A
s web developers it is easy to find ourselves in situations where we spend un-
healthy amounts of time with the refresh button in our browsers. You know the drill:
type some code in your text editor, Alt+Tab to the browser, hit F5. Lather, rinse,
repeat. This sort of manual testing is time-consuming, error-prone, and irrepro-
ducible. Given that our web applications are expected to run on a vast combination
of browsers and platforms, testing them all manually will inevitably become an
impossible task. So we focus on a few combinations and perform the occasional
check-up on the broader selection. The end result is an unsatisfactory development
process and possibly brittle solutions.
Over the yearslots of tools have emerged to improve our lives as web developers.
We now have developer tools for all the major browsers, there are several JavaScript
debuggers to choose from, and even IDEs to spot typos and other mistakes. Spend-
ing some time in Firefox’s Firebug plugin interacting with an application sure beats
those pesky alerts, but we’re still stuck with a manual, error-prone, and time-
consuming debugging process.
Humans are lazy, programmers even more so. When manual processes slow us
down, we seek to automate the manual behavior, allowing us to spend our time doing
something meaningful. In fact, as web developers, our job is more often than not
to automate some tedious task in order to improve business value. Online banking
is a great example—instead of going to the bank, standing in line and interacting
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Automated Testing
with another human to move some cash from account A to account B, we simply

log in from the comfort of our couch and get the job done in a couple of minutes.
Saves us time and saves the bank time.
Automated testing provides a solution to the manual testing process. Instead
of filling out that form one more time and hitting submit to see if the client-side
validations trigger as expected, we can instruct software to perform this test for
us. The advantages are obvious: given a convenient way to run the automated test
we can test in numerous browsers with a single effort, we can rerun the test at any
later stage, and the test may even run on some schedule that requires no manual
interaction whatsoever.
Automated software testing has been around for quite a while, even for
JavaScript. JsUnit dates back to 2001, Selenium came along in 2004, and since then
an incredible amount of tools have emerged. Still, automated testing seems to have
less momentum in the JavaScript/web development community than most other
programming communities. In this chapter we’ll investigate one means to automate
software testing, the unit test, and how it applies to the world of JavaScript.
1.1 The Unit Test
A unit test is a piece of code that tests a piece of production code. It does so
by setting up one or a few more objects in a known state, exercising them (e.g.,
calling a method), and then inspecting the result, comparing it to the expected
outcome.
Unit tests are stored on disk and should be easy and fast to run; if tests are
hard or slow to run, developers are less likely to run them. Unit tests should test
software components in isolation. They should also run isolated—no test should
ever depend on another test, tests should be able to run simultaneously and in any
order. In order to test components in isolation, it is sometimes necessary to mock
or stub their dependencies. We will discuss mocking and stubbing in context in
Part III, Real-World Test-Driven Development in JavaScript and in more detail in
Chapter 16, Mocking and Stubbing.
Having unit tests stored on disk, and usually stored in version control along
with the production code, means we can run tests at any time:

• When the implementation is complete, to verify its correct behavior
• When the implementation changes, to verify its behavior is intact
• When new units are added to the system, to verify it still fulfills its intended
purpose
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1.1 The Unit Test
5
1.1.1 Unit Testing Frameworks
Unit tests are usually written using a unit testing framework, although that is not
strictly necessary. In this chapter we’ll focus on the concept of unit tests, working
through the different aspects of writing and running them. We’ll defer the discussion
of actual testing frameworks for JavaScript to Chapter 3, Tools of the Trade.
It’s likely that you’ve already written more than a few unit tests, even if you
have never done any structured unit testing. Whenever you pop up a console in
a browser (e.g., Firebug, Safari’s Inspector or others) to debug or play live with
your code, you probably issue some statements and inspect the resulting state of
the involved objects. In many cases this is unit testing, only it isn’t automated and
it’s not reproducible. We’ll work through an example of this kind of testing and
gradually formalize it as an xUnit test case.
xUnit is a common way to refer to test frameworks that are either a direct port
of JUnit, or more loosely based on the ideas and concepts in it—or, more correctly,
the ideas and concepts in SUnit, the Smalltalk testing framework. Kent Beck, the
father of extreme programming, played an integral part in the creation of both these
frameworks, and even though SUnit was the first implementation, JUnit has done
the most in terms of popularizing the pattern.
1.1.2 strftime for JavaScript Dates
Many programming languages provide a strftime function or similar. It operates
on a date or timestamp, accepts a format string, and produces a formatted string

that represents the date. For example, in Ruby, strftime is available as a method
on time and date objects, and Listing 1.1 shows an example of its use.
Listing 1.1 Time#strftime in Ruby
Time.now.strftime("Printed on %m/%d/%Y")
#=> "Printed on 09/09/2010"
Listing 1.2 shows an early attempt at implementing strftime for JavaScript.
It’s implemented on Date.prototype which makes it available as a method on
all date objects. Don’t despair should you find it hard to understand all the details
of the code in this chapter. Concepts are more important than the actual code, and
most advanced techniques will be discussed in Part II, JavaScript for Programmers.
Listing 1.2 Starting point for strftime for JavaScript
Date.prototype.strftime = (function () {
function strftime(format) {
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var date = this;
return (format + "").replace(/%([a-zA-Z])/g,
function (m, f) {
var formatter = Date.formats && Date.formats[f];
if (typeof formatter == "function") {
return formatter.call(Date.formats, date);
} else if (typeof formatter == "string") {
return date.strftime(formatter);
}
return f;
});
}

// Internal helper
function zeroPad(num) {
return (+num < 10 ? "0" : "") + num;
}
Date.formats = {
// Formatting methods
d: function (date) {
return zeroPad(date.getDate());
},
m: function (date) {
return zeroPad(date.getMonth() + 1);
},
y: function (date) {
return date.getYear() % 100;
},
Y: function (date) {
return date.getFullYear();
},
// Format shorthands
F: "%Y-%m-%d",
D: "%m/%d/%y"
};
return strftime;
}());
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1.1 The Unit Test
7
Date.prototype.strftime mainly consists of two parts: the replace

function which takes care of replacing format specifiers with their corresponding
values, and the Date.formats object which is a collection of helpers. It can be
broken down as follows:
• Date.formats is an object with format specifiers as keys and methods to
extract the corresponding data from a date as values
• Some format specifiers are convenient shortcuts to longer formats
• String.prototype.replace is used with a regexp that matches format
specifiers
• The replacer function checks if a given specifier is available on Date.
formats and uses it if it is, otherwise the specifier is left untouched (i.e.,
returned directly)
How would we go about testing this method? One way is to include the script
in our web page and use it where we need it and then verify manually if the website
displays dates correctly. If it doesn’t work, we probably won’t get a lot of hints as to
why, and are left debugging. A slightly more sophisticated approach (although not
by much) is to load it in a web page and pop open a console and play around with
it. Perhaps something like the session in Listing 1.3.
Listing 1.3 Manually checking code in Firebug
>>> var date = new Date(2009, 11, 5);
>>> date.strftime("%Y");
"2009"
>>> date.strftime("%m");
"12"
>>> date.strftime("%d");
"05"
>>> date.strftime("%y");
"9"
Uh-oh. Our Firebug session indicates all is not well with our strftime. This
means we’ll have to investigate and rerun the test to verify that it’s working. That’s
more manual labor. We can do better. Let’s create a minimal HTML page where we

load in the source script along with another script where we add some test code.
This way we can inspect the result of changes without having to retype the tests.
Listing 1.4 shows the HTML page that we’ll use to test.
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Listing 1.4 A HTML test page
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
" /><html lang="en">
<head>
<title>Date.prototype.strftime test</title>
<meta http-equiv="content-type"
content="text/html;charset=utf-8">
</head>
<body>
<script type="text/javascript" src=" /src/strftime.js">
</script>
<script type="text/javascript" src="strftime_test.js">
</script>
</body>
</html>
We then copy our console session into a new file, shown in Listing 1.5, which
will serve as the test file. To log results we’ll simply use console.log, which is
available in most modern browsers, and logs to the browser’s JavaScript console.
Listing 1.5 strftime test.js
var date = new Date(2009, 11, 5);
console.log(date.strftime("%Y"));
console.log(date.strftime("%m"));

console.log(date.strftime("%d"));
console.log(date.strftime("%y"));
console.log(date.strftime("%F"));
We now have a reproducible test case. We can then attend to the failure:
"%y" does not zero pad the number it returns. It turns out we simply forgot
to wrap the method call in a zeroPad() call. Listing 1.6 shows the updated
Date.formats.y method.
Listing 1.6 Zero-pad year
Date.formats = {
//
y: function (date) {
return zeroPad(date.getYear() % 100);
}
//
};
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1.2 Assertions
9
Now we can immediately rerun the test file in a browser and inspect the console
to verify that the change fixed the “y” format specifier. In all its simplicity, we’ve
now written a unit test. We’re targeting the smallest unit possible in JavaScript—the
function. You have probably done something like this many times without being
aware of the fact that it is a unit test.
While automating the process of creating test objects and calling some methods
on them is nice, we still need to manually check which calls are OK and which are
not. For a unit test to be truly automated, it needs to be self-checking.
1.2 Assertions
At the heart of a unit test is the assertion. An assertion is a predicate that states the

programmer’s intended state of a system. When debugging the broken “y” format
in the previous section, we carried out a manual assertion: when the strftime
method is called on a date from 2009 with the format of "%y", we expect it to
return the string "09". If it doesn’t, our system is not working correctly. Assertions
are used in unit tests to perform these checks automatically. When an assertion
fails, the test is aborted and we’re notified of the failure. Listing 1.7 shows a simple
assert function.
Listing 1.7 A simple assert function
function assert(message, expr) {
if (!expr) {
throw new Error(message);
}
assert.count++;
return true;
}
assert.count = 0;
The assert function simply checks that its second argument is truthy (i.e.,
any value except false, null, undefined, 0, "", and NaN). If it is, it incre-
ments the assertion counter, otherwise an error is thrown, using the first argument
as error message. We can leverage assert in our tests from before, as seen in
Listing 1.8.
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Listing 1.8 Testing with assert
var date = new Date(2009, 9, 2);
try {
assert("%Y should return full year",

date.strftime("%Y") === "2009");
assert("%m should return month",
date.strftime("%m") === "10");
assert("%d should return date",
date.strftime("%d") === "02");
assert("%y should return year as two digits",
date.strftime("%y") === "09");
assert("%F should act as %Y-%m-%d",
date.strftime("%F") === "2009-10-02");
console.log(assert.count + " tests OK");
} catch (e) {
console.log("Test failed: " + e.message);
}
This requires slightly more typing, but the test now speaks for itself and is able
to verify itself. The manual labor has been reduced from inspecting each and every
outcome to simply inspecting the final status reported by the test.
1.2.1 Red and Green
In the world of unit testing, “red” and “green” are often used in place of “failure”
and “success,” respectively. Having tests go red or green makes the outcome even
clearer to interpret, and demands less effort on our part. Listing 1.9 provides a
simplified output function which uses the DOM to display messages in color.
Listing 1.9 Outputting messages in color
function output(text, color) {
var p = document.createElement("p");
p.innerHTML = text;
p.style.color = color;
document.body.appendChild(p);
}
// console.log can now be replaced with
output(assert.count + " tests OK", "#0c0");

// and, for failures:
output("Test failed: " + e.message, "#c00");
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1.3 Test Functions, Cases, and Suites
11
1.3 Test Functions, Cases, and Suites
The test we have built so far has several assertions, but because the assert function
throws an error when a test fails, we won’t know whether or not tests following a
failing test fail or succeed. For more fine-grained feedback, we can organize our test
into test functions. Each test function should exercise only one unit, but it may do
so using one or more assertions. For complete control, we can also require each test
to only test one specific behavior of a single unit. This means there will be many
tests for each function, but they’ll be short and easy to understand, and the test as
a whole will provide to-the-point feedback.
A set of related test functions/methods is referred to as a test case. In the case
of the strftime function, we can imagine a test case for the whole method, with
each test testing a specific behavior of the function through one or more assertions.
Test cases are usually organized in test suites in more complex systems. Listing 1.10
shows a very simple testCase function. It accepts a string name and an object
with test methods. Every property whose name starts with the word “test” is run as
a test method.
Listing 1.10 A simple testCase function
function testCase(name, tests) {
assert.count = 0;
var successful = 0;
var testCount = 0;
for (var test in tests) {
if (!/^test/.test(test)) {

continue;
}
testCount++;
try {
tests[test]();
output(test, "#0c0");
successful++;
} catch (e) {
output(test + " failed: " + e.message, "#c00");
}
}
var color = successful == testCount ? "#0c0" : "#c00";
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output("<strong>" + testCount + " tests, " +
(testCount - successful) + " failures</strong>",
color);
}
Listing 1.11 uses testCase to restructure the strftime test into a test case.
Listing 1.11 strftime test case
var date = new Date(2009, 9, 2);
testCase("strftime test", {
"test format specifier %Y": function () {
assert("%Y should return full year",
date.strftime("%Y") === "2009");
},
"test format specifier %m": function () {

assert("%m should return month",
date.strftime("%m") === "10");
},
"test format specifier %d": function () {
assert("%d should return date",
date.strftime("%d") === "02");
},
"test format specifier %y": function () {
assert("%y should return year as two digits",
date.strftime("%y") === "09");
},
"test format shorthand %F": function () {
assert("%F should act as %Y-%m-%d",
date.strftime("%F") === "2009-10-02");
}
});
The tests have so far been distinct and simple enough that we end up with one
assertion in each test. The test case now groups all the tests into a single object, but
the date object is still being created outside, which is unnatural as it’s an integral
part of the test. We could create a new object inside each test, but since we can
create it the same way for all of them, that would lead to unnecessary duplication.
A better option would be to gather common setup code in a single place.
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