Training Guide: Programming in HTML5 with JavaScript and CSS3 Ebook

Lesson 1: Creating JavaScript objects CHAPTER 6 269

Remember that the this keyword references the object that owns the current code. The

way the test is currently written, the this keyword references the global object, and getInfo

will still be a global variable. To solve the problem, the new keyword must be used to create

an object from this class, as shown in the modified test code.

test("Encapsulation Test", function () {

expect(2);

var car1 = new Vehicle(2000, 'Ford', 'Fusion');

var car2 = new Vehicle(2010, 'BMW', 'Z4');

var expected = 'Vehicle: 2000 Ford Fusion';

var actual = car1.getInfo();

equal(actual, expected, 'Expected value: ' + expected +

' Actual value: ' + actual);

expected = 2000;

actual = year;

equal(actual, expected, 'Expected value: ' + expected +

' Actual value: ' + actual);

});

Notice that a new variable is defined, car1, and it is assigned the object that is created by

using the new keyword. After that, another new variable is defined, car2, and it is assigned the

second Vehicle object created by using the new keyword. Two instances of the Vehicle class

are being created, which means that two Vehicle objects are being constructed. Each instance

has its own data and its own copy of the getInfo method. The getInfo method is public but

has access to the private data. A method that is public but has access to private data is called

a privileged method.

Figure 6-3 shows the test results. Notice that the first assertion passed, which proves that

there are separate object instances, each having its own data. The second assertion failed. The

failure message states that the year is undefined, which proves that the year is not directly

accessible from the test, which is in the global namespace. Instead, year, in addition to make

and model, is encapsulated in the object.

You have now created a class and constructed objects from the class, but there’s more

to cover in the Vehicle function that is being used as a class. The Vehicle function is known

as a constructor function. The new keyword created an object and executed the constructor

function to initialize the object by creating the year, make, and model private variables and

the public getInfo variable. Each instance has these four variables, and memory is allocated

for them. That’s what you want for the data, but is that what you want for the getInfo variable

that references a function? The answer is that it depends on what you are trying to accom-

plish with your code.

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