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ptg 8.3 Object Literals 187 8.3 Object Literals Object literals enable you to create objects that support many features without directly invoking a function. When a function acts as constructor you have to keep track of the order of arguments that will be passed, and so on. Not so with object literals. They are similar to hashes in other languages using key/value pairs to represent fields. The fields can be nested. The basic syntax for an object literal is: 1. A colon separates the property name from its value. 2. A comma separates each set of name/value pairs from the next set. 3. The comma should be omitted from the last name/value pair. 1 Even with nested key/value pairs, the last key/value pair does not have a comma. 4. The entire object is enclosed in curly braces. The value assigned to a property can be of any data type, including array literals and object literals (arrays are covered in Chapter 9). Using the new operator with the Object() constructor or an object literal is both simple and logical, but the biggest short- coming is that the code is not reusable; that is, you would have to retype the code to use it again within the program, whereas you can use a constructor function to create mul- tiple instances of an object. In Example 8.7, one soldier object is created as an object literal, but we cannot create two soldier objects unless we redefine another entirely new object. (See JSON in Chapter 18, “An Introduction to Ajax (with JSON),” for a good rea- son to use object literals.) Figure 8.7 Two objects of the Distance class call the calculate() method. 1. Firefox will not complain if the last field is terminated with a comma; Internet Explorer will raise an exception. From the Library of WoweBook.Com ptg 188 Chapter 8 • Objects In the next example, two methods are created for the object literal. Once an object literal has been defined you can later add more properties to it as demonstrated in Example 8.7. FORMAT var object = { property1: value, property2: value }; EXAMPLE var area = { length: 15, width: 5 }; EXAMPLE 8.7 <html> <head><title>working with literal objects</title> <script type="text/javascript"> 1 var soldier ={ 2 name: undefined, 3 rank: "captain", 4 picture: "keeweeboy.jpg", 5 fallIn: function() { alert("At attention, arms at the side, head and eyes forward."); }, 6 fallOut: function() { alert("Drop out of formation, step back, about face!"); } }; </script> </head> <body> <big> <script type="text/javascript"> 7 soldier.name="Tina Savage"; // Assign value to object // property 8 document.write("The soldier's name is ", soldier.name,".<br />"); document.write(soldier.name+"'s rank is ", soldier.rank+."<br />”); 9 document.write("<img src='"+soldier.picture+"'>") 10 soldier.fallIn(); //call object’s methods 11 soldier.fallOut(); </script> </big> </body> </html> From the Library of WoweBook.Com ptg 8.3 Object Literals 189 EXPLANATION 1 A literal object called soldier is defined with properties and methods. 2 The property name is separated from its value with a colon. The value is undefined at this point. 3 The property is rank. Its value is a string, “captain”. 4 This property is picture; its value is a JPEG file. 5 The property fallIn is assigned an anonymous function that will serve as a method for this object. 6 The property fallOut is assigned an anonymous function that will server as anoth- er method for this object. 7 The object’s property name is assigned a value, “Tina Savage”. It is initially assigned “undefined”. 8 In this and the next line the value of the name and rank properties is printed. 9 A picture of the soldier is assigned to the img tag’s src property and displayed in the document. 10 The method for the object, fallIn(), is called. Note that by appending the () oper- ator to the fallIn property, the function assigned to it on line 5 is called and exe- cuted. 11 The method for the object, fallOut(), is called. By appending the () operator to the fallOut property, the function assigned to it on line 6 is called and executed. The output is shown in Figure 8.8. Figure 8.8 A literal object with properties and methods. From the Library of WoweBook.Com ptg 190 Chapter 8 • Objects EXAMPLE 8.8 <html> <head><title>Object Literals</title> <script type = "text/javascript"> 1 var Car = { // Create a Car object 2 make:undefined, year:2006, price:undefined, 3 owner:{ name:"Henry Lee", cell_phone: "222-222-2222", 4 address:{street: "10 Main", city: "SF", state: "CA" } }, dealer: "SF Honda", 5 display: function() {details="Make: "+Car.make+"\n"; details += "Year: "+Car.year+"\n"; details += "Price: $"+Car.price+"\n"; details += "Owner: "+ Car.owner.name+"\n"; alert(details); } }; </script> </head> <body> <script type="text/javascript"> 6 Car.make="Honda Civic"; // Assign value 7 Car.year=2009; // Update the year 8 Car.price=30000; 9 Car.display(); </script> </body> </html> EXPLANATION 1 An object literal Car is created and initialized. 2 The properties for the Car object are assigned. Properties are separated from their corresponding values with a colon and each property/value pair is separated by a comma, except the last one. Watch the commas. Internet Explorer is very partic- ular about this rule. 3 The owner property contains another set of property/value pairs. The value of the owner property is another nested object literal with key/value pairs enclosed in curly braces. From the Library of WoweBook.Com ptg 8.4 Manipulating Objects 191 8.4 Manipulating Objects 8.4.1 The with Keyword The with keyword is used as a kind of shorthand for referencing an object’s properties or methods. The object specified as an argument to with becomes the current object for the duration of the block that follows. The properties and methods for the object can be used without naming the object or using the dot syntax. A caveat: This shorthand might save typing, but it will also take a lot longer to run because JavaScript checks each variable within the with block to see whether or not it is a property of the object it references. 4 The owner has an address field that contains its set of property/value pairs. 5 This property will act as the object’s method. It is assigned an inline function de- fined to display properties of the object. 6 With object literals there is no constructor function. This line assigns a value to an undefined Car property called make. 7 This Car’s year property is assigned a new value. 8 The Car object’s price property, originally undefined, is assigned a value. 9 The Car object’s method called details() is called to display the properties of the object. Be sure to observe the syntax differences; that is, an equals sign rather than colon between property and value, and an optional semicolon rather than comma at the end of the assignment. The output is shown in Figure 8.9. Figure 8.9 Nesting property/value pairs in an object literal. Output of Example 8.8 literal objects. EXPLANATION ( CONTINUED) From the Library of WoweBook.Com ptg 192 Chapter 8 • Objects FORMAT with (object){ < properties used without the object name and dot> } EXAMPLE with(employee){ document.write(name, ssn, address); } EXAMPLE 8.9 <html> <head><title>The with Keyword</title> <script type = "text/javascript"> 1 function book(title, author, publisher){ 2 this.title = title; // Properties this.author = author; this.publisher = publisher; 3 this.show = show; // Define a method } 4 function show(){ 5 with(this){ // The with keyword with this 6 var info = "The title is " + title; info += "\nThe author is " + author; info += "\nThe publisher is " + publisher; 7 alert(info); } } </script> </head> <body bgcolor="lightblue"> <script type = "text/javascript"> 8 var childbook = new book("A Child's Garden of Verses", "Robert Lewis Stevenson", "Little Brown"); 9 var adultbook = new book("War and Peace", "Leo Tolstoy", "Penguin Books"); 10 childbook.show(); // Call method for child's book 11 adultbook.show(); // Call method for adult's book </script> </body> </html> From the Library of WoweBook.Com ptg 8.4 Manipulating Objects 193 EXPLANATION 1 The book constructor function is defined with its properties. 2 The book object is described with three properties: title, author, and publisher. 3 The book object’s property is assigned the name of a function. This property will serve as a method for the object. 4 A function called show is defined. 5 The with keyword will allow you to reference the properties of the object without using the name of the object and the dot or the this keyword. (See “The Math Ob- ject” on page 241 in Chapter 9.) 6 A variable called info is assigned the property values of a book object. The with keyword allows you to specify the property name without a reference to the object (and dot) preceding it. 7 The alert box displays the properties for a book object. 8 The constructor function is called and returns an instance of a new book object called childbook. 9 The constructor function is called and returns an instance of another book object called adultbook. 10 The show() method is called for the childbook object (see Figure 8.10). 11 The show() method is called for the adultbook object (see Figure 8.11). Figure 8.10 The childbook object and its properties. From the Library of WoweBook.Com ptg 194 Chapter 8 • Objects 8.4.2 The for/in Loop JavaScript provides the for/in loop, which can be used to iterate through a list of object properties or array elements. The for/in loop reads: for each property in an object (or for each element in an array) get the name of each property (element), in turn, and for each of the properties (elements), execute the statements in the block that follows. The for/in loop is a convenient mechanism for looping through the properties of an object. Figure 8.11 The adultbook object and its properties. FORMAT for(var property_name in object){ statements; } EXAMPLE 8.10 <html> <head><title>User-defined objects</title> <script type = "text/javascript"> 1 function book(title, author, publisher){ 2 this.title = title; this.author = author; this.publisher = publisher; 3 this.show=show; // Define a method for the object } From the Library of WoweBook.Com ptg 8.4 Manipulating Objects 195 4 function show(obj, name){ // Function to show the object's properties var result = ""; 5 for (var prop in obj){ 6 result += name + "."+ prop +"="+ obj[prop] + "<br />"; } 7 return result; } </script> </head> <body bgcolor="lightblue"> <script type="text/javascript"> 8 myBook = new book("JavaScript by Example", "Ellie", "Prentice Hall"); 9 document.write("<br /><b>" + myBook.show(myBook, "myBook")); </script> </body> </html> EXPLANATION 1 The function called book will define the properties and methods for a book object. The function is a template for the new object. An instance of a new book object will be created when this constructor is called. 2 This is the first property defined for the book object. The this keyword refers to the current object. 3 A show property of the object is assigned a function name also called show, thus creating a method for the object. 4 The function called show() is defined, tasked to display all the properties of the object. 5 The special for/in loop executes a set of statements for each property of the object. 6 The name and value of each property is concatenated and assigned to a variable called result. The value obj[prop] is used to key into each of the property values of the book object. 7 The value of the variable result is sent back to the caller. Each time through the loop, another property and value are displayed. 8 A new book object called myBook is created (instantiated). 9 The properties for the book object are shown in the browser window; see Figure 8.12. Notice how the method and its definition are displayed. EXAMPLE 8.10 (CONTINUED) From the Library of WoweBook.Com ptg 196 Chapter 8 • Objects 8.5 Extending Objects with Prototypes In object-oriented languages, like Java and C++, the object, along with its properties (state) and methods (behavior), is bundled up into a container called a class. These pro- gramming languages all allow classes to reuse and extend an existing class; that is, inherit commonly used state and behaviors from other classes. A new class, called a sub- class, can be derived from an existing class. For example, if you have a Pet class, let’s say all pets have an owner, a gender, and some methods for setting and getting the proper- ties. We could create subclasses of the Pet, such as a Dog and Cat, because both the Dog and Cat share the same properties as a Pet. The code for the Pet can be extended to the Dog and the Cat by using inheritance. Then each subclass can be refined further by add- ing its own specific features. Let’s look at an example of how JavaScript might use inheritance. A constructor func- tion called divStyle() has been defined. It will be used to create div containers to style a Web page. The constructor defines the properties for a div, for example, the borders, pad- ding, background color, and so on, and has a divGet() method to display the div in the document. Later we want to make divStyle objects but change or add properties; perhaps change the width of the border or set a border color. We can reuse or extend the divStyle code by using inheritance. Section 8.2 explained that unlike Java and C++, JavaScript doesn’t have a class mech- anism per se, but inheritance involves reusing or extending a class. With JavaScript we can simulate a class with a constructor function and the new keyword. And to imple- ment inheritance JavaScript extends a the class with a prototype. (For those of you who are Java or C++ programmers, JavaScript does not use keywords like extended, protected, private, public, etc.). How does prototype inheritance work? JavaScript functions are automatically given an empty prototype object. If the function acts as a constructor function for a class of objects, the prototype object can be used to extend the class. Each object created for the class is also given two properties, a constructor property and a prototype property. The constructor property is a reference to the function that created this object, and the prototype property a reference to its prototype object. This prop- erty allows the object to share properties and methods. Figure 8.12 The book object’s properties. From the Library of WoweBook.Com . "text /javascript& quot;> 1 var Car = { // Create a Car object 2 make:undefined, year:2006, price:undefined, 3 owner:{ name:"Henry Lee", cell_phone: " ;222 -222 -222 2", 4. <body bgcolor="lightblue"> <script type="text /javascript& quot;> 8 myBook = new book(" ;JavaScript by Example", "Ellie", "Prentice Hall"); 9. 8.2 explained that unlike Java and C++, JavaScript doesn’t have a class mech- anism per se, but inheritance involves reusing or extending a class. With JavaScript we can simulate a class with
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