4 JavaScript: Introduction to Scripting

In Chapters 2 and 3, we introduced XHTML and Cascading Style Sheets (CSS). In this chapter, we begin our introduction to the JavaScript¹ scripting language, which facilitates a disciplined approach to designing computer programs that enhance the functionality and appearance of web pages.²

Chapters 4–9 present a detailed discussion of JavaScript—the de facto standard client-side scripting language for web-based applications, due to its highly portable nature. Our treatment of JavaScript introduces client-side scripting (used in Chapters 4–11), which makes web pages more dynamic and interactive.

We now introduce JavaScript programming and present examples that illustrate several important features of JavaScript. Each example is carefully analyzed one line at a time. In Chapters 5–6, we present a detailed treatment of program development and program control in JavaScript.

Before you can run code examples with JavaScript on your computer, you may need to change your browser’s security settings. By default, Internet Explorer 7 prevents scripts on your local computer from running, displaying a yellow warning bar at the top of the window instead. To allow scripts to run in files on your computer, select Internet Options from the Tools menu. Click the Advanced tab and scroll down to the Security section of the Settings list. Check the box labeled Allow active content to run in files on My Computer (Fig. 4.1). Click OK and restart Internet Explorer. XHTML documents on your own computer that contain JavaScript code will now run properly. Firefox has JavaScript enabled by default.

Fig. 4.1 | Enabling JavaScript in Internet Explorer 7

1. Don’t confuse the scripting language JavaScript with the programming language Java (from Sun Microsystems, Inc.). Java is a full-fledged object-oriented programming language. It can be used to develop applications that execute on a range of devices—from the smallest devices (such as cell phones and PDAs) to supercomputers. Java is popular for developing large-scale distributed enterprise applications and web applications. JavaScript is a browser-based scripting language developed by Netscape and implemented in all major browsers.

2. JavaScript was originally created by Netscape. Both Netscape and Microsoft have been instrumental in the standardization of JavaScript by ECMA International as ECMAScript. For more infomation on ECMAScript visit www.ecma-international.org/publications/standards/ECMA-262.htm.

4.2 Simple Program: Displaying a Line of Text in a Web Page

JavaScript uses notations that are familiar to programmers. We begin by considering a simple script (or program) that displays the text "Welcome to JavaScript Programming!" in the body of an XHTML document. All major web browsers contain JavaScript interpreters, which process the commands written in JavaScript. The JavaScript code and its output in Internet Explorer are shown in Fig. 4.2.

Fig. 4.2 | Displaying a line of text. (Part 1 of 2.)

Fig. 4.2 | Displaying a line of text. (Part 2 of 2.)

This program illustrates several important JavaScript features. We consider each line of the XHTML document and script in detail. As in the preceding chapters, we have given each XHTML document line numbers for your convenience; the line numbers are not part of the XHTML document or of the JavaScript programs. Lines 12–13 do the “real work” of the script, namely, displaying the phrase Welcome to JavaScript Programming! in the web page.

Line 8 indicates the beginning of the <head> section of the XHTML document. For the moment, the JavaScript code we write will appear in the <head> section. The browser interprets the contents of the <head> section first, so the JavaScript programs we write there execute before the <body> of the XHTML document displays. In later chapters on JavaScript and in the chapters on dynamic HTML, we illustrate inline scripting, in which JavaScript code is written in the <body> of an XHTML document.

Line 10 uses the <script> tag to indicate to the browser that the text which follows is part of a script. The type attribute specifies the type of file as well as the scripting language used in the script—in this case, a text file written in javascript. Both Internet Explorer and Firefox use JavaScript as the default scripting language.

Line 11 contains the XHTML opening comment tag <!--. Some older web browsers do not support scripting. In such browsers, the actual text of a script often will display in the web page. To prevent this from happening, many script programmers enclose the script code in an XHTML comment, so that browsers that do not support scripts will simply ignore the script. The syntax used is as follows:

<script type = "text/javascript">
   
</script>

When a browser that does not support scripts encounters the preceding code, it ignores the <script> and </script> tags and the script code in the XHTML comment. Browsers that do support scripting will interpret the JavaScript code as expected. [Note: Some browsers require the JavaScript single-line comment // (see Section 4.4 for an explanation) before the ending XHTML comment delimiter (-->) to interpret the script properly. The opening HTML comment tag (<!--) also serves as a single-line comment delimiter in JavaScript, therefore it does not need to be commented.]

Portability Tip 4.1

Some browsers do not support the <script>…</script> tags. If your document is to be rendered with such browsers, enclose the script code between these tags in an XHTML comment, so that the script text does not get displayed as part of the web page. The closing comment tag of the XHTML comment (-->) is preceded by a JavaScript comment (//) to prevent the browser from trying to interpret the XHTML comment as a JavaScript statement.

Lines 12–13 instruct the browser’s JavaScript interpreter to perform an action, namely, to display in the web page the string of characters contained between the double quotation (“) marks. A string is sometimes called a character string, a message or a string literal. We refer to characters between double quotation marks as strings. Individual white-space characters between words in a string are not ignored by the browser. However, if consecutive spaces appear in a string, browsers condense them to a single space. Also, in most cases, browsers ignore leading white-space characters (i.e., white space at the beginning of a string).

Software Engineering Observation 4.1

Strings in JavaScript can be enclosed in either double quotation marks (“) or single quotation marks (’).

Lines 12–13 use the browser’s document object, which represents the XHTML document the browser is currently displaying. The document object allows you to specify text to display in the XHTML document. The browser contains a complete set of objects that allow script programmers to access and manipulate every element of an XHTML document. In later chapters, we overview some of these objects as we discuss the Document Object Model (DOM).

An object resides in the computer’s memory and contains information used by the script. The term object normally implies that attributes (data) and behaviors (methods) are associated with the object. The object’s methods use the attributes to perform useful actions for the client of the object (i.e., the script that calls the methods). A method may require additional information (arguments) to perform its action; this information is enclosed in parentheses after the name of the method in the script. In lines 12–13, we call the document object’s writeln method to write a line of XHTML markup in the XHTML document. The parentheses following the method name writeln contain the one argument that method writeln requires (in this case, the string of XHTML that the browser is to display). Method writeln instructs the browser to display the argument string. If the string contains XHTML elements, the browser interprets these elements and renders them on the screen. In this example, the browser displays the phrase Welcome to JavaScript Programming! as an h1-level XHTML heading, because the phrase is enclosed in an h1 element.

The code elements in lines 12–13, including document.writeln, its argument in the parentheses (the string) and the semicolon (;), together are called a statement. Every statement ends with a semicolon (also known as the statement terminator), although this practice is not required by JavaScript. Line 15 indicates the end of the script.

Good Programming Practice 4.1

Always include a semicolon at the end of a statement to terminate the statement. This notation clarifies where one statement ends and the next statement begins.

Common Programming Error 4.1

Forgetting the ending </script> tag for a script may prevent the browser from interpreting the script properly and may prevent the XHTML document from loading or rendering properly.

The </head> tag in line 16 indicates the end of the <head> section. Also in line 16, the tags <body> and </body> specify that this XHTML document has an empty body. Line 17 indicates the end of this XHTML document.

We are now ready to view our XHTML document in a web browser—open it in Internet Explorer or Firefox. If the script contains no syntax errors, it should produce the output shown in Fig. 4.2.

Common Programming Error 4.2

JavaScript is case sensitive. Not using the proper uppercase and lowercase letters is a syntax error—a violation of the rules of writing correct statements in the programming language. The JavaScript interpreter in Internet Explorer reports all syntax errors by indicating in a separate popup window that a runtime error has occurred (i.e., a problem occurred while the interpreter was running the script). [Note: To enable this feature in IE7, select Internet Options… from the Tools menu. In the Internet Options dialog that appears, select the Advanced tab and click the checkbox labelled Display a notification about every script error under the Browsing category. Firefox has an error console that reports JavaScript errors and warnings. It is accessible by choosing Error Console from the Tools menu.]

4.3 Modifying Our First Program

This section continues our introduction to JavaScript programming with two examples that modify the example in Fig. 4.2.

Displaying a Line of Colored Text

A script can display Welcome to JavaScript Programming! several ways. Figure 4.3 uses two JavaScript statements to produce one line of text in the XHTML document. This example also displays the text in a different color, using the CSS color property.

Fig. 4.3 | Printing one line with separate statements. (Part 1 of 2.)

Fig. 4.3 | Printing one line with separate statements. (Part 2 of 2.)

Most of this document is identical to Fig. 4.2, so we concentrate only on lines 12–14 of Fig. 4.3, which display one line of text in the XHTML document. The first statement uses document method write to display a string. Unlike writeln, write does not position the output cursor in the XHTML document at the beginning of the next line after writing its argument. [Note: The output cursor keeps track of where the next character appears in the XHTML document, not where the next character appears in the web page as rendered by the browser.] The next character written in the XHTML document appears immediately after the last character written with write. Thus, when lines 13–14 execute, the first character written, “W,” appears immediately after the last character displayed with write (the > character inside the right double quote in line 12). Each write or writeln statement resumes writing characters where the last write or writeln statement stopped writing characters. So, after a writeln statement, the next output appears on the beginning of the next line. In effect, the two statements in lines 12–14 result in one line of XHTML text. Remember that statements in JavaScript are separated by semicolons (;). Therefore, lines 13–14 represent only one complete statement. JavaScript allows large statements to be split over many lines. However, you cannot split a statement in the middle of a string. The + operator (called the “concatenation operator” when used in this manner) in line 13 joins two strings together and is explained in more detail later in this chapter.

Common Programming Error 4.3

Splitting a statement in the middle of a string is a syntax error.

Note that the characters " (in line 12) are not displayed in the browser. The backslash () in a string is an escape character. It indicates that a “special” character is to be used in the string. When a backslash is encountered in a string of characters, the next character is combined with the backslash to form an escape sequence. The escape sequence " is the double-quote character, which causes a double-quote character to be inserted into the string. We use this escape sequence to insert double quotes around the attribute value for style without terminating the string. Note that we could also have used single quotes for the attribute value, as in document.write( "<h1 style = 'color: magenta'>" );, because the single quotes do not terminate a double-quoted string. We discuss escape sequences in greater detail momentarily.

It is important to note that the preceding discussion has nothing to do with the actual rendering of the XHTML text. Remember that the browser does not create a new line of text unless the browser window is too narrow for the text being rendered or the browser encounters an XHTML element that explicitly starts a new line—for example, <br /> to start a new line or <p> to start a new paragraph.

Common Programming Error 4.4

Many people confuse the writing of XHTML text with the rendering of XHTML text. Writing XHTML text creates the XHTML that will be rendered by the browser for presentation to the user.

Displaying Multiple Lines of Text

In the next example, we demonstrate that a single statement can cause the browser to display multiple lines by using line-break XHTML tags (<br />) throughout the string of XHTML text in a write or writeln method call. Figure 4.4 demonstrates the use of line-break XHTML tags. Lines 12–13 produce three separate lines of text when the browser renders the XHTML document.

Fig. 4.4 | Printing on multiple lines with a single statement.

Displaying Text in an Alert Dialog

The first several programs in this chapter display text in the XHTML document. Sometimes it is useful to display information in windows called dialogs (or dialog boxes) that “pop up” on the screen to grab the user’s attention. Dialogs typically display important messages to users browsing the web page. JavaScript allows you easily to display a dialog box containing a message. The program in Fig. 4.5 displays Welcome to JavaScript Programming! as three lines in a predefined dialog called an alert dialog.

Fig. 4.5 | Alert dialog displaying multiple lines.

Line 12 in the script uses the browser’s window object to display an alert dialog. The argument to the window object’s alert method is the string to display. Executing the preceding statement displays the dialog shown in the first window of Fig. 4.5. The title bar of the dialog contains the string Windows Internet Explorer to indicate that the browser is presenting a message to the user. The dialog provides an OK button that allows the user to dismiss (i.e., close) the dialog by clicking the button. To dismiss the dialog, position the mouse cursor (also called the mouse pointer) over the OK button and click the mouse. Firefox’s alert dialog looks similar, but the title bar contains the text [JavaScript Application].

Common Programming Error 4.5

Dialogs display plain text; they do not render XHTML. Therefore, specifying XHTML elements as part of a string to be displayed in a dialog results in the actual characters of the tags being displayed.

Note that the alert dialog contains three lines of plain text. Normally, a dialog displays the characters in a string exactly as they appear between the double quotes. Note, however, that the dialog does not display the characters . The escape sequence is the newline character. In a dialog, the newline character causes the cursor (i.e., the current screen position indicator) to move to the beginning of the next line in the dialog. Some other common escape sequences are listed in Fig. 4.6. The , and escape sequences in the table do not affect XHTML rendering unless they are in a pre element (this element displays the text between its tags in a fixed-width font exactly as it is formatted between the tags, including leading white-space characters and consecutive white-space characters). The other escape sequences result in characters that will be displayed in plain text dialogs and in XHTML.

Fig. 4.6 | Some common escape sequences.

Common Programming Error 4.6

XHTML elements in an alert dialog’s message are not interpreted as XHTML. This means that using <br />, for example, to create a line break in an alert box is an error. The string <br /> will simply be included in your message.

4.4 Obtaining User Input with prompt Dialogs

Scripting gives you the ability to generate part or all of a web page’s content at the time it is shown to the user. A script can adapt the content based on input from the user or other variables, such as the time of day or the type of browser used by the client. Such web pages are said to be dynamic, as opposed to static, since their content has the ability to change. The next two subsections use scripts to demonstrate dynamic web pages.

4.4.1 Dynamic Welcome Page

Our next script builds on prior scripts to create a dynamic welcome page that obtains the user’s name, then displays it on the page. The script uses another predefined dialog box from the window object—a prompt dialog—which allows the user to input a value that the script can use. The program asks the user to input a name, then displays the name in the XHTML document. Figure 4.7 presents the script and sample output. [Note: In later Java-Script chapters, we obtain input via GUI components in XHTML forms, as introduced in Chapter 2.]

Fig. 4.7 | Prompt box used on a welcome screen. (Part 1 of 2.)

Fig. 4.7 | Prompt box used on a welcome screen. (Part 2 of 2.)

Line 12 is a declaration that contains the JavaScript keyword var. Keywords are words that have special meaning in JavaScript. The keyword var at the beginning of the statement indicates that the word name is a variable. A variable is a location in the computer’s memory where a value can be stored for use by a program. All variables have a name, type and value, and should be declared with a var statement before they are used in a program. Although using var to declare variables is not required, we’ll see in Chapter 7, JavaScript: Functions, that var sometimes ensures proper behavior of a script.

The name of a variable can be any valid identifier. An identifier is a series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ($) that does not begin with a digit and is not a reserved JavaScript keyword. [Note: A complete list of keywords can be found in Fig. 5.1.] Identifiers may not contain spaces. Some valid identifiers are Welcome, $value, _value, m_inputField1 and button7. The name 7button is not a valid identifier, because it begins with a digit, and the name input field is not valid, because it contains a space. Remember that JavaScript is case sensitive—uppercase and lowercase letters are considered to be different characters, so name, Name and NAME are different identifiers.

Good Programming Practice 4.2

Choosing meaningful variable names helps a script to be “self-documenting” (i.e., easy to understand by simply reading the script, rather than having to read manuals or extended comments).

Good Programming Practice 4.3

By convention, variable-name identifiers begin with a lowercase first letter. Each subsequent word should begin with a capital first letter. For example, identifier itemPrice has a capital P

in its second word, Price.

Common Programming Error 4.7

Splitting a statement in the middle of an identifier is a syntax error.

Declarations end with a semicolon (;) and can be split over several lines with each variable in the declaration separated by a comma—known as a comma-separated list of variable names. Several variables may be declared either in one declaration or in multiple declarations.

In line 12, a single-line comment that begins with the characters // states the purpose of the variable in the script. This form of comment is called a single-line comment because it terminates at the end of the line in which it appears. A // comment can begin at any position in a line of JavaScript code and continues until the end of the line.

Good Programming Practice 4.4

Declare each variable on a separate line. This format allows for easy insertion of a descriptive comment next to each declaration—a widely followed professional coding standard.

Another comment notation facilitates writing multiline comments. For example,

/* This is a multiline
comment. It can be
split over many lines. */

is a multiline comment spread over several lines. Such comments begin with the delimiter /* and end with the delimiter */. All text between the delimiters of the comment is ignored by the interpreter.

Common Programming Error 4.8

Forgetting one of the delimiters of a multiline comment is a syntax error.

Common Programming Error 4.9

Nesting multiline comments (i.e., placing a multiline comment between the delimiters of another multiline comment) is a syntax error.

JavaScript adopted comments delimited with /* and */ from the C programming language and single-line comments delimited with // from the C++ programming language. JavaScript programmers generally prefer C++-style single-line comments over C-style comments. Throughout this book, we use C++-style single-line comments.

Line 14 is a comment indicating the purpose of the statement in the next line. Line 15 calls the window object’s prompt method, which displays the dialog in Fig. 4.8. The dialog allows the user to enter a string representing the user’s name.

Fig. 4.8 | Prompt dialog displayed by the window object’s prompt method.

The argument to prompt specifies a message telling the user what to type in the text field. An optional second argument, separated from the first by a comma, may specify the default string displayed in the text field; our code does not supply a second argument. In this case, Internet Explorer displays the default value undefined, while Firefox and most other browsers leave the text field empty. The user types characters in the text field, then clicks the OK button to submit the string to the program. We normally receive input from a user through a GUI component such as the prompt dialog, as in this program, or through an XHTML form GUI component, as we’ll see in later chapters.

The user can type anything in the text field of the prompt dialog. For this program, whatever the user enters is considered the name. If the user clicks the Cancel button, no string value is sent to the program. Instead, the prompt dialog submits the value null, a JavaScript keyword signifying that a variable has no value. Note that null is not a string literal, but rather a predefined term indicating the absence of value. Writing a null value to the document, however, displays the word null in the web page.

The statement in line 15 assigns the value returned by the window object’s prompt method (a string containing the characters typed by the user—or the default value or null if the Cancel button is clicked) to variable name by using the assignment operator, =. The statement is read as, “name gets the value returned by window.prompt( "Please enter your name" ).” The = operator is called a binary operator because it has two operands—name and the result of the expression window.prompt( "Please enter your name" ). The expression to the right of the assignment operator is always evaluated first.

Good Programming Practice 4.5

Place spaces on either side of a binary operator. This format makes the operator stand out and makes the program more readable.

Lines 17–18 use document.writeln to display the new welcome message. The expression inside the parentheses uses the operator + to “add” a string (the literal "<h1>Hello,"), the variable name (the string that the user entered in line 15) and another string (the literal ", welcome to JavaScript programming!</h1>"). JavaScript has a version of the + operator for string concatenation that enables a string and a value of another data type (including another string) to be combined. The result of this operation is a new (and normally longer) string. If we assume that name contains the string literal "Jim", the expression evaluates as follows: JavaScript determines that the two operands of the first + operator (the string "<h1>Hello," and the value of variable name) are both strings, then concatenates the two into one string. Next, JavaScript determines that the two operands of the second + operator (the result of the first concatenation operation, the string "<h1>Hello, Jim", and the string ", welcome to JavaScript programming!</h1>") are both strings and concatenates the two. This results in the string "<h1>Hello, Jim, welcome to JavaScript programming!</h1>". The browser renders this string as part of the XHTML document. Note that the space between Hello, and Jim is part of the string "<h1>Hello,".

As we’ll illustrate later, the + operator used for string concatenation can convert other variable types to strings if necessary. Because string concatenation occurs between two strings, JavaScript must convert other variable types to strings before it can proceed with the operation. For example, if a variable age has the integer value 21, then the expression "my age is" + age evaluates to the string "my age is 21". JavaScript converts the value of age to a string and concatenates it with the existing string literal "my age is".

After the browser interprets the <head> section of the XHTML document (which contains the JavaScript), it then interprets the <body> (lines 22–24) and renders the XHTML. Notice that the XHTML page is not rendered until the prompt is dismissed because the prompt pauses execution in the head, before the body is processed. If you click your browser’s Refresh (Internet Explorer) or Reload (Firefox) button after entering a name, the browser will reload the XHTML document, so that you can execute the script again and change the name. [Note: In some cases, it may be necessary to hold down the Shift key while clicking the Refresh or Reload button, to ensure that the XHTML document reloads properly. Browsers often save a recent copy of a page in memory, and holding the Shift key forces the browser to download the most recent version of a page.]

4.4.2 Adding Integers

Our next script illustrates another use of prompt dialogs to obtain input from the user. Figure 4.9 inputs two integers (whole numbers, such as 7, –11, 0 and 31914) typed by a user at the keyboard, computes the sum of the values and displays the result.

Fig. 4.9 | Addition script. (Part 1 of 2.)

Fig. 4.9 | Addition script. (Part 2 of 2.)

Lines 12–16 declare the variables firstNumber, secondNumber, number1, number2 and sum. Single-line comments state the purpose of each of these variables. Line 19 employs a prompt dialog to allow the user to enter a string representing the first of the two integers that will be added. The script assigns the first value entered by the user to the variable firstNumber. Line 22 displays a prompt dialog to obtain the second number to add and assign this value to the variable secondNumber.

As in the preceding example, the user can type anything in the prompt dialog. For this program, if the user either types a noninteger value or clicks the Cancel button, a logic error will occur, and the sum of the two values will appear in the XHTML document as NaN (meaning not a number). A logic error is caused by syntactically correct code that produces an undesired result. In Chapter 9, JavaScript: Objects, we discuss the Number object and its methods that can determine whether a value is not a number.

Recall that a prompt dialog returns to the program as a string the value typed by the user. Lines 25–26 convert the two strings input by the user to integer values that can be used in a calculation. Function parseInt converts its string argument to an integer. Line 25 assigns to the variable number1 the integer that function parseInt returns. Line 26 assigns an integer value to variable number2 in a similar manner. Any subsequent references to number1 and number2 in the program use these integer values. [Note: We refer to parseInt as a function rather than a method because we do not precede the function call with an object name (such as document or window) and a dot (.). The term method means that the function belongs to a particular object. For example, method writeln belongs to the document object and method prompt belongs to the window object.]

Line 28 calculates the sum of the variables number1 and number2 using the addition operator, +, and assigns the result to variable sum by using the assignment operator, =. Notice that the + operator can perform both addition and string concatenation. In this case, the + operator performs addition, because both operands contain integers. After line 28 performs this calculation, line 31 uses document.writeln to display the result of the addition on the web page. Lines 33 and 34 close the script and head elements, respectively. Lines 35–37 render the body of XHTML document. Use your browser’s Refresh or Reload button to reload the XHTML document and run the script again.

Common Programming Error 4.10

Confusing the + operator used for string concatenation with the + operator used for addition often leads to undesired results. For example, if integer variable y has the value 5, the expression “y + 2 = “ + y + 2 results in “y + 2 = 52”, not “y + 2 = 7”, because first the value of y (i.e., 5) is concatenated with the string “y + 2 = “, then the value 2 is concatenated with the new, larger string “y + 2 = 5”. The expression “y + 2 = “ + (y + 2) produces the string “y + 2 = 7” because the parentheses ensure that y + 2 is executed mathematically before it is conveted to a string.

4.5 Data Types in JavaScript

Unlike its predecessor languages C, C++ and Java, JavaScript does not require variables to have a declared type before they can be used in a program. A variable in JavaScript can contain a value of any data type, and in many situations JavaScript automatically converts between values of different types for you. For this reason, JavaScript is referred to as a loosely typed language. When a variable is declared in JavaScript, but is not given a value, the variable has an undefined value. Attempting to use the value of such a variable is normally a logic error.

When variables are declared, they are not assigned values unless specified by you. Assigning the value null to a variable indicates that it does not contain a value.

4.6 Arithmetic

Many scripts perform arithmetic calculations. The arithmetic operators use various special symbols that are not used in algebra. The asterisk (*) indicates multiplication; the percent sign (%) is the remainder operator, which will be discussed shortly. The arithmetic operators are binary operators—each operates on two operands. For example, the expression sum + value contains the binary operator + and the two operands sum and value.

JavaScript provides the remainder operator, %, which yields the remainder after division. [Note: The % operator is known as the modulus operator in some programming languages.] The expression x % y yields the remainder after x is divided by y. Thus, 17 % 5 yields 2 (i.e., 17 divided by 5 is 3, with a remainder of 2), and 7.4 % 3.1 yields 1.2. In later chapters, we consider applications of the remainder operator, such as determining whether one number is a multiple of another. There is no arithmetic operator for exponentiation in JavaScript. (Chapter 6, JavaScript: Control Statements II, shows how to perform exponentiation in JavaScript using the Math object’s pow method.)

4.7 Decision Making: Equality and Relational Operators

This section introduces a version of JavaScript’s if statement that allows a program to make a decision based on the truth or falsity of a condition. If the condition is met (i.e., the condition is true), the statement in the body of the if statement is executed. If the condition is not met (i.e., the condition is false), the statement in the body of the if statement is not executed. We’ll see an example shortly. [Note: Other versions of the if statement are introduced in Chapter 5, JavaScript: Control Statements I.]

Conditions in if statements can be formed by using the equality operators and relational operators summarized in Fig. 4.10. The relational operators all have the same level of precedence and associate from left to right. The equality operators both have the same level of precedence, which is lower than the precedence of the relational operators. The equality operators also associate from left to right.

Common Programming Error 4.11

It is a syntax error if the operators ==, !=, >= and <= contain spaces between their symbols, as in = =, ! =, > = and < =, respectively.

Common Programming Error 4.12

Reversing the operators !=, >= and <=, as in =!, => and =<, respectively, is a syntax error.

Common Programming Error 4.13

Confusing the equality operator, ==, with the assignment operator, =, is a logic error. The equality operator should be read as “is equal to,” and the assignment operator should be read as “gets” or “gets the value of.” Some people prefer to read the equality operator as “double equals” or “equals equals.”

Fig. 4.10 | Equality and relational operators.

The script in Fig. 4.11 uses four if statements to display a time-sensitive greeting on a welcome page. The script obtains the local time from the user’s computer and converts it from 24-hour clock format (0–23) to a 12-hour clock format (0–11). Using this value, the script displays an appropriate greeting for the current time of day. The script and sample output are shown in Fig. 4.11.

Fig. 4.11 | Using equality and relational operators. (Part 1 of 2.)

Fig. 4.11 | Using equality and relational operators. (Part 2 of 2.)

Lines 12–14 declare the variables used in the script. Remember that variables may be declared in one declaration or in multiple declarations. If more than one variable is declared in a single declaration (as in this example), the names are separated by commas (,). This list of names is referred to as a comma-separated list. Once again, note the comment at the end of each line, indicating the purpose of each variable in the program. Also note that some of the variables are assigned a value in the declaration—JavaScript allows you to assign a value to a variable when the variable is declared.

Line 13 sets the variable now to a new Date object, which contains information about the current local time. In Section 4.2, we introduced the document object, an object that encapsulates data pertaining to the current web page. Programmers may choose to use other objects to perform specific tasks or obtain particular pieces of information. Here, we use JavaScript’s built-in Date object to acquire the current local time. We create a new instance of an object by using the new operator followed by the type of the object, Date, and a pair of parentheses. Some objects require that arguments be placed in the parentheses to specify details about the object to be created. In this case, we leave the parentheses empty to create a default Date object containing information about the current date and time. After line 13 executes, the variable now refers to the new Date object. [Note: We did not need to use the new operator when we used the document and window objects because these objects always are created by the browser.] Line 14 sets the variable hour to an integer equal to the current hour (in a 24-hour clock format) returned by the Date object’s get-Hours method. Chapter 9 presents a more detailed discussion of the Date object’s attributes and methods, and of objects in general. As in the preceding example, the script uses window.prompt to allow the user to enter a name to display as part of the greeting (line 17).

To display the correct time-sensitive greeting, the script must determine whether the user is visiting the page during the morning, afternoon or evening. The first if statement (lines 20–21) compares the value of variable hour with 12. If hour is less than 12, then the user is visiting the page during the morning, and the statement at line 21 outputs the string "Good morning". If this condition is not met, line 21 is not executed. Line 24 determines whether hour is greater than or equal to 12. If hour is greater than or equal to 12, then the user is visiting the page in either the afternoon or the evening. Lines 25–36 execute to determine the appropriate greeting. If hour is less than 12, then the JavaScript interpreter does not execute these lines and continues to line 38.

The brace { in line 25 begins a block of statements (lines 27–35) that are executed if hour is greater than or equal to 12—to execute multiple statements inside an if construct, enclose them in curly braces. Line 27 subtracts 12 from hour, converting the current hour from a 24-hour clock format (0–23) to a 12-hour clock format (0–11). The if statement (line 30) determines whether hour is now less than 6. If it is, then the time is between noon and 6 PM, and line 31 outputs the beginning of an XHTML h1 element ("<h1>Good Afternoon,"). If hour is greater than or equal to 6, the time is between 6 PM and midnight, and the script outputs the greeting "Good Evening" (lines 34–35). The brace } in line 36 ends the block of statements associated with the if statement in line 24. Note that if statements can be nested, i.e., one if statement can be placed inside another if statement. The if statements that determine whether the user is visiting the page in the afternoon or the evening (lines 30–31 and lines 34–35) execute only if the script has already established that hour is greater than or equal to 12 (line 24). If the script has already determined the current time of day to be morning, these additional comparisons are not performed. (Chapter 5 presents a more in-depth discussion of blocks and nested if statements.) Finally, lines 38–39 output the rest of the XHTML h1 element (the remaining part of the greeting), which does not depend on the time of day.

Good Programming Practice 4.6

Include comments after the closing curly brace of control statements (such as if statements) to indicate where the statements end, as in line 36 of Fig. 4.11.

Note that there is no semicolon (;) at the end of the first line of each if statement. Including such a semicolon would result in a logic error at execution time. For example,

if ( hour <12 ) ;
document.write( "<h1>Good Morning, " );

would actually be interpreted by JavaScript erroneously as

if ( hour <12 )
;
document.write("<h1>Good Morning, " );

where the semicolon on the line by itself—called the empty statement—is the statement to execute if the condition in the if statement is true. When the empty statement executes, no task is performed in the program. The program then continues with the next statement, which executes regardless of whether the condition is true or false. In this example, "<h1>Good Morning, " would be printed regardless of the time of day.

Common Programming Error 4.14

Placing a semicolon immediately after the right parenthesis of the condition in an if statement is normally a logic error. The semicolon would cause the body of the if statement to be empty, so the if statement itself would perform no action, regardless of whether its condition was true. Worse yet, the intended body statement of the if statement would now become a statement in sequence after the if statement and would always be executed.

Common Programming Error 4.15

Leaving out a condition in a series of if statements is normally a logic error. For instance, checking if hour is greater than 12 or less than 12, but not if hour is equal to 12, would mean that the script takes no action when hour is equal to 12. Always be sure to handle every possible condition.

Note the use of spacing in lines 38–39 of Fig. 4.11. Remember that white-space characters, such as tabs, newlines and spaces, are normally ignored by the browser. So, statements may be split over several lines and may be spaced according to your preferences without affecting the meaning of a program. However, it is incorrect to split identifiers and string literals. Ideally, statements should be kept small, but it is not always possible to do so.

The chart in Fig. 4.12 shows the precedence of the operators introduced in this chapter. The operators are shown from top to bottom in decreasing order of precedence. Note that all of these operators, with the exception of the assignment operator, =, associate from left to right. Addition is left associative, so an expression like x + y + z is evaluated as if it had been written as (x + y) + z. The assignment operator, =, associates from right to left, so an expression like x = y = 0 is evaluated as if it had been written as x = (y = 0), which first assigns the value 0 to variable y, then assigns the result of that assignment, 0, to x.

Fig. 4.12 | Precedence and associativity of the operators discussed so far.

4.8 Web Resources

www.deitel.com/javascript

The Deitel JavaScript Resource Center contains links to some of the best JavaScript resources on the web. There you’ll find categorized links to JavaScript tutorials, tools, code generators, forums, books, libraries, frameworks and more.

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Table of Contents for 4 JavaScript: Introduction to Scripting

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