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CD-52 Part II . JavaScript Tutorial Listing 7-1: Calling a Function from an Event Handler

Parameters (also known as arguments) provide a mechanism for handing off a value from one statement to another by way of a function call. If no parameters occur in the function definition, both the function definition and call to the function have only empty sets of parentheses (as shown in Chapter 5, Listing 5-8). When a function receives parameters, it assigns the incoming values to the variable names specified in the function definition s parentheses. Consider the following script segment: function sayHiToFirst(a, b, c) { alert( Say hello, + a) } sayHiToFirst( Gracie , George , Harry ) sayHiToFirst( Larry , Moe , Curly ) After the function is defined in the script, the next statement calls that very function, passing three strings as parameters. The function definition automatically assigns the strings to variables a, b, and c. Therefore, before the alert()statement inside the function ever runs, aevaluates to Gracie, bevaluates to George, and c evaluates to Harry. In the alert()statement, only the a value is used and the alert reads Say hello, Gracie When the user closes the first alert, the next call to the function occurs. This time through, different values are passed to the function and assigned to a, b, and c. The alert dialog box reads Say hello, Larry Unlike other variables that you define in your script, function parameters do not use the varkeyword to initialize them. They are automatically initialized whenever the function is called.
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CD-51Chapter 7 .Programming Fundamentals, Part II Functions In Chapter 5, you saw a preview of the JavaScript function. A function is a definition of a set of deferred actions. Functions are invoked by event handlers or by statements elsewhere in the script. Whenever possible, good functions are designed for reuse in other documents. They can become building blocks you use over and over again. If you have programmed before, you can see parallels between JavaScript functions and other languages subroutines. But unlike some languages that distinguish between procedures (which carry out actions) and functions (which carry out actions and return values), only one classification of routine exists for JavaScript. A function is capable of returning a value to the statement that invoked it, but this is not a requirement. However, when a function does return a value, the calling statement treats the function call like any expression plugging in the returned value right where the function call is made. I will show some examples in a moment. Formal syntax for a function is as follows: function functionName ( [parameter1]…[,parameterN] ) { statement[s] } Names you assign to functions have the same restrictions as names you assign HTML elements and variables. You should devise a name that succinctly describes what the function does. I tend to use multiword names with the interCap (internally capitalized) format that start with a verb because functions are action items, even if they do nothing more than get or set a value. Another practice to keep in mind as you start to create functions is to keep the focus of each function as narrow as possible. It is possible to generate functions that are literally hundreds of lines long. Such functions are usually difficult to maintain and debug. Chances are that you can divide the long function into smaller, more tightly focused segments. Function parameters In Chapter 5, you saw how an event handler invokes a function by calling the function by name. Any call to a function, including one that comes from another JavaScript statement, works the same way: a set of parentheses follows the function name. You also can define functions so they receive parameter values from the calling statement. Listing 7-1 shows a simple document that has a button whose onClick event handler calls a function while passing text data to the function. The text string in the event handler call is in a nested string a set of single quotes inside the double quotes required for the entire event handler attribute.
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CD-50 Part II . JavaScript Tutorial About Repeat Loops Repeat loops in real life generally mean the repetition of a series of steps until some condition is met, thus enabling you to break out of that loop. Such was the case earlier in this chapter when you looked through a bushel of tomatoes for the one that came closest to your ideal tomato. The same can be said for driving around the block in a crowded neighborhood until a parking space opens up. A repeat loop lets a script cycle through a sequence of statements until some condition is met. For example, a JavaScript data validation routine might inspect every character that you enter into a form text field to make sure that each one is a number. Or if you have a collection of data stored in a list, the loop can check whether an entered value is in that list. Once that condition is met, the script can then break out of the loop and continue with the next statement after the loop construction. The most common repeat loop construction used in JavaScript is called the for loop. It gets its name from the keyword that begins the construction. A forloop is a powerful device because you can set it up to keep track of the number of times the loop repeats itself. The formal syntax of the forloop is as follows: for ([initial expression]; [condition]; [update expression]) { statement[s] inside loop } The square brackets mean that the item is optional. However, until you get to know the forloop better, I recommend designing your loops to utilize all three items inside the parentheses. The initial expression portion usually sets the starting value of a counter. The condition the same kind of condition you saw for if constructions defines the condition that forces the loop to stop going around and around. Finally, the update expression is a statement that executes each time all of the statements nested inside the construction complete running. A common implementation initializes a counting variable, i, increments the value of i by one each time through the loop, and repeats the loop until the value of i exceeds some maximum value, as in the following: for (var i = startValue; i <= maxValue; i++) { statement[s] inside loop } Placeholders startValue and maxValuerepresent any numeric values, including explicit numbers or variables holding numbers. In the update expression is an operator you have not seen yet. The ++operator adds 1 to the value of i each time the update expression runs at the end of the loop. If startValueis 1, the value of i is 1 the first time through the loop, 2 the second time through, and so on. Therefore, if maxValueis 10, the loop repeats itself 10 times (in other words, as long as iis less than or equal to 10). Generally speaking, the statements inside the loop use the value of the counting variable in their execution. Later in this lesson, I show how the variable can play a key role in the statements inside a loop. At the same time, you see how to break out of a loop prematurely and why you may need to do this in a script.
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CD-49Chapter 7 .Programming Fundamentals, Part II The following example assumes that a variable, myAge, has had its value set earlier in the script (exactly how is not important for this example). The condition expression compares the value myAge against a numeric value of 18. if (myAge < 18) { alert( Sorry, you cannot vote. ) } The data type of the value inside myAge must be a number so that the proper comparison (via the Note: In case you are looking for affordable and reliable webhost to host and run your j2ee application check Vision J2ee Web Hosting services.

CD-48 Part II . JavaScript Tutorial for an acceptable morsel. Your last stop in the store is the checkout aisle. Paper or plastic? the clerk asks. One more decision to make. What you choose impacts how you get the groceries from the car to the kitchen as well as your recycling habits. In your trip to the store, you go through the same kinds of decisions and repeti tions that your JavaScript programs also encounter. If you understand these frame works in real life, you can now look into the JavaScript equivalents and the syntax required to make them work. Control Structures In the vernacular of programming, the kinds of statements that make decisions and loop around to repeat themselves are called control structures. A control struc ture directs the execution flow through a sequence of script statements based on simple decisions and other factors. An important part of a control structure is the condition. Just as you may travel different routes to work depending on certain conditions (for example, nice weather, nighttime, attending a soccer game), so, too, does a program sometimes have to branch to an execution route if a certain condition exists. Each condition is an expression that evaluates to true or false one of those Boolean data types mentioned in Chapter 6. The kinds of expressions commonly used for conditions are expressions that include a comparison operator. You do the same in real life: If it is true that the outdoor temperature is less than freezing, then you put on a coat before going outside. In programming, however, the comparisons are strictly com parisons of number or string values. JavaScript provides several kinds of control structures for different programming situations. Three of the most common control structures you ll use are if construc tions, if…elseconstructions, and for loops. Chapter 39 covers in great detail other common control structures you should know, some of which were introduced only in Navigator 4 and Internet Explorer 4. For this tutorial, however, you need to learn about the three common ones just mentioned. if constructions The simplest program decision is to follow a special branch or path of the pro gram if a certain condition is true. Formal syntax for this construction follows. Items in italics get replaced in a real script with expressions and statements that fit the situation. if (condition) { statement[s] if true } Don t worry about the curly braces yet. Instead, get a feel for the basic structure. The keyword, if, is a must. In the parentheses goes an expression that evaluates to a Boolean value. This is the condition being tested as the program runs past this point. If the condition evaluates to true, then one or more statements inside the curly braces execute before continuing on with the next statement after the closing brace. If the condition evaluates to false, then the statements inside the curly brace are ignored and processing continues with the next statement after the clos ing brace.
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Programming Fundamentals, Part II Your tour of programming fundamentals continues in this chapter with subjects that have more intriguing possibilities. For example, I show you how programs make decisions and why a program must sometimes repeat statements over and over. Before you re finished here, you will learn how to use one of the most powerful information holders in the JavaScript language: the array. Decisions and Loops Every waking hour of every day you make decisions of some kind most of the time you probably don t even realize it. Don t think so? Well, look at the number of decisions you make at the grocery store, from the moment you enter the store to the moment you clear the checkout aisle. No sooner do you enter the store than you are faced with a decision. Based on the number and size of items you intend to buy, do you pick up a hand-carried basket or attempt to extri cate a shopping cart from the metallic conga line near the front of the store? That key decision may have impact later when you see a special offer on an item that is too heavy to put into the hand basket. Next, you head for the food aisles. Before entering an aisle, you compare the range of goods stocked in that aisle against items on your shopping list. If an item you need is likely to be found in this aisle, you turn into the aisle and start looking for the item; otherwise, you skip the aisle and move to the head of the next aisle. Later, you reach the produce section in search of a juicy tomato. Standing in front of the bin of tomatoes, you begin inspecting them one by one picking one up, feeling its firm ness, checking the color, looking for blemishes or signs of pests. You discard one, pick up another, and continue this process until one matches the criteria you set in your mind 7 CHAPTER …. In This Chapter How control structures make decisions How to define functions Where to initialize variables efficiently What those darned curly braces are all about The basics of data arrays ….
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CD-45Chapter 6 .Programming Fundamentals, Part I

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5. What does the term concatenate mean in the context of JavaScript programming? …
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CD-44 Part II . JavaScript Tutorial Exercises 1. Which of the following are valid variable declarations or initializations? Explain why each one is or is not valid. If an item is invalid, how do you fix it so that it is? a. my_name = Cindy b. var how many = 25 c. var zipCode = document.form1.zip.value d. var 1address = document.nameForm.address1.value 2. For each of the statements in the following sequence, write down how the someVal expression evaluates after the statement executes in JavaScript. var someVal = 2 someVal = someVal + 2 someVal = someVal * 10 someVal = someVal + 20 someVal = Robert 3. Name the two JavaScript functions that convert strings to numbers. How do you give the function a string value to convert to a number? 4. Type and load the HTML page and script shown in Listing 6-1. Enter a three- digit number into the top two fields and click the Add button. Examine the code and explain what is wrong with the script. How do you fix the script so the proper sum is displayed in the output field? Listing 6-1: What s Wrong with This Page?
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CD-43Chapter 6 .Programming Fundamentals, Part I Arithmetic operators It may seem odd to talk about text strings in the context of arithmetic operators, but you have already seen the special case of the plus (+) operator when one or more of the operands is a string. The plus operator instructs JavaScript to concatenate (pronounced kon-KAT-en-eight), or join, two strings together precisely where you place the operator. The string concatenation operator doesn t know about words and spaces, so the programmer must make sure that any two strings to be joined have the proper word spacing as part of the strings even if that means adding a space: firstName = John lastName = Doe fullName = firstName + + lastName JavaScript uses the same plus operator for arithmetic addition. When both operands are numbers, JavaScript knows to treat the expression as an arithmetic addition rather than a string concatenation. The standard math operators for addition, subtraction, multiplication, and division (+, -, *, /) are built into JavaScript. Comparison operators Another category of operator helps you compare values in scripts whether two values are the same, for example. These kinds of comparisons return a value of the Boolean type true or false. Table 6-2 lists the comparison operators. The operator that tests whether two items are equal consists of a pair of equal signs to distinguish it from the single equal sign assignment operator. Table 6-2 JavaScript Comparison Operators Symbol Description == Equals != Does not equal > Is greater than >= Is greater than or equal to < Is less than <= Is less than or equal to Where comparison operators come into greatest play is in the construction of scripts that make decisions as they run. A cook does this in the kitchen all the time: If the sauce is too watery, add a bit of flour. You see comparison operators in action in the next chapter.
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CD-42 Part II . JavaScript Tutorial Even though the second expression passes the string version of a floating-point number to the function, the value returned by the function is an integer. No round ing of the value occurs here (although other math functions can help with that if necessary). The decimal and everything to its right are simply stripped off. The parseFloat() function returns an integer if it can; otherwise, it returns a floating-point number as follows: parseFloat( 42 ) // result = 42 parseFloat( 42.33 ) // result = 42.33 Because these two conversion functions evaluate to their results, you simply insert the entire function wherever you need a string value converted to a number. Therefore, modifying an earlier example in which one of three values was a string, the complete expression can evaluate to the desired result: 3 + 3 + parseInt( 3 ) // result = 9 Converting numbers to strings You ll have less need for converting a number to its string equivalent than the other way around. As you saw in the previous section, JavaScript gravitates toward strings when faced with an expression containing mixed data types. Even so, it is good practice to perform data type conversions explicitly in your code to prevent any potential ambiguity. The simplest way to convert a number to a string is to take advantage of JavaScript s string tendencies in addition operations. By adding an empty string to a number, you convert the number to its string equivalent: ( + 2500) // result = 2500 ( + 2500).length // result = 4 In the second example, you can see the power of expression evaluation at work. The parentheses force the conversion of the number to a string. A string is a JavaScript object that has properties associated with it. One of those properties is the length property, which evaluates to the number of characters in the string. Therefore, the length of the string 2500 is 4. Note that the length value is a num ber, not a string. Operators You will use lots of operators in expressions. Earlier, you used the equal sign (=) as an assignment operator to assign a value to a variable. In the preceding exam ples with strings, you used the plus symbol (+) to join two strings. An operator gen erally performs some kind of calculation (operation) or comparison with two values (the value on each side of an operator is called an operand) to reach a third value. In this lesson, I briefly describe two categories of operators arithmetic and com parison. Chapter 40 covers many more operators, but once you understand the basics here, the others are easier to grasp.
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