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Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 1
Chapter 2
How to code a JavaScript application
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 2
Objectives
Applied
Using a text editor like Notepad++ or TextWrangler, enter and edit XHTML, CSS, and JavaScript files.
Using Firefox as your web browser, display any error messages in the Error Console when you test a JavaScript application.
Given the specifications for a JavaScript application that requires only the skills and language elements presented in this chapter, code, test, and debug the application.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 3
Objectives (continued)
Knowledge
Describe the use of the two types of JavaScript comments.
Describe the type of error that can occur if you split a JavaScript statement in the wrong place.
List the primary rules for creating a JavaScript identifier.
Describe the three primitive data types used in JavaScript.
Describe the use of the \n escape sequence in a string literal.
Describe the rules for evaluating an arithmetic expression, including order of precedence and the use of parentheses.
Describe the use of variable declarations and assignment statements.
Describe the use of objects, properties, methods, and object chaining.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 4
Objectives (continued) Describe the ways that number, string, Boolean, and Date objects
are created.
Given the name of a common method for a window, document, Number, String, Boolean, or Textbox object, describe the function of the method.
Describe the rules for evaluating a conditional expression, including the use of the isNan function, the order of precedence for logical operators, and the use of parentheses.
Describe the flow of control for an if, while, or for statement.
Describe the creation and use of functions.
Describe the creation and use of event handlers for the click event of a button element and the load event of the window object.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 5
Notepad++ with an auto-completion list
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 6
Basic skills for using Notepad++ How to open, save, close, and start files
How to change the style for comments
How to use the auto-completion feature
How to let Notepad++ know which language you’re working with
Note These skills are presented in figure 2-1.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 7
The Sales Tax application in the Firefox browser
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 8
How to test a web page Open the web page in your browser.
Enter the required input data, and run the application.
How to retest a web page after you change the source code To reload the edited file for the page, click the Reload or Refresh
button.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 9
If the Reload or Refresh button doesn’t clear browser memory… you may need to close the browser window and reopen the
application.
If the application doesn’t do anything when you test it… it either has syntax errors or other types of coding errors.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 10
The Firefox Error Console with an error displayed
How to display the Firefox Error Console Use the ToolsError Console command or press Ctrl+Shift+J.
How to display the source code Click on the link in the Error Console.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 11
The source code that’s displayed when you click on the link
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 12
How to load JavaScript from an external file <script type="text/javascript" src="sales_tax.js"></script>
How to embed JavaScript in the head of an XHTML document <head> ... <script type="text/javascript"> var $ = function (id) { return document.getElementById(id); } </script> ... </head>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 13
How to embed JavaScript in the body of an HTML document <p>© <script type="text/javascript"> var today = new Date(); document.writeln( today.getFullYear() ); </script> Mike's Bait and Tackle Shop</p>
How to load a script from a different web server <script type="text/javascript" src="http://www.google-analytics.com/urchin.js"> </script>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 14
How to use the noscript tag in the body of an XHTML document <script type="text/javascript"> var today = new Date(); document.writeln( today.getFullYear() ); </script> <noscript>2009</noscript>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 15
The basic syntax rules for JavaScript JavaScript is case-sensitive.
JavaScript statements end with a semicolon.
JavaScript ignores extra whitespace in statements.
Single-line comments begin with two forward slashes.
Multi-line comments begin with /* and end with */.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 16
A single-line comment nextYear = thisYear + 1; // Add 1 to this year
A multi-line comment /* The following line determines what the next year is by adding 1 to the current year */ nextYear = thisYear + 1;
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 17
How to split a statement across multiple lines Split a statement after an arithmetic or relational operator such as:
+, -, *, /, =, ==, >, or <.
Split a statement after an opening brace ( { ), bracket ( [ ), or parenthesis.
Split a statement after a closing brace ( } ).
Do not split a statement after an identifier, a value, or the return keyword.
Do not split a statement after a closing bracket ( ] ) or parenthesis.
A split statement that results in an error return "Hello";
A split statement that works correctly nextYear = thisYear + 1;
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 18
Rules for creating identifiers in JavaScript Identifiers can only contain letters, numbers, the underscore, and
the dollar sign.
Identifiers can’t start with a number.
Identifiers are case-sensitive.
Identifiers can be any length.
Identifiers can’t be the same as reserved words.
Avoid using global properties and methods as identifiers.
Avoid using names that are similar to reserved words, global properties, or global methods.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 19
Valid identifiers in JavaScript subtotal index_1 $ taxRate calculate_click $log
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 20
Examples of number values 15 // an integer -21 // a negative integer 21.5 // a floating-point value -124.82 // a negative floating-point value -3.7e-9 // a floating-point value // equivalent to -0.0000000037
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 21
The primitive data types Number
String
Boolean
Terms integer
floating-point value
exponent
empty string
escape sequence
Boolean value
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 22
Examples of string values "JavaScript" // a string with double quotes 'String Data' // a string with single quotes "" // an empty string
How the \n escape sequence can be used to start a new line in a string
"A valid variable name\ncannot start with a number." // represents the string: A valid variable name // cannot start with a number.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 23
The two Boolean values true // equivalent to true, yes, or on false // equivalent to false, no, or off
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 24
Common arithmetic operators + - * / % ++ --
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 25
The order of precedence for arithmetic expressions
Order Operators
1 ++
2 --
3 * / %
4 + -
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 26
Precedence and the use of parentheses 3 + 4 * 5 // Result is 23 (3 + 4) * 5 // Result is 35 13 % 4 + 9 // Result is 10 13 % (4 + 9) // Result is 0
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 27
Terms arithmetic expression
arithmetic operator
order of precedence
concatenate
concatenation operator
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 28
How to declare variables without assigning values to them var subtotal; // declares a variable named subtotal var lastName, state, zipCode; // declares three variables
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 29
The assignment operators = += -= *= /= %=
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 30
How to declare variables and assign values var subtotal = 74.95; // subtotal is 74.95 var salesTax = subtotal * .1; // salesTax is 7.495 var isValid = false; // Boolean value is false var zipCode = "93711", state = "CA"; // two assignments var firstName = "Ray", lastName = "Harris"; var fullName = lastName + ", " + firstName; // fullName is "Harris, Ray"
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 31
How to code compound assignment statements var subtotal = 24.50; subtotal += 75.50; // subtotal is 100 subtotal *= .9; // subtotal is 90 (100 * .9) var firstName = "Ray", lastName = "Harris"; var fullName = lastName; // fullName is "Harris" fullName += ", "; // fullName is "Harris, " fullName += firstName; // fullName is "Harris, Ray" var months = 120, message = "Months: "; message += months; // message is "Months: 120"
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 32
Terms variable
declare a variable
assignment statement
assignment operator
string literal
numeric literal
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 33
The syntax for creating a new object new ObjectType()
Examples that create new objects var today = new Date(); // creates a Date object var states = new Array(); // creates an Array object
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 34
The syntax for accessing a property of an object objectName.propertyName
Examples that access a property of an object alert(window.screenX); // Displays the width of the user's screen window.location = "http://www.murach.com"; // Loads the murach.com home page
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 35
The syntax for calling a method of an object objectName.methodName(parameters)
Examples that call a method of an object // Stores a reference to the XHTML element // with the id "rate" var rateTextbox = document.getElementById("rate");
// Gets the full year and writes it to the web page document.writeln( today.getFullYear() );
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 36
Examples of object chaining // Uses the alert method to display the value property // of the DOM object that is returned // by the getElementById method. alert( document.getElementById("rate").value );
// Uses the alert method to display the location property // of the current page // after it has been converted to lowercase letters // by the toLowerCase method. alert( window.location.toLowerCase() );
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 37
Terms object
property
method
call a method
parameter (or argument)
object chaining
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 38
One property of the window object location
Common methods of the window object alert(message) prompt(message,default) parseInt(string) parseFloat(string)
Examples that use these properties and methods alert("Invalid entry."); // displays "Invalid entry." var userEntry = prompt(errorMessage,100); // accepts user entry
parseInt("12345.67"); // returns the integer 12345 parseFloat("12345.67"); // returns the floating point // value 12345.67 parseFloat("Ray Harris"); // returns NaN
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 39
Common methods of the document object getElementById(id) write(text) writeln(text)
Examples that use these methods // gets the DOM object that represents // the XHTML element with "rate" as its id // and stores it in a variable named rateTextbox var rateTextbox = document.getElementById("rate");
// writes the string in the message variable to the document document.writeln(message);
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 40
Notes on using the window and document objects The window object is the global object when JavaScript is used in a
web browser.
JavaScript lets you omit the object name and period when referring to the window object.
The document object is the object that allows you to work with the DOM.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 41
One method of the Number object toFixed(digits)
Examples var balance = 2384.55678; // creates a number object named balance alert ( balance.toFixed(2) ); // displays 2384.56 // balance is still 2384.55678 balance = parseFloat( balance.toFixed(2) ); // balance is now 2384.56
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 42
Methods of the String object substr(start,length) toLowerCase() toUpperCase()
Examples var guest = "Ray Harris"; // creates a string object named guest alert ( guest.toUpperCase() ); // displays "RAY HARRIS" alert ( guest.substr(0,3) ); // displays "Ray" guest = guest.substr(4,6); // guest is now "Harris"
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 43
Methods of the Date object toDateString() getMonth() getDate() getFullYear()
Examples var today = new Date(); // creates a Date object named today // assume the current date is 09/20/2008 alert ( today.toDateString() ); // displays Sat Sep 20 2008 alert ( today.getFullYear() ); // displays 2008 alert ( today.getMonth() ); // displays 8, not 9, for September
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 44
Common properties of the Textbox object value disabled
Two XHTML <input> tags that create text boxes <input type="text" id="rate" /> <input type="text" id="salesTax" disabled="disabled" />
How to get the text box value in two statements // Store a reference to the text box var rateTextbox = document.getElementById("rate"); // Get the value and convert it to a number var rate = parseFloat( rateTextbox.value );
How to get the value with with object chaining var rate = parseFloat(document.getElementById("rate").value);
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 45
How to enable a text box document.getElementById("salesTax").disabled = false;
How to display a value in a text box // Assign an empty string to a text box document.getElementById("salesTax").value = ""; // Assign the value of a variable named salesTax // to a text box document.getElementById("salesTax").value = salesTax.toFixed(2);
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 46
One method of the Textbox object focus()
How to move the cursor to a text box document.getElementById("investment").focus();
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 47
The relational operators == != < <= > >=
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 48
The syntax of the global isNaN method isNaN(expression)
Examples of the isNaN method isNaN("Harris") // Returns true since "Harris" is not a number isNaN("123.45") // Returns false since "123.45" // can be converted to a number
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 49
The logical operators in order of precedence
Operator Description
! NOT
&& AND
|| OR
How the logical operators work Both tests with the AND operator must be true for the overall test
to be true.
At least one test with the OR operator must be true for the overall test to be true.
The NOT operator switches the result of the expression to the other Boolean value.
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 50
Terms to know conditional expression
relational operator
compound conditional expression
logical operator
One common programming error Confusing the assignment operator ( = )
with the equality operator ( ==).
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 51
An if statement with an else clause if ( age >= 18 ) { alert ("You may vote."); } else { alert ("You are not old enough to vote."); }
An if statement with else if and else clauses if ( isNaN(rate) ) { alert ("You did not provide a number for the rate."); } else if ( rate < 0 ) { alert ("The rate may not be less than zero."); } else { alert ("The rate is: " + rate + "."); }
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 52
An if statement with a compound conditional expression
if ( isNaN(userEntry) || userEntry <= 0 ) { alert ("Please enter a valid number > zero."); }
An if statement that tests a Boolean value if ( invalidFlag ) { alert ("All entries must be numeric values > 0."); }
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 53
A nested if statement if ( isNaN(totalMonths) || totalMonths <= 0 ) { alert ("Please enter a number of months > zero."); } else { var years = parseInt ( totalMonths / 12 ); var months = totalMonths % 12; if ( years == 0 ) { alert ( "The investment time is " + months + " months."); } else if ( months == 0 ) { alert ( "The investment time is " + years + " years."); } else { var message = "The investment time is " + years + " years "; message += "and " + months + " months."; alert(message); } }
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 54
A while loop that adds 1 through 5 var sumOfNumbers = 0; var loops = 5; var counter = 1; while (counter <= loops) { sumOfNumbers += counter; counter++; } alert(sumOfNumbers); // displays 15
A for loop that adds 1 through 5 var sumOfNumbers = 0; var loops = 5; for (var counter = 1; counter <= loops; counter++) { sumOfNumbers += counter; } alert(sumOfNumbers); // displays 15
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 55
A while loop that gets a user entry userEntry = prompt("Please enter a number:", 100); while ( isNaN( userEntry ) ) { alert( "You did not enter a number."); userEntry = prompt("Please enter a number:", 100); }
A for loop that calculates the future value of a monthly investment
var monthlyInvestment = 100; // investment is $100 var monthlyRate = .01; // interest rate is 12% var months = 120; // 120 months is 10 years var futureValue = 0; // futureValue starts at 0 for ( i = 1; i <= months; i++ ) { futureValue = ( futureValue + monthlyInvestment ) * (1 + monthlyRate); }
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 56
A function with no parameters var showYear = function () { var today = new Date(); alert( today.getFullYear() ); }
How to call a function that doesn’t return a value showYear(); // displays the current year
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 57
A function with one parameter that returns a value var $ = function ( id ) { return document.getElementById( id ); }
How to call a function that returns a value var taxRate = $("taxRate");
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 58
A function with two parameters that doesn’t return a value
var displayEmail = function ( username, domain ) { document.write( username + "@" + domain); }
How to call a function with two parameters that doesn’t return a value
displayEmail( "mike", "murach.com");
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 59
A function with two parameters that returns a value
var calculateTax = function ( subtotal, taxRate ) { var tax = subtotal * taxRate; tax = parseFloat( tax.toFixed(2) ); return tax; }
How to call a function with two parameters that returns a value
var subtotal = 74.95; var taxRate = 0.07; var salesTax = calculateTax( subtotal, taxRate ); // returns 5.25
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 60
Terms to know function
return statement
call a function
return undefined
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 61
Common events
Object Event
button onclick
window onload
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 62
An application with an onclick event handler <head> <script type="text/javascript"> // This function receives an id and returns an XHTML object. var $ = function ( id ) { return document.getElementById( id ); } // This is the onclick event handler named display_click. var display_click = function () { alert( "You clicked the button."); } // This is the onload event handler that assigns the // display_click handler to the click event of the button. window.onload = function () { $("btnDisplay").onclick = display_click; } </script> </head> <body> <p><input type="button" value="Display Message" id="btnDisplay" /></p>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 63
The web browser after the Display Message button is clicked
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 64
The Future Value application in a web browser
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 65
The XHTML for the Future Value application <head> <title>Future Value Calculator</title> <link rel="stylesheet" type="text/css" href="future_value.css" /> <script type="text/javascript" src="future_value.js"></script> </head>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 66
The XHTML (continued) <body> <h1>Future Value Calculator</h1> <p>Enter the values below and click "Calculate".</p> <label for="investment">Monthly Investment:</label> <input type="text" id="investment" /><br /> <label for="rate">Annual Interest Rate:</label> <input type="text" id="rate" />%<br /> <label for="years">Number of Years:</label> <input type="text" id="years" /><br /> <label for="futureValue">Future Value:</label> <input type="text" id="futureValue" disabled="disabled" /><br /> <label> </label> <input type="button" id="calculate" value="Calculate" /><br /> </body>
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 67
The JavaScript for the Future Value application var $ = function (id) { return document.getElementById(id); } window.onload = function () { $("calculate").onclick = calculate_click; $("investment").focus(); }
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 68
The JavaScript (continued) var calculate_click = function () { // Get the user entries from the first three text boxes. var investment = parseFloat( $("investment").value ); var annualRate = parseFloat( $("rate").value ); var years = parseInt( $("years").value ); // Set the value of the fourth text box. $("futureValue").value = ""; // Test the three input values for validity. if (isNaN(investment) || investment <= 0) { alert("Investment must be a valid number\nand greater than zero."); } else if(isNaN(annualRate) || annualRate <= 0) { alert("Annual rate must be a valid number\nand greater than zero."); } else if(isNaN(years) || years <= 0) { alert("Years must be a valid number\nand greater than zero.");
Murach’s JavaScript, C2 © 2009, Mike Murach & Associates, Inc. Slide 69
The JavaScript (continued) // If all input values are valid, calculate future value. } else { var monthlyRate = annualRate / 12 / 100; var months = years * 12; var futureValue = 0; for ( i = 1; i <= months; i++ ) { futureValue = ( futureValue + investment ) * (1 + monthlyRate); } // Set the value of the fourth text box to the future value $("futureValue").value = futureValue.toFixed(2); } }