Bài giảng Web Technologies and e-Services - Bài 4: Javascript

Javascript 1 Content Client-side programming with JavaScript § scripts vs. programs ØJavaScript vs. JScript vs. VBScript Øcommon tasks for client-side scripts § JavaScript Ødata types & expressions Øcontrol statements Øfunctions & libraries Østrings & arrays ØDate, document, navigator, user-defined classes 2 Client-Side Programming v HTML is good for developing static pages § can specify text/image layout, presentation, links, § Web page looks the same each time it is accessed v Client-side programming § programs are written in a separate programming (or scripting) language e.g., JavaScript, JScript, VBScript § programs are embedded in the HTML of a Web page, with (HTML) tags to identify the program component e.g., § the browser executes the program as it loads the page, integrating the dynamic output of the program with the static content of HTML § could also allow the user (client) to input information and process it, might be used to validate input before it’s submitted to a remote server 3 Scripts vs. Programs v A scripting language is a simple, interpreted programming language § scripts are embedded as plain text, interpreted by application § simpler execution model: don't need compiler or development environment § saves bandwidth: source code is downloaded, not compiled executable § platform-independence: code interpreted by any script-enabled browser § but: slower than compiled code, not as powerful/full-featured JavaScript: the first Web scripting language, developed by Netscape in 1995 syntactic similarities to Java/C++, but simpler, more flexible in some respects, limited in others (loose typing, dynamic variables, simple objects) JScript: Microsoft version of JavaScript, introduced in 1996 • same core language, but some browser-specific differences • fortunately, IE, Netscape, Firefox, etc. can (mostly) handle both VBScript: client-side scripting version of Microsoft Visual Basic 4 Common Scripting Tasks v adding dynamic features to Web pages § validation of form data (probably the most commonly used application) § image rollovers § time-sensitive or random page elements § handling cookies v defining programs with Web interfaces § utilize buttons, text boxes, clickable images, prompts, etc v limitations of client-side scripting § since script code is embedded in the page, it is viewable to the world § for security reasons, scripts are limited in what they can do e.g., can't access the client's hard drive § since they are designed to run on any machine platform, scripts do not contain platform specific commands 5 JavaScript v JavaScript code can be embedded in a Web page using tags § the output of JavaScript code is displayed as if directly entered in HTML JavaScript Page // silly code to demonstrate output document.write("Hello world!"); document.write(" How are " + " you? "); Here is some static text as well. document.write displays text in the page text to be displayed can include HTML tags the tags are interpreted by the browser when the text is displayed as in C++/Java, statements end with ; but a line break might also be interpreted as the end of a statement (depends upon browser) JavaScript comments similar to C++/Java // starts a single line comment /**/ enclose multi-line comments view page 6 JavaScript Data Types & Variables v JavaScript has only three primitive data types String : "foo" 'how do you do?' "I said 'hi'." "" Number: 12 3.14159 1.5E6 Boolean : true false *Find info on Null, Undefined Data Types and Variables var x, y; x= 1024; y=x; x = "foobar"; document.write("x = " + y + ""); document.write("x = " + x + ""); assignments are as in C++/Java message = "howdy"; pi = 3.14159; variable names are sequences of letters, digits, and underscores that start with a letter or an underscore variables names are case-sensitive you don't have to declare variables, will be created the first time used, but it’s better if you use var statements var message, pi=3.14159; variables are loosely typed, can be assigned different types of values (Danger!) view page 7 JavaScript Operators & Control Statements Folding Puzzle var distanceToSun = 93.3e6*5280*12; var thickness = .002; var foldCount = 0; while (thickness < distanceToSun) { thickness *= 2; foldCount++; } document.write("Number of folds = " + foldCount); standard C++/Java operators & control statements are provided in JavaScript • +, -, *, /, %, ++, --, • ==, !=, , = • &&, ||, !,===,!== • if , if-else, switch • while, for, do-while, PUZZLE: Suppose you took a piece of paper and folded it in half, then in half again, and so on. How many folds before the thickness of the paper reaches from the earth to the sun? *Lots of information is available online view page 8 JavaScript Math Routines Random Dice Rolls var roll1 = Math.floor(Math.random()*6) + 1; var roll2 = Math.floor(Math.random()*6) + 1; document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll1 + ".gif‘ alt=‘dice showing ‘ + roll1 />"); document.write("  "); document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll2 + ".gif‘ alt=‘dice showing ‘ + roll2 />"); the built-in Math object contains functions and constants Math.sqrt Math.pow Math.abs Math.max Math.min Math.floor Math.ceil Math.round Math.PI Math.E Math.random function returns a real number in [0..1) view page 9 Interactive Pages Using Prompt Interactive page var userName = prompt("What is your name?", ""); var userAge = prompt("Your age?", ""); var userAge = parseFloat(userAge); document.write("Hello " + userName + ".") if (userAge < 18) { document.write(" Do your parents know " + "you are online?"); } else { document.write(" Welcome friend!"); } The rest of the page... crude user interaction can take place using prompt 1st argument: the prompt message that appears in the dialog box 2nd argument: a default value that will appear in the box (in case the user enters nothing) the function returns the value entered by the user in the dialog box (a string) if value is a number, must use parseFloat (or parseInt) to convert forms will provide a better interface for interaction (later) view page 10 User-Defined Functions v function definitions are similar to C++/Java, except: § no return type for the function (since variables are loosely typed) § no variable typing for parameters (since variables are loosely typed) § by-value parameter passing only (parameter gets copy of argument) function isPrime(n) // Assumes: n > 0 // Returns: true if n is prime, else false { if (n < 2) { return false; } else if (n == 2) { return true; } else { for (var i = 2; i <= Math.sqrt(n); i++) { if (n % i == 0) { return false; } } return true; } } Can limit variable scope to the function. if the first use of a variable is preceded with var, then that variable is local to the function for modularity, should make all variables in a function local 11 Function Example Prime Tester function isPrime(n) // Assumes: n > 0 // Returns: true if n is prime { // CODE AS SHOWN ON PREVIOUS SLIDE } testNum = parseFloat(prompt("Enter a positive integer", "7")); if (isPrime(testNum)) { document.write(testNum + " is a prime number."); } else { document.write(testNum + " is not a prime number."); } Function definitions (usually) go in the section section is loaded first, so then the function is defined before code in the is executed (and, therefore, the function can be used later in the body of the HTML document) view page 12 Another Example Random Dice Rolls Revisited function randomInt(low, high) // Assumes: low <= high // Returns: random integer in range [low..high] { return Math.floor(Math.random()*(high-low+1)) + low; } roll1 = randomInt(1, 6); roll2 = randomInt(1, 6); document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll1 + ".gif'/>"); document.write("  "); document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll2 + ".gif'/>"); recall the dynamic dice page could define a function for generating random numbers in a range, then use whenever needed easier to remember, promotes reuse view page 13 JavaScript Libraries better still: if you define functions that may be useful to many pages, store in a separate library file and load the library when needed load a library using the SRC attribute in the SCRIPT tag (put nothing between the beginning and ending tags) <script type="text/javascript" src="random.js"> 14 Library Example Random Dice Rolls Revisited <script type="text/javascript" src="random.js"> roll1 = randomInt(1, 6); roll2 = randomInt(1, 6); document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll1 + ".gif'/>"); document.write("  "); document.write("<img src='"+ "~martin/teaching/CS443/Images/die" + roll2 + ".gif'/>"); view page 15 JavaScript Objects v an object defines a new type (formally, Abstract Data Type) § encapsulates data (properties) and operations on that data (methods) v a String object encapsulates a sequence of characters, enclosed in quotes properties include • length : stores the number of characters in the string methods include • charAt(index): returns the character stored at the given index (as in C++/Java, indices start at 0) • substring(start, end) : returns the part of the string between the start (inclusive) and end (exclusive) indices • toUpperCase() : returns copy of string with letters uppercase • toLowerCase() : returns copy of string with letters lowercase to create a string, assign using new or (in this case) just make a direct assignment (new is implicit) word = new String("foo"); word = "foo"; properties/methods are called exactly as in C++/Java • word.length word.charAt(0) 16 String example: Palindromes function strip(str) // Assumes: str is a string // Returns: str with all but letters removed { var copy = ""; for (var i = 0; i < str.length; i++) { if ((str.charAt(i) >= "A" && str.charAt(i) <= "Z") || (str.charAt(i) >= "a" && str.charAt(i) <= "z")) { copy += str.charAt(i); } } return copy; } function isPalindrome(str) // Assumes: str is a string // Returns: true if str is a palindrome, else false { str = strim(str.toUpperCase()); for(var i = 0; i < Math.floor(str.length/2); i++) { if (str.charAt(i) != str.charAt(str.length-i-1)) { return false; } } return true; } suppose we want to test whether a word or phrase is a palindrome noon Radar Madam, I'm Adam. A man, a plan, a canal: Panama! must strip non-letters out of the word or phrase make all chars uppercase in order to be case-insensitive finally, traverse and compare chars from each end 17 Palindrome Checker function strip(str) { // CODE AS SHOWN ON PREVIOUS SLIDE } function isPalindrome(str) { // CODE AS SHOWN ON PREVIOUS SLIDE } text = prompt("Enter a word or phrase", "Madam, I'm Adam"); if (isPalindrome(text)) { document.write("'" + text + "' is a palindrome."); } else { document.write("'" + text + "' is not a palindrome."); } view page 18 JavaScript Arrays v arrays store a sequence of items, accessible via an index since JavaScript is loosely typed, elements do not have to be the same type § to create an array, allocate space using new (or can assign directly) items = new Array(10); // allocates space for 10 items items = new Array(); // if no size given, will adjust dynamically items = [0,0,0,0,0,0,0,0,0,0]; // can assign size & values [] § to access an array element, use [] (as in C++/Java) for (i = 0; i < 10; i++) { items[i] = 0; // stores 0 at each index } § the length property stores the number of items in the array for (i = 0; i < items.length; i++) { document.write(items[i] + ""); // displays elements } 19 Array Example Dice Statistics <script type="text/javascript" src=""> numRolls = 60000; diceSides = 6; rolls = new Array(dieSides+1); for (i = 1; i < rolls.length; i++) { rolls[i] = 0; } for(i = 1; i <= numRolls; i++) { rolls[randomInt(1, dieSides)]++; } for (i = 1; i < rolls.length; i++) { document.write("Number of " + i + "'s = " + rolls[i] + ""); } suppose we want to simulate dice rolls and verify even distribution keep an array of counters: initialize each count to 0 each time you roll X, increment rolls[X] display each counter view page 20 Arrays (cont.) 21 • Arrays have predefined methods that allow them to be used as stacks, queues, or other common programming data structures. var stack = new Array(); stack.push("blue"); stack.push(12); // stack is now the array ["blue", 12] stack.push("green"); // stack = ["blue", 12, "green"] var item = stack.pop(); // item is now equal to "green" var q = [1,2,3,4,5,6,7,8,9,10]; item = q.shift(); // item is now equal to 1, remaining // elements of q move down one position // in the array, e.g. q[0] equals 2 q.unshift(125); // q is now the array [125,2,3,4,5,6,7,8,9,10] q.push(244); // q = [125,2,3,4,5,6,7,8,9,10,244] Date Object v String & Array are the most commonly used objects in JavaScript § other, special purpose objects also exist v the Date object can be used to access the date and time § to create a Date object, use new & supply year/month/day/ as desired today = new Date(); // sets to current date & time newYear = new Date(2002,0,1); //sets to Jan 1, 2002 12:00AM § methods include: newYear.getFullYear() can access individual components of a date newYear.getMonth() number (0, 11) newYear.getDay() number (1, 31) newYear.getHours() number (0, 23) newYear.getMinutes() number (0, 59) newYear.getSeconds() number (0, 59) newYear.getMilliseconds() number (0, 999) 22 Date Example Time page Time when page was loaded: now = new Date(); document.write("" + now + ""); time = "AM"; hours = now.getHours(); if (hours > 12) { hours -= 12; time = "PM" } else if (hours == 0) { hours = 12; } document.write("" + hours + ":" + now.getMinutes() + ":" + now.getSeconds() + " " + time + ""); by default, a date will be displayed in full, e.g., Sun Feb 03 22:55:20 GMT-0600 (Central Standard Time) 2002 can pull out portions of the date using the methods and display as desired here, determine if "AM" or "PM" and adjust so hour between 1-12 10:55:20 PM view page 23 Another Example Time page Elapsed time in this year: now = new Date(); newYear = new Date(2012,0,1); secs = Math.round((now-newYear)/1000); days = Math.floor(secs / 86400); secs -= days*86400; hours = Math.floor(secs / 3600); secs -= hours*3600; minutes = Math.floor(secs / 60); secs -= minutes*60 document.write(days + " days, " + hours + " hours, " + minutes + " minutes, and " + secs + " seconds."); you can add and subtract Dates: the result is a number of milliseconds here, determine the number of seconds since New Year's day (note: January is month 0) divide into number of days, hours, minutes and seconds view page 24 Document Object Internet Explorer, Firefox, Opera, etc. allow you to access information about an HTML document using the document object Documentation page document.write(document.URL); document.write(document.lastModified); document.write() method that displays text in the page document.URL property that gives the location of the HTML document document.lastModified property that gives the date & time the HTML document was last changed view page 25 Navigator Object Dynamic Style Page if (navigator.appName == "Netscape") { document.write('<link rel=stylesheet '+ 'type="text/css" href="Netscape.css">'); } else { document.write('<link rel=stylesheet ' + 'type="text/css" href="MSIE.css">'); } Here is some text with a <a href="javascript:alert('GO AWAY')">link. a {text- decoration:none; font- size:larger; color:red; font- family:Arial} a:hover {color:blue} <!-- Netscape.css --> a {font- family:Arial; color:white; background- color:red} navigator.appNam e property that gives the browser name navigator.appVer sion property that gives the browser version view page 26 User-Defined Objects v can define new objects, but the notation can be somewhat awkward § simply define a function that serves as a constructor § specify data fields & methods using this § no data hiding: can't protect data or methods // CS443 Die.js 11.10.2011 // // Die class definition //////////////////////////////////////////// function Die(sides) { this.numSides = sides; this.numRolls = 0; this.roll = roll; // define a pointer to a function } function roll() { this.numRolls++; return Math.floor(Math.random()*this.numSides) + 1; } define Die function (i.e., the object's constructor) initialize data fields in the function, preceded with "this" similarly, assign method to separately defined function (which uses this to access data) 27 Object Example 28 Dice page <script type="text/javascript" src="Die.js"> die6 = new Die(6); die8 = new Die(8); roll6 = -1; // dummy value to start loop roll8 = -2; // dummy value to start loop while (roll6 != roll8) { roll6 = die6.roll(); roll8 = die8.roll(); document.write("6-sided: " + roll6 + "    " + "8-sided: " + roll8 + ""); } document.write("Number of rolls: " + die6.numRolls); create a Die object using new (similar to String and Array) here, the argument to Die initializes numSides for that particular object each Die object has its own properties (numSides & numRolls) Roll(), when called on a particular Die, accesses its numSides property and updates its NumRolls view page JavaScript and HTML validators •In order to use an HTML validator, and not get error messages from the JavaScript portions, you must “mark” the JavaScipt sections in a particular manner. Otherwise the validator will try to interpret the script as HTML code. •To do this, you can use a markup like the following in your inline code (this isn’t necessary for scripts stored in external files). // <![CDATA[ document.write(“The quick brown fox jumped over the lazy dogs.”); // **more code here, etc. // ]]> 29 30 •Since the (new) XHTML standard is written as an XML application, validators such as the one from the W3C are actually attempting to check an XML document for the correct structure. •The two tags together form an XML directive, meaning to interpret the data between them as literal (non-parsed) “character data”. An XML validator will effectively ignore the data between these two tags, meaning that any symbols that would result in an invalid document structure are ignored and do not result in an error message from the validator. •Because we are using these tags inside of a JavaScript block, and they are not JavaScript commands, we precede each of them with a (JavaScript) comment marker, hence the two forward slashes before each tag. More to learn v Accessing elements on the page using JavaScript functions v JavaScript and forms v Events, capturing user input v The Document Object Model, and manipulating the webpage 31 32 Numbers v In JavaScript, all numbers are floating point v Special predefined numbers: § Infinity, Number.POSITIVE_INFINITY -- the result of dividing a positive number by zero § Number.NEGATIVE_INFINITY -- the result of dividing a negative number by zero § NaN, Number.NaN (Not a Number) -- the result of dividing 0/0 • NaN is unequal to everything, even itself • There is a global isNaN() function § Number.MAX_VALUE -- the largest representable number § Number.MIN_VALUE -- the smallest (closest to zero) representable number 32 33 Strings and characters v In JavaScript, string is a primitive type v Strings are surrounded by either single quotes or double quotes v There is no “character” type v Special characters are: \0 NUL \b backspace \f form feed \n newline \r carriage return \t horizontal tab \v vertical tab \' single quote \" double quote \\ backslash \xDD Unicode hex DD \xDDDD Unicode hex DDDD 33 34 Some string methods v charAt(n) § Returns the nth character of a string v concat(string1, ..., stringN) § Concatenates the string arguments to the recipient string v indexOf(substring) § Returns the position of the first character of substring in the recipient string, or -1 if not found v indexOf(substring, start) § Returns the position of the first character of substring in the given string that begins at or after position start, or -1 if not found v lastIndexOf(substring), lastIndexOf(substring, start) § Like indexOf, but searching starts from the end of the recipient string 34 35 More string methods v match(regexp) § Returns an array containing the results, or null if no match is found § On a successful match: • If g (global) is set, the array contains the matched substrings • If g is not set: • Array location 0 contains the matched text • Locations 1... contain text matched by parenthesized groups • The array index property gives the first matched position v replace(regexp, replacement) § Returns a new string that has the matched substring replaced with the replacement v search(regexp) § Returns the position of the first matched substring in the given string, or -1 if not found. 35 36 boolean v The boolean values are true and false v When converted to a boolean, the following values are also false: § 0 § "0" and '0' § The empty string, '' or "" § undefined § null § NaN 36 37 undefined and null v There are special values undefined and null v undefined is the only value of its “type” § This is the value of a variable that has been declared but not defined, or an object property that does not exist § void is an operator that, applied to any value, returns the value undefined v null is an “object” with no properties v null and undefined are == but not === 37 38 Arrays v As in C and Java, there are no “true” multidimensional arrays § However, an array can contain arrays § The syntax for array reference is as in C and Java v Example: var a = [ ["red", 255], ["green", 128] ]; var b = a[1][0]; // b is now "green" var c = a[1]; // c is now ["green", 128] var d = c[1]; // d is now 128 38 39 Determining types v The unary operator typeof returns one of the following strings: "number", "string", "boolean", "object", "undefined", and "function" § typeof null is "object" § If myArray is an array, typeof myArray is "object" 39 40 Wrappers and conversions v JavaScript has “wrapper” objects for when a primitive value must be treated as an object § var s = new String("Hello"); // s is now a String § var n = new Number(5); // n is now a Number § var b = new Boolean(true); // b is now a Boolean § Because JavaScript does automatic conversions as needed, wrapper objects are hardly ever needed v JavaScript has no “casts,” but conversions can be forced § var s = x + ""; // s is now a string § var n = x + 0; // n is now a number § var b = !!x; // b is now a boolean § Because JavaScript does automatic conversions as needed, explicit conversions are hardly ever needed 40 41 Variables v Every variable is a property of an object v When JavaScript starts, it creates a global object v In client-side JavaScript, the window is the global object § It can be referred to as window or as this § The “built-in” variables and methods are defined here v There can be more than one “global” object § For example, one frame can refer to another frame with code such as parent.frames[1] v Local variables in a function are properties of a special call object 41 42 HTML names in JavaScript v In HTML the window is the global object § It is assumed that all variables are properties of this object, or of some object descended from this object § The most important window property is document v HTML form elements can be referred to by document.forms[formNumber].elements[elementNumber] v Every HTML form element has a name attribute § The name can be used in place of the array reference § Hence, if • • Then instead of document.forms[0].elements[0] • you can say document.myForm.myButton 42 43 Global and local variables v A variable is local to a function if § It is a formal parameter of the function § It is declared with var inside the function (e.g. var x = 5) v Otherwise, variables are global v Specifically, a variable is global if § It is declared outside any function (with or without var) § It is declared by assignment inside a function (e.g. x = 5) 43 44 Functions and methods v When a function is a property of an object, we call it a “method” § A method can be invoked by either of call(object, arg1, ..., argN) or apply(object, [arg1, ..., argN]) § call and apply are defined for all functions • call takes any number of arguments • apply takes an array of arguments § Both allow you to invoke a function as if it were a method of some other object, object § Inside the function, the keyword this refers to the object 44 Thank you for your attention! 46