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JavaScript_III.pptx

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JavaScript_III.pptx

  1. 1. JavaScript
  2. 2. AboutJavaScript  JavaScript is notJava, or even related to Java  The original name for JavaScript was “LiveScript”  The name was changed when Java became popular  Statements in JavaScript resemble statements in Java, because both languages borrowed heavily from the C language  JavaScript should be fairly easy for Java programmers to learn  However, JavaScript is a complete, full-featured, complex language  JavaScript is seldom used to write complete “programs”  Instead, small bits of JavaScript are used to add functionality to HTML pages  JavaScript is often used in conjunction with HTML “forms”  JavaScript is reasonably platform-independent
  3. 3. What is JavaScript? • JavaScript is a programming language for use in HTML pages • Invented in 1995 at Netscape Corporation (LiveScript) • JavaScript has nothing to do with Java • JavaScript programs are run by an interpreter built into the user's web browser (not on the server)
  4. 4. UsingJavaScript in a browser  JavaScript code is included within <script> tags: – <script type="text/javascript"> document.write("<h1>Hello World!</h1>") ; </script>  Notes:  The type attribute is to allow you to use other scripting languages (but JavaScript is the default)  This simple code does the same thing as just putting <h1>Hello World!</h1> in the same place in the HTML document  The semicolon at the end of the JavaScript statement is optional  You need semicolons if you put two or more statements on the same line  It’s probably a good idea to keep using semicolons
  5. 5. Dealing with old browsers  Some old browsers do not recognize script tags  These browsers will ignore the script tags but will display the included JavaScript  To get old browsers to ignore the whole thing, use: <script type="text/javascript"> <!-- document.write("Hello World!") //--> </script>  The <!-- introduces an HTML comment  To get JavaScript to ignore the HTML close comment, -->, the // starts a JavaScript comment, which extends to the end of the line
  6. 6. Where to put JavaScript  JavaScript can be put in the <head> or in the <body> of an HTML document  JavaScript functions should be defined in the <head>  This ensures that the function is loaded before it is needed  JavaScript in the <body> will be executed as the page loads  JavaScript can be put in a separate .js file – <script src="myJavaScriptFile.js"></script>  Put this HTML wherever you would put the actual JavaScript code  An external .js file lets you use the same JavaScript on multiple HTML pages  The external .js file cannot itself contain a <script> tag  JavaScript can be put in HTML form object, such as a button  This JavaScript will be executed when the form object is used
  7. 7. Where does JavaScript Fit In? • • Recall 1. client opens connection to server 2. client sends request to server 3. server sends response to client 4. client and server close connection What about Step 5? 5. Client renders (displays) the response received from server • Step 5 involves displaying HTML • And running any JavaScript code within the HTML
  8. 8. What can JavaScript Do? • JavaScript can dynamically modify an HTML page • JavaScript can react to user input • JavaScript can validate user input • JavaScript can be used to create cookies (yum!) • JavaScript is a full-featured programming language • JavaScript user interaction does not require any communication with the server
  9. 9. Pros and Cons of JavaScript • • Pros: – Allows more dynamic HTML pages, even complete web applications Cons: – Requires a JavaScript-enabled browser – Requires a client who trusts the server enough to run the code the server provides • JavaScript has some protection in place but can still cause security problems for clients – Remember JavaScript was invented in 1995 and web- browsers have changed a lot since then
  10. 10. Using JavaScript in your HTML • JavaScript can be inserted into documents by using the SCRIPT tag <html> <head> <title>Hello World in JavaScript</title> </head> <body> <script type="text/javascript"> document.write("Hello World!"); </script> </body> </html>
  11. 11. Where to Put your Scripts • You can have any number of scripts • Scripts can be placed in the HEAD or in the BODY – In the HEAD, scripts are run before the page is displayed – In the BODY, scripts are run as the page is displayed • In the HEAD is the right place to define functions and variables that are used by scripts within the BODY
  12. 12. Using JavaScript in your HTML <html> <head> <title>Hello World in JavaScript</title> <script type="text/javascript"> function helloWorld() { document.write("Hello World!"); } </script> </head> <body> <script type="text/javascript"> helloWorld(); </script> </body> </html>
  13. 13. External Scripts • Scripts can also be loaded from an external file • This is useful if you have a complicated script or set of subroutines that are used in several different documents <script src="myscript.js"></script>
  14. 14. JavaScript is not Java  By now you should have realized that you already know a great deal of JavaScript  So far we have talked about things that are the same as in Java  JavaScript has some features that resemble features in Java:  JavaScript has Objects and primitive data types  JavaScript has qualified names; for example, document.write("Hello World");  JavaScript has Events and event handlers  Exception handling in JavaScript is almost the same as in Java  JavaScript has some features unlike anything in Java:  Variable names are untyped: the type of a variable depends on the value it is currently holding  Objects and arrays are defined in quite a different way  JavaScript has with statements and a new kind of for statement
  15. 15. Primitive data types  JavaScript has three “primitive” types: number, string, and boolean  Everything else is an object  Numbers are always stored as floating-point values  Hexadecimal numbers begin with 0x  Some platforms treat 0123 as octal, others treat it as decimal  Strings may be enclosed in single quotes or double quotes  Strings can contains n (newline), "(double quote), etc.  Booleans are either true or false – 0, "0", empty strings, undefined, null, and NaN are false , other values are true
  16. 16. JavaScriptVariables  JavaScript has variables that you can declare with the optional var keyword  Variables declared within a function are local to that function  Variables declared outside of any function are global variables var myname = "Pat Morin";
  17. 17. Variables  Variables are declared with a var statement: – var pi = 3.1416, x, y, name = "Dr. Dave" ;  Variables names must begin with a letter or underscore  Variable names are case-sensitive  Variables are untyped (they can hold values of any type)  The word var is optional (but it’s good style to use it)  Variables declared within a function are local to that function (accessible only within that function)  Variables declared outside a function are global (accessible from anywhere on the page)
  18. 18. JavaScriptOperators and Constructs • JavaScript has most of the operators we're used to from C/Java – Arithmetic (+, - , *, /, %) – Assignment (=, +=, -=, *= /=, %=, ++, --) – Logical (&&, ||, !) – Comparison (<, >, <=, >=, ==) • Note: + also does string concatentation • Constructs: – if, else, while, for, switch, case
  19. 19. Operators, I  Because most JavaScript syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it  Arithmetic operators: + - * / % ++ --  Comparison operators: < <= == != >= >  Logical operators: && || ! (&& and || are short-circuit operators)  Bitwise operators: & | ^ ~ << >> >>>  Assignment operators: += -= *= /= %= <<= >>= >>>= &= ^= |=
  20. 20. Operators, II  String operator: +  The conditional operator: condition ? value_if_true : value_if_false  Special equality tests: – == and != try to convert their operands to the same type before performing the test – === and !== consider their operands unequal if they are of different types  Additional operators (to be discussed): new typeof void delete
  21. 21. Simple User Interaction • There are three built-in methods of doing simple user interaction – alert(msg) alerts the user that something has happened – confirm(msg) asks the user to confirm (or cancel) something – prompt(msg, default) asks the user to enter some text alert("There's a monster on the wing!"); confirm("Are you sure you want to do that?"); prompt("Enter you name", "Adam");
  22. 22. The for…in statement  You can loop through all the properties of an object with for (variable in object) statement;  Example: for (var prop in course) { document.write(prop + ": " + course[prop]); }  Possible output: teacher: Dr. Dave number: CIT597  The properties are accessed in an undefined order  If you add or delete properties of the object within the loop, it is undefined whether the loop will visit those properties  Arrays are objects; applied to an array, for…in will visit the “properties” 0, 1, 2, …  Notice that course["teacher"] is equivalent to course.teacher  You must use brackets if the property name is in a variable
  23. 23. The with statement • with (object) statement ; uses the object as the default prefix for variables in the statement  For example, the following are equivalent: – with (document.myForm) { result.value = compute(myInput.value) ; } – document.myForm.result.value = compute(document.myForm.myInput.value);  One of my books hints at mysterious problems resulting from the use of with, and recommends against ever using it
  24. 24. JavaScript Functions • JavaScript lets you define functions using the function keyword • Functions can return values using the return keyword function showConfirm() { confirm("Are you sure you want to do that?"); }
  25. 25. Functions  Functions should be defined in the <head> of an HTML page, to ensure that they are loaded first  The syntax for defining a function is: function name(arg1, …, argN) { statements }  The function may contain return value; statements  Any variables declared within the function are local to it  The syntax for calling a function is just name(arg1, …, argN)  Simple parameters are passed by value, objects are passed by reference
  26. 26. Regular expressions  A regular expression can be written in either of two ways:  Within slashes, such as re = /ab+c/  With a constructor, such as re = new RegExp("ab+c")  Regular expressions are almost the same as in Perl or Java (only a few unusual features are missing)  string.match(regexp) searches string for an occurrence of regexp  It returns null if nothing is found  If regexp has the g (global search) flag set, match returns an array of matched substrings  If g is not set, match returns an array whose 0th element is the matched text, extra elements are the parenthesized subexpressions, and the index property is the start position of the matched substring
  27. 27. Warnings  JavaScript is a big, complex language  We’ve only scratched the surface  It’s easy to get started in JavaScript, but if you need to use it heavily, plan to invest time in learning it well  Write and test your programs a little bit at a time  JavaScript is not totally platform independent  Expect different browsers to behave differently  Write and test your programs a little bit at a time  Browsers aren’t designed to report errors  Don’t expect to get any helpful error messages  Write and test your programs a little bit at a time
  28. 28. JavaScriptArrays • JavaScript has arrays that are indexed starting at 0 • Special version of for works with arrays <script type="text/javascript"> var colors = new Array(); colors[0] = "red"; colors[1] = "green"; colors[2] = "blue"; colors[3] = "orange"; colors[4] = "magenta"; colors[5] = "cyan"; for (var i in colors) { document.write("<div style="background-color:" + colors[i] + ";">" + colors[i] + "</div>n"); } </script>
  29. 29. Array literals  You don’t declare the types of variables in JavaScript  JavaScript has array literals, written with brackets and commas  Example: color = ["red", "yellow", "green", "blue"];  Arrays are zero-based: color[0] is "red"  If you put two commas in a row, the array has an “empty” element in that location  Example: color = ["red", , , "green", "blue"]; • color has 5 elements  However, a single comma at the end is ignored  Example: color = ["red", , , "green", "blue”,]; still has 5 elements
  30. 30. Four ways to create an array  You can use an array literal: var colors = ["red", "green", "blue"];  You can use new Array() to create an empty array: – var colors = new Array();  You can add elements to the array later: colors[0] = "red"; colors[2] = "blue"; colors[1]="green";  You can use new Array(n) with a single numeric argument to create an array of that size – var colors = new Array(3);  You can use new Array(…) with two or more arguments to create an array containing those values: – var colors = new Array("red","green", "blue");
  31. 31. The length of an array  If myArray is an array, its length is given by myArray.length  Array length can be changed by assignment beyond the current length  Example: var myArray = new Array(5); myArray[10] = 3;  Arrays are sparse, that is, space is only allocated for elements that have been assigned a value  Example: myArray[50000] = 3; is perfectly OK  But indices must be between 0 and 232-1  As in C and Java, there are no two-dimensional arrays; but you can have an array of arrays: myArray[5][3]
  32. 32. Arrays and objects  Arrays are objects • car = { myCar: "Saturn", 7: "Mazda" } – car[7] is the same as car.7 – car.myCar is the same as car["myCar"]  If you know the name of a property, you can use dot notation: car.myCar  If you don’t know the name of a property, but you have it in a variable (or can compute it), you must use array notation: car.["my" + "Car"]
  33. 33. Array functions  If myArray is an array, – myArray.sort() sorts the array alphabetically – myArray.sort(function(a, b) { return a - b; }) sorts numerically – myArray.reverse() reverses the array elements – myArray.push(…) adds any number of new elements to the end of the array, and increases the array’s length – myArray.pop() removes and returns the last element of the array, and decrements the array’s length – myArray.toString() returns a string containing the values of the array elements, separated by commas
  34. 34. JavaScript Events • JavaScript can be made to respond to user events • Common Events: – onload and onunload : when a page is first visited or left – onfocus, onblur, onchange : events pertaining to form elements – onsubmit : when a form is submitted – onmouseover, onmouseout : for "menu effects"
  35. 35. Exception Handling • JavaScript also has try, catch, and throw keywords for handling JavaScript errors try { runSomeCode(); } catch(err) { var txt="There was an error on this page.nn" + "Error description: " + err.description + "nn" alert(txt) }
  36. 36. Exception handling, I  Exception handling in JavaScript is almost the same as in Java •throw expression creates and throws an exception  The expression is the value of the exception, and can be of any type (often, it's a literal String) •try { statements to try } catch (e) { // Notice: no type declaration for e exception-handling statements } finally { // optional, as usual code that is always executed }  With this form, there is only one catch clause
  37. 37. Exception handling, II •try { statements to try } catch (e if test1) { exception-handling for the case that test1 is true } catch (e if test2) { exception-handling for when test1 is false and test2 is true } catch (e) { exception-handling for when both test1and test2 are false } finally { // optional, as usual code that is always executed }  Typically, the test would be something like e == "InvalidNameException"
  38. 38. Comments in JavaScript • Comments in JavaScript are delimited with // and /* */ as in Java and C++
  39. 39. JavaScriptObjects • JavaScript is object-oriented and uses the same method-calling syntax as Java • We have already seen this with the document object document.write("Hello World!");
  40. 40. Built-In JavaScriptObjects • Some basic objects are built-in to JavaScript – String – Date – Array – Boolean – Math
  41. 41. Object literals  You don’t declare the types of variables in JavaScript  JavaScript has object literals, written with this syntax: – { name1 : value1 , ... , nameN : valueN }  Example (from Netscape’s documentation): – car = {myCar: "Saturn", 7: "Mazda", getCar: CarTypes("Honda"), special: Sales}  The fields are myCar, getCar, 7 (this is a legal field name) , and special • "Saturn" and "Mazda" are Strings • CarTypes is a function call • Sales is a variable you defined earlier  Example use: document.write("I own a " + car.myCar);
  42. 42. Three ways to create an object  You can use an object literal: – var course = { number: "CIT597", teacher="Dr. Dave" }  You can use new to create a “blank” object, and add fields to it later: – var course = new Object(); course.number = "CIT597"; course.teacher = "Dr. Dave";  You can write and use a constructor: – function Course(n, t) { // best placed in <head> this.number = n; this.teacher = t; } – var course = new Course("CIT597", "Dr. Dave");
  43. 43. JavaScriptStrings • A String object is created every time you use a string literal (just like in Java) • Have many of the same methods as in Java – charAt, concat, indexOf, lastIndexOf, match, replace, search, slice, split, substr, substring, toLowerCase, toUpperCase, valueOf • There are also some HTML specific methods – big, blink, bold, fixed, fontcolor, fontsize, italics, link, small, strike, sub, sup • Don't use the HTML methods (use CSS instead) – This is the worst kind of visual formatting
  44. 44. JavaScript Dates • The Date class makes working with dates easier • A new date is initialized with the current date • Dates can be compared and incremented var myDate = new Date(); myDate.setFullYear(2007,2,14); var today = new Date(); var nextWeek = today + 7; if (nextWeek > today) { alert("You have less than one week left"); }
  45. 45. JavaScriptArrays and Booleans • We have already seen the Array class • The Boolean class encapsulates a boolean value
  46. 46. The JavaScript Math Class • The Math class encapsulates many commonly- used mathematical entities and formulas • These are all class methods – abs, acos, asin, atan, atan2, ceil, cos, exp, floor, log, max, min, pow, random, round, sin, sqrt, tan • These are all class methods – E, LN2, LN10, LOG2E, LOG10E, PI, SQRT1_2, SQRT2 if (Math.cos(Math.PI) != 0) { alert("Something is wrong with Math.cos"); }
  47. 47. JavaScript and the DOM • The Document Object Model (DOM) is a specification that determines a mapping between programming language objects and the elements of an HTML document • Not specific to JavaScript
  48. 48. HTML DOMObjects • Environment objects – Window, Navigator, Screen, History, Location, Document • HTML objects – Anchor, Area, Base, Body, Button, Event, Form, Frame, Frameset, Iframe, Image, Checkbox, FileUpload, Hidden, Password, Radio, Reset, Submit, Text, Link, Meta, Object, Option, Select, Style, Table, TableCell, TableRow, TextArea
  49. 49. HTML DOM: Document • • The Document object represents an HTML document and can be used to access all documents in a page A Document contains several collections – anchors, forms, images, links • Has several properties – body, cookie, domain, lastModified, referrer, title, URL • Has several useful methods – getElementById, getElementsByName, getElementsByTagName, write, writeln, open, close
  50. 50. HTML DOM: Document • An instance of Document is already created for you, called document function changeF() { var cText = document.getElementById("c"); var fText = document.getElementById("f"); ... } ... <input type="text" id="c" onchange="changeC()">C <input type="text" id="f" onchange="changeF()">F
  51. 51. HTML DOM: Form Elements • One of the most common uses of JavaScript is for form validation • Several HTML DOM classes encapsulate form elements – Form, Button, Checkbox, Hidden, Password, Text, Radio, Reset, Submit, Textarea • Warning: Using JavaScript is not a substitute for validating form data in CGI scripts
  52. 52. HTML DOM:Text •  A text entry field (input type="text") is encapsulated by aText object  Variables  – value, maxLength, id, size, name, tabindex, readOnly  Changing these variables has an immediate effect on the displayed data
  53. 53. HTML DOM:The DocumentTree • Accessing elements and changing their properties lets us do simple things like form validation, data transfer etc • HTML DOM lets us do much more • We can create, delete, and modify parts of the HTML document • For this we need to understand the Document Tree
  54. 54. HTML DOM:The DocumentTree
  55. 55. Navigating the DocumentTree • • With JavaScript we can navigate the document tree • document.getElementById(), getElementsByName(), and getElementsByTagName() return nodes in the document tree Information can be obtained from – nodeName – The tag name – nodeValue – The the text of a text node – nodeType – The kind of node
  56. 56. ThankYou

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