JavaScript

1. 5 JavaScript

2. Overview • A "scripting" language for HTML pages - a scripting language is a lightweight programming language • Embed code in HTML pages so they are downloaded directly to browser • The browser interprets and executes the script (it is not compiled) • Was designed to add interactivity to HTML pages • Everyone can use JavaScript without purchasing a license • Supported by all major browsers

3. … Overview • Do not declare data types for variables (loose typing) • Dynamic binding – object references checked at runtime • Scripts can manipulate "browser objects:" • HTML form elements • Images • Frames • etc. • For security – cannot write to disk (when run on a client)

4. Abilities • Generating HTML content dynamically • Monitoring and responding to user events • Validate forms before submission • Manipulate HTTP cookies • Interact with the frames and windows of the browser • Customize pages to suit users

5. It is not Java • JavaScript is not Java, or even related to Java • The original name for JavaScript was “LiveScript” • The name was changed when Java became popular • Released in the Fall of 1995 • 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 • 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

6. … It is not 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

7. Scripting • The entire script is stored in memory as plain text • When requested by the user the applicable portion of the script is executed by fetching the associated machine instructions from a library • Tends to be a bit slower than compiling programs • However, there is no burden on the author to compile anything • Errors are not obvious when scripting; only rigorous testing will find errors

8. History • Built into Netscape Navigator since v2.0 (early 1996) • Developed independently of Java • Proprietary, but submitted as standard and built into Microsoft IE 3.0 and later • Standardized by ECMA (European Computer Manufacture’s Association) into ECMAscript • EMCAscript joins JavaScript and Jscript to one standard

9. Javascript has many names • In Netscape it’s Javascript • In Internet Explorer it’s JScript • IE also supports it’s own VB Script, a Visual Basic scripting language • VB Script won’t work in Netscape • There is also ECMAscript • A variation of Javascript 1.1 • Open Standard • Promoted by European Computer Manufacturers Association (ECMA) • JScript is essentially ECMAscript in IE 4.0+

10. Javascript Versions • 1.0 – Original version, largely obsolete • Supported in Navigator 2.0 • Buggy version of it supported in IE 3.0 as JScript • 1.1 • Improved array processing • Supported in Navigator 3.0 as JScript, some discrepancies • 1.2 • Supports regular expressions, new statements • Supported in Navigator 4.0 • 1.3 • Fixed some problems with dates, introduced in Navigator 4.06 • ECMAScript • First supported in IE 4.0, also in Navigator 4.06 • Largely the same as Javascript 1.1

11. Dynamic HTML HTML CSS Java Script Java Script HTML HTML CSS HTML

12. Web Architecture for JavaScript "CLIENT" "SERVER" Desktop access Remotehost Web browser Web (HTTP) Server HTML Page: <SCRIPT> …code..… </SCRIPT> Internet HTML/HTTP TCP/IP HTML/HTTP TCP/IP built-in JavaScript interpreter HTML pages w/ embedded script

13. Client and Server • JavaScript can be used • On the client side • On the server • More lightweight and reliable on clients than Java (Applets) • Useful for developing interactive interface (Dynamic HTML)

14. Sample Things you can Do with JavaScript • Auto email • Rename a button • Change background color • View URL • Set timer • Open a window • Display date

15. Sample Things you can Do with JavaScript • Print page • Save and access a cookie • Sniff the browser • Data validation • Print a page • Preload images • Flyout/Dropdown menus

16. Example • 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

17. 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

18. 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

19. 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

20. Variables • Variables are declared with a var statement: • var pi = 3.1416, x, y, name = "Dr. ABC" ; • 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)

21. 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:& | ^ (XOR) ~ (NOT) << >> (Shifts binary bits to right, discarding bits shifted off) >>> (Shifts binary bits to right, discarding bits shifted off and shifting in zeros from left.) • Assignment operators:+= -= *= /= %= <<= >>= >>>= &= ^= |=

22. 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 Using x=3 and y="3": 1) x==y Result: returns true 2) x===y Result: returns false • Additional operators:new typeof void delete

23. Comments • Comments are as in C or Java: • Between // and the end of the line • Between /* and */

24. Statements, I • Most JavaScript statements are also borrowed from C • Assignment: greeting = "Hello, " + name; • Compound statement:{ statement; ...; statement } • If statements:if (condition) statement; if (condition) statement; else statement; • Familiar loop statements:while (condition) statement; do statement while (condition); for (initialization; condition; increment) statement;

25. Statements, II • The switch statement:switch (expression){ case label :statement; break; case label :statement; break; ... default : statement; } • Other familiar statements: • break; • continue; • The empty statement, as in ;; or { }

26. 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 eexception-handling statements} finally { // optional, as usualcode that is always executed} • With this form, there is only one catch clause

27. 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 test1 andtest2 are false} finally { // optional, as usualcode that is always executed} • Typically, the test would be something likee == "InvalidNameException"

28. 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: • car = {myCar: "Toyota", 7: "Mazda", getCar: CarTypes("Honda"), special: Sales} • The fields are myCar, getCar, 7 (this is a legal field name) , and special • "Toyota" 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);

29. Three ways to create an object • You can use an object literal: • var course = { number: "CS450", teacher: "Dr. ABC" } • You can use new to create a “blank” object, and add fields to it later: • var course = new Object();course.number = "CS450";course.teacher = "Dr. ABC"; • 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("CS450", "Dr. ABC");

30. 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. Some browsers recognize this as 6 elements.

31. 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");

32. 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]

33. Arrays and objects • Arrays are objects • car = { myCar: "Toyota", 7: "Mazda" } • car[7] is the same as car.7 • car.myCar is the same as car["myCar"] • Using quotes (single or double) is a must for myCar but not for 7. • 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"]

34. 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

35. The for…instatement • 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. ABCnumber: CS450 • 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…inwill 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

36. 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

37. 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

38. Debugging • If you mess up on the syntax you will get a Javascript Error • Netscape • You will see a notification of an error on the status bar in the bottom left corner • You type “javascript:” in the URL field to pinpoint the error • Internet Explorer • By default a tiny little Javascript error message appears at the bottom left corner of the browser in yellow. Usually you won’t see it. • Can be explicitly disabled under Tools/Internet Options • Recommend under Tools/Internet Options/Advanced/Browsing to uncheck “Disable Script Debugging” and to check “Display a Notification about every script error” while doing development

39. Fixing Javascript Errors • If possible use the debugging tool to locate the line containing the error • Errors can be hard to find and fix • “code a little, test a little” strategy • Often errors are due to things that are easy to overlook, like not closing a quote

40. Numbers • In JavaScript, all numbers are floating point • 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

41. Strings and characters • In JavaScript, string is a primitive type • Strings are surrounded by either single quotes or double quotes • There is no “character” type • Special characters are: \0 NUL \b backspace \f form feed \n newline \r carriage return \thorizontal tab \v vertical tab \' single quote \" double quote \\ backslash \xDD Unicode hex DD \xDDDD Unicode hex DDDD

42. Some string methods • charAt(n) • Returns the nth character of a string • concat(string1, ..., stringN) • Concatenates the string arguments to the recipient string • indexOf(substring) • Returns the position of the first character of substring in the recipient string, or -1 if not found • 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 • lastIndexOf(substring), lastIndexOf(substring, start) • Like indexOf, but searching starts from the end of the recipient string

43. More string methods • 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 • replace(regexp, replacement) • Returns a new string that has the matched substring replaced with the replacement • search(regexp) • Returns the position of the first matched substring in the given string, or -1 if not found.

44. boolean • The boolean values are true and false • When converted to a boolean, the following values are also false: • 0 • "0" and '0' • the empty string, '' or "" • undefined • null • NaN

45. Arrays • 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 • 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

46. Input • Programming languages need to start with some data and manipulate it • Confirm asks a yes or no question in a dialog box • Prompt prompts the user to type in some information into a text field inside the dialog box • Sources of data can include: • Files • Databases • User (keyboard & mouse typically) • Variable assignments (ex: pi=3.14159) • Javascript objects • Example: date object • Example: • User_name = prompt(“What is your name?”, “Enter your name here”);

47. Output • After a program manipulates the input data with various statements it usually creates an output of some kind • Source of output may include: • Files • Database • Display or Printer • Devices (sound card, modems etc) • Javascript Objects • Via Object Methods

48. Simple Output • document is an object (not a class) representing the current document • write is a method on the document object that let’s you write any text to the browser window at the current location of the cursor • Warning: if invoked as part of a form action output will appear in a new window • Example: • document.write(“Hello world!”);

49. alert method • A dialog box containing information can be written by using the window.alert method • Example: • alert(“This brings up an annoying non-modal dialog box. The user can’t do anything until they click OK.”);

50. HTML names in JavaScript • In HTML the window is the global object • It is assumed that all variables are properties of this object, or of some object decended from this object • The most important window property is document • HTML form elements can be referred to by document.forms[formNumber].elements[elementNumber] • Every HTML form element has a name attribute • The name can be used in place of the array reference • Hence, if • <form name="myForm"> <input type="button" name="myButton" ...> • Then instead of document.forms[0].elements[0] • you can say document.myForm.myButton