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					  JavaScript

Language Fundamentals




                        1-Sep-11
     About JavaScript
   JavaScript is not Java, or even related to Java
       The original name for JavaScript was “LiveScript”
       The name was changed when Java became popular
       Now that Microsoft no longer likes Java, its name for their
        JavaScript dialect is “Active Script”
   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 reasonably platform-independent


                                                                             2
     Using JavaScript 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


                                                                                 3
        JavaScript isn’t always available
   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
   Some users turn off JavaScript
       Use the <noscript>message</noscript> to display a message in place of
        whatever the JavaScript would put there
                                                                                   4
        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 an HTML form object, such as a button
       This JavaScript will be executed when the form object is used
                                                                                 5
         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
             Since you can’t be sure, avoid octal altogether!
   Strings may be enclosed in single quotes or double quotes
        Strings can contain \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

                                                                           6
     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)
       local variables must be declared using var
   Variables declared outside a function are global
    (accessible from anywhere on the page)
                                                                   7
      Operators, I

   Most JavaScript syntax is borrowed from C
   Arithmetic operators (all numbers are floating-point):
       +    -    *     /     %    ++    --
   Comparison operators:
       <    <=      ==     !=    >=    >
   Logical operators:
       &&      ||     !     (&& and || are short-circuit operators)
   Bitwise operators:
       &     |     ^     ~    <<    >>     >>>
   Assignment operators:
       += -= *= /= %= <<= >>= >>>= &= ^= |=

                                                                      8
        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
       new typeof              void      delete

                                                                     9
        Comments
   Comments are as in C++ or Java:
       Between // and the end of the line
       Between /* and */
   Java’s javadoc comments, /** ... */, are treated just the
    same as /* ... */ comments; they have no special
    meaning in JavaScript




                                                                10
        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;

                                                                    11
         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 { }


                                               12
        JavaScript is not Java
   By now you should have realized that you already know a
    great deal of JavaScript

   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

                                                                            13
        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


                                                                                       14
       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"


                                                                       15
        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);

                                                                                16
        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;     // keyword "this" is required, not optional
           this.teacher = t;
        }
       var course = new Course("CIT597", "Dr. Dave");


                                                                              17
        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



                                                                                    18
     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");


                                                                    19
     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]


                                                                 20
        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"]


                                                               21
        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

                                                                       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
        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

                                                                     24
        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



                                                                                   25
        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


                                                                            26
   Example
<html>
<head>
<title> JavaScript 1 </title>
<script language="JavaScript">
document.write("<h1>This is it. </h1>");
 document.write(Date());
</script>
</head>
<body>
This is the rest of the page.
</body>
</html>




                                           27
Modified example
<html>
<head>
<title> JavaScript 1 </title>
<script language="JavaScript">
<!--
var rawDate = Date();
var mon = rawDate.substr(4,3);

document.write("The date is");
document.write(rawDate, "<br>")
document.write("The month is ", mon);
//-->
</script>
</head>
<body>
<br>
This is the rest of the page.
</body>
</html>
                                        28
    Using built-in functions, variables
<html> <head> <title> JavaScript 1 </title>
<script language="JavaScript">
                                            Variable set to what
<!--
                                            Date() returns.
var rawDate = Date();
var mon = rawDate.substr(4,3);
                                                       Extract the
document.write("The month is ");
                                                       month.
document.write(mon);
//-->
</script> </head>
<body>
<br>
This is the rest of the page.
</body> </html>



                                                                     29
<html>
<head><title>Form example </title>
<script language="JavaScript">
function verify(f)
{

if (f.lname.value == null || f.address.value == null)
{   alert("Form needs a last name and an address");
    return false;
}
if (f.lname.value == "" || f.address.value == "")
{   alert("Form needs a last name and an address");
    return false;
}
return true;

}
</script> </head>

                                                        30
             <body>
             <h1> Address Information </h1> <br>
             <form method=post enctype="text/plain" action="mailto:snell@cs.byu.edu"
             onSubmit="return verify(this);">

Javascript   First Name: <input type="text" name="fname"> <br>
Event        Last Name: <input type="text" name="lname"> <br>
             Street Address: <input type="text" name="address" size=30> <br>
             Town/City: <input type="text" name="city"> <br>
             State: <select name="state" size=1> <br>
             <option value="NY" selected> Utah
             <option value="NY" selected> Idaho
             <option value="NY" selected> New York
             <option value="NJ"> New Jersey
             <option value="CT"> Connecticut
             <option value="PA"> Pennsylvania
             </select> <br>
             Status: <input type="radio" name="status" value="R"> Returning client
             <input type="radio" name="status" value="N"> New client
             <hr> Thank you <p>
             <input type="submit" value="Send information">

             </form> </body> </html>
                                                                                       31
        Javscript Events
   Events are actions that can be detected by Javascript
   Every element on a web page has certain events which
    can trigger Javascript functions
   Often placed within the HTML tag
       <tag attribute1 attribute2 onEventName="javascript code;">
       <a href="" onMouseOver="popupFunc();">
   The set of all events which may occur and the page
    elements on which they can occur is part of the
    Document Object Model(DOM) not Javascript
       Browsers don’t necessarily share the same set of events

                                                                     32
          Common Javascript Events
    Event          Occurs when...                                               Event Handler
   click          User clicks on form element or link                          onClick
   change         User changes value of text, textarea, or select element      onChange
   focus User gives form element input focus                          onFocus
   blur           User removes input focus from form element                   onBlur
   mouseover      User moves mouse pointer over a link or anchor               onMouseOver
   mouseout       User moves mouse pointer off of link or anchor               onMouseOut
   select         User selects form element's input field                      onSelect
   submit         User submits a form                                          onSubmit
   resize         User resizes the browser window                              onResize
   load           User loads the page in the Navigator                         onLoad
   unload         User exits the page                                 onUnload




                                                                                           33
        The Document Object Model
   When a document is loaded in the web browser, a number of
    objects are created.
       Most commonly used are window and document
   Window
       open(), close(), alert(), confirm(), prompt()
   Document
       Contains arrays which store all the components of your page
       You can access and call methods on the components using the arrays
       An object may also be accessed by its name
            document.myform.address.value = “123 Main”
            document.myform.reset()
       Can also search for element by name or id
            document.getElementById(“myelementid”)
            document.getElementsByName(“myelementname”)


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