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    • Introduction To Javascript

      What is JavaScript?



      • JavaScript was designed to add interactivity to HTML pages
      • JavaScript is a scripting language
      • A scripting language is a lightweight programming language
      • JavaScript is usually embedded directly into HTML pages
      • JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
      • Everyone can use JavaScript without purchasing a license

      Are Java and JavaScript the same?




      NO!Java and JavaScript are two completely different languages in both concept and design!Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.


      What can a JavaScript do?



      • JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
      • JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
      • JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
      • JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
      • JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
      • JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
      • JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer

      Put a JavaScript into an HTML page



      The example below shows how to use JavaScript to write text on a web page:
      Example

      Code:
      <html>
      <body>
      <script type="text/javascript">
      document.write("Hello World!");
      </script>
      </body>
      </html>
      The example below shows how to add HTML tags to the JavaScript:
      Example
      Code:
      <html>
      <body>
      <script type="text/javascript">
      document.write("<h1>Hello World!</h1>");
      </script>
      </body>
      </html>
      The document.write command is a standard JavaScript command for writing output to a page.
      By entering the document.write command between the <script> and </script> tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page:

      Code:
      <html>
      <body>
      <script type="text/javascript">
      document.write("Hello World!");
      </script>
      </body>
      </html>

      Where to Put the JavaScript


      JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, or at a later event, such as when a user clicks a button. When this is the case we put the script inside a function, you will learn about functions in a later chapter.
      Scripts in <head>
      Scripts to be executed when they are called, or when an event is triggered, are placed in functions.
      Put your functions in the head section, this way they are all in one place, and they do not interfere with page content.
      Example


      Code:
      <html>
      <head>
      <script type="text/javascript">
      function message()
      {
      alert("This alert box was called with the onload event");
      }
      </script>
      </head>
      
      <body onload="message()">
      </body>
      </html>

      Scripts in <body>
      If you don't want your script to be placed inside a function, or if your script should write page content, it should be placed in the body section.
      Example

      Code:
      <html>
      <head>
      </head>
      
      <body>
      <script type="text/javascript">
      document.write("This message is written by JavaScript");
      </script>
      </body>
      
      </html>
      Scripts in <head> and <body>
      You can place an unlimited number of scripts in your document, so you can have scripts in both the body and the head section.
      Example

      Code:
      <html>
      <head>
      <script type="text/javascript">
      function message()
      {
      alert("This alert box was called with the onload event");
      }
      </script>
      </head>
      
      <body onload="message()">
      <script type="text/javascript">
      document.write("This message is written by JavaScript");
      </script>
      </body>
      
      </html>

      Using an External JavaScript



      If you want to run the same JavaScript on several pages, without having to write the same script on every page, you can write a JavaScript in an external file.
      Save the external JavaScript file with a .js file extension.
      Note: The external script cannot contain the <script></script> tags!
      To use the external script, point to the .js file in the "src" attribute of the <script> tag:
      Example

      Code:
      <html>
      <head>
      <script type="text/javascript" src="xxx.js"></script>
      </head>
      <body>
      </body>
      </html>

      JavaScript is Case Sensitive


      Unlike HTML, JavaScript is case sensitive - therefore watch your capitalization closely when you write JavaScript statements, create or call variables, objects and functions.

      JavaScript Code


      JavaScript code (or just JavaScript) is a sequence of JavaScript statements.
      Each statement is executed by the browser in the sequence they are written.
      This example will write a heading and two paragraphs to a web page:
      Example

      Code:
      <script type="text/javascript">
      document.write("<h1>This is a heading</h1>");
      document.write("<p>This is a paragraph.</p>");
      document.write("<p>This is another paragraph.</p>");
      </script>

      JavaScript Blocks


      JavaScript statements can be grouped together in blocks.
      Blocks start with a left curly bracket {, and ends with a right curly bracket }.
      The purpose of a block is to make the sequence of statements execute together.
      This example will write a heading and two paragraphs to a web page:
      Example

      Code:
      <script type="text/javascript">
      {
      document.write("<h1>This is a heading</h1>");
      document.write("<p>This is a paragraph.</p>");
      document.write("<p>This is another paragraph.</p>");
      }
      </script>

      JavaScript Operators


      = is used to assign values.
      + is used to add values.

      The assignment operator = is used to assign values to JavaScript variables.
      The arithmetic operator + is used to add values together.
      y=5;
      z=2;
      x=y+z;
      The value of x, after the execution of the statements above is 7.
      ________________________________________

      JavaScript Arithmetic Operators


      Arithmetic operators are used to perform arithmetic between variables and/or values.
      Given that y=5, the table below explains the arithmetic operators:

      Operator Description Example Result
      + Addition x=y+2 x=7
      - Subtraction x=y-2 x=3
      * Multiplication x=y*2 x=10
      / Division x=y/2 x=2.5
      % Modulus (division remainder) x=y%2 x=1
      ++ Increment x=++y x=6
      -- Decrement x=--y x=4

      JavaScript Assignment Operators


      Assignment operators are used to assign values to JavaScript variables.
      Given that x=10 and y=5, the table below explains the assignment operators:


      Operator Example Same As Result
      = x=y x=5
      += x+=y x=x+y x=15
      -= x-=y x=x-y x=5
      *= x*=y x=x*y x=50
      /= x/=y x=x/y x=2
      %= x%=y x=x%y x=0

      The + Operator Used on Strings
      The + operator can also be used to add string variables or text values together.
      To add two or more string variables together, use the + operator.
      txt1="What a very";
      txt2="nice day";
      txt3=txt1+txt2;
      After the execution of the statements above, the variable txt3 contains "What a verynice day".
      To add a space between the two strings, insert a space into one of the strings:
      txt1="What a very ";
      txt2="nice day";
      txt3=txt1+txt2;
      or insert a space into the expression:
      txt1="What a very";
      txt2="nice day";
      txt3=txt1+" "+txt2;
      After the execution of the statements above, the variable txt3 contains:
      "What a very nice day"
      ________________________________________
      Adding Strings and Numbers
      The rule is: If you add a number and a string, the result will be a string!
      Example
      x=5+5;
      document.write(x);

      x="5"+"5";
      document.write(x);

      x=5+"5";
      document.write(x);

      x="5"+5;
      document.write(x);

      Comparison Operators


      Comparison operators are used in logical statements to determine equality or difference between variables or values.
      Given that x=5, the table below explains the comparison operators:


      Operator Description Example
      == is equal to x==8 is false
      === is exactly equal to (value and type) x===5 is true
      x==="5" is false
      != is not equal x!=8 is true
      > is greater than x>8 is false
      < is less than x<8 is true
      >= is greater than or equal to x>=8 is false
      <= is less than or equal to x<=8 is true


      How Can it be Used
      Comparison operators can be used in conditional statements to compare values and take action depending on the result:
      if (age<18) document.write("Too young");
      You will learn more about the use of conditional statements in the next chapter of this tutorial.
      ________________________________________

      Logical Operators


      Logical operators are used to determine the logic between variables or values.
      Given that x=6 and y=3, the table below explains the logical operators:


      Operator Description Example
      && and (x < 10 && y > 1) is true
      || or (x==5 || y==5) is false
      ! not !(x==y) is true


      Conditional Operator


      JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.
      Syntax
      Code:
      variablename=(condition)?value1:value2
      Example
      Code:
      greeting=(visitor=="PRES")?"Dear President ":"Dear ";
      Conditional Statements

      Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.
      In JavaScript we have the following conditional statements:


      • if statement - use this statement to execute some code only if a specified condition is true
      • if...else statement - use this statement to execute some code if the condition is true and another code if the condition is false
      • if...else if....else statement - use this statement to select one of many blocks of code to be executed
      • switch statement - use this statement to select one of many blocks of code to be executed
      ________________________________________

      If Statement


      Use the if statement to execute some code only if a specified condition is true.
      Syntax

      Code:
      if (condition)
        {
        code to be executed if condition is true
        }
      Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!
      Example

      Code:
      <script type="text/javascript">
      //Write a "Good morning" greeting if
      //the time is less than 10
      
      var d=new Date();
      var time=d.getHours();
      
      if (time<10)
        {
        document.write("<b>Good morning</b>");
        }
      </script>
      Notice that there is no ..else.. in this syntax. You tell the browser to execute some code only if the specified condition is true.
      ________________________________________

      If...else Statement


      Use the if....else statement to execute some code if a condition is true and another code if the condition is not true.
      Syntax

      Code:
      if (condition)
        {
        code to be executed if condition is true
        }
      else
        {
        code to be executed if condition is not true
        }
      Example

      Code:
      <script type="text/javascript">
      //If the time is less than 10, you will get a "Good morning" greeting.
      //Otherwise you will get a "Good day" greeting.
      
      var d = new Date();
      var time = d.getHours();
      
      if (time < 10)
        {
        document.write("Good morning!");
        }
      else
        {
        document.write("Good day!");
        }
      </script>

      If...else if...else Statement



      Use the if....else if...else statement to select one of several blocks of code to be executed.
      Syntax

      [code]
      if (condition1)
      {
      code to be executed if condition1 is true
      }
      else if (condition2)
      {
      code to be executed if condition2 is true
      }
      else
      {
      code to be executed if condition1 and condition2 are not true
      }
      [code]
      Example

      Code:
      <script type="text/javascript">
      var d = new Date()
      var time = d.getHours()
      if (time<10)
        {
        document.write("<b>Good morning</b>");
        }
      else if (time>10 && time<16)
        {
        document.write("<b>Good day</b>");
        }
      else
        {
        document.write("<b>Hello World!</b>");
        }
      </script>

      The JavaScript Switch Statement


      Use the switch statement to select one of many blocks of code to be executed.
      Syntax

      Code:
      switch(n)
      {
      case 1:
        execute code block 1
        break;
      case 2:
        execute code block 2
        break;
      default:
        code to be executed if n is different from case 1 and 2
      }
      This is how it works: First we have a single expression n (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.
      Example

      Code:
      <script type="text/javascript">
      //You will receive a different greeting based
      //on what day it is. Note that Sunday=0,
      //Monday=1, Tuesday=2, etc.
      
      var d=new Date();
      theDay=d.getDay();
      switch (theDay)
      {
      case 5:
        document.write("Finally Friday");
        break;
      case 6:
        document.write("Super Saturday");
        break;
      case 0:
        document.write("Sleepy Sunday");
        break;
      default:
        document.write("I'm looking forward to this weekend!");
      }
      </script>

      JavaScript has three kind of popup boxes: Alert box, Confirm box, and Prompt box.



      Alert Box


      An alert box is often used if you want to make sure information comes through to the user.
      When an alert box pops up, the user will have to click "OK" to proceed.
      Syntax

      alert("sometext");

      Example
      Code:
      <html>
      <head>
      <script type="text/javascript">
      function show_alert()
      {
      alert("I am an alert box!");
      }
      </script>
      </head>
      <body>
      
      <input type="button" onclick="show_alert()" value="Show alert box" />
      
      </body>
      </html>

      Confirm Box


      A confirm box is often used if you want the user to verify or accept something.
      When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed.
      If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false.
      Syntax

      confirm("sometext");

      Example

      Code:
      <html>
      <head>
      <script type="text/javascript">
      function show_confirm()
      {
      var r=confirm("Press a button");
      if (r==true)
        {
        alert("You pressed OK!");
        }
      else
        {
        alert("You pressed Cancel!");
        }
      }
      </script>
      </head>
      <body>
      
      <input type="button" onclick="show_confirm()" value="Show confirm box" />
      
      </body>
      </html>

      Prompt Box


      A prompt box is often used if you want the user to input a value before entering a page.
      When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value.
      If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null.
      Syntax
      prompt("sometext","defaultvalue");

      Example

      Code:
      <html>
      <head>
      <script type="text/javascript">
      function show_prompt()
      {
      var name=prompt("Please enter your name","Harry Potter");
      if (name!=null && name!="")
        {
        document.write("Hello " + name + "! How are you today?");
        }
      }
      </script>
      </head>
      <body>
      
      <input type="button" onclick="show_prompt()" value="Show prompt box" />
      
      </body>
      </html>

      JavaScript Function


      To keep the browser from executing a script when the page loads, you can put your script into a function.
      A function contains code that will be executed by an event or by a call to the function.
      You may call a function from anywhere within a page (or even from other pages if the function is embedded in an external .js file).
      Functions can be defined both in the <head> and in the <body> section of a document. However, to assure that a function is read/loaded by the browser before it is called, it could be wise to put functions in the <head> section.
      ________________________________________
      How to Define a Function

      Syntax
      function functionname(var1,var2,...,varX)
      {
      some code
      }
      The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.
      Note: A function with no parameters must include the parentheses () after the function name.
      Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.
      ________________________________________
      JavaScript Function Example
      Example
      Code:
      <html>
      <head>
      <script type="text/javascript">
      function displaymessage()
      {
      alert("Hello World!");
      }
      </script>
      </head>
      
      <body>
      <form>
      <input type="button" value="Click me!" onclick="displaymessage()" />
      </form>
      </body>
      </html>
      If the line: alert("Hello world!!") in the example above had not been put within a function, it would have been executed as soon as the page was loaded. Now, the script is not executed before a user hits the input button. The function displaymessage() will be executed if the input button is clicked.
      You will learn more about JavaScript events in the JS Events chapter.
      ________________________________________

      The return Statement


      The return statement is used to specify the value that is returned from the function.
      So, functions that are going to return a value must use the return statement.
      The example below returns the product of two numbers (a and b):
      Example
      Code:
      <html>
      <head>
      <script type="text/javascript">
      function product(a,b)
      {
      return a*b;
      }
      </script>
      </head>
      
      <body>
      <script type="text/javascript">
      document.write(product(4,3));
      </script>
      
      </body>
      </html>

      JavaScript Loops


      Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
      In JavaScript, there are two different kind of loops:
      • for - loops through a block of code a specified number of times
      • while - loops through a block of code while a specified condition is true
      ________________________________________

      The for Loop


      The for loop is used when you know in advance how many times the script should run.
      Syntax
      Code:
      for (var=startvalue;var<=endvalue;var=var+increment)
      {
      code to be executed
      }
      Example


      The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs.
      Note: The increment parameter could also be negative, and the <= could be any comparing statement.
      Example
      Code:
      <html>
      <body>
      <script type="text/javascript">
      var i=0;
      for (i=0;i<=5;i++)
      {
      document.write("The number is " + i);
      document.write("<br />");
      }
      </script>
      </body>
      </html>

      The while Loop


      The while loop loops through a block of code while a specified condition is true.
      Syntax
      Code:
      while (var<=endvalue)
        {
        code to be executed
        }
      Note: The <= could be any comparing operator.
      Example
      The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs:
      Example

      Code:
      <html>
      <body>
      <script type="text/javascript">
      var i=0;
      while (i<=5)
        {
        document.write("The number is " + i);
        document.write("<br />");
        i++;
        }
      </script>
      </body>
      </html>

      The do...while Loop


      The do...while loop is a variant of the while loop. This loop will execute the block of code ONCE, and then it will repeat the loop as long as the specified condition is true.
      Syntax
      Code:
      do
        {
        code to be executed
        }
      while (var<=endvalue);
      Example
      The example below uses a do...while loop. The do...while loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested:
      Example

      Code:
      <html>
      <body>
      <script type="text/javascript">
      var i=0;
      do
        {
        document.write("The number is " + i);
        document.write("<br />");
        i++;
        }
      while (i<=5);
      </script>
      </body>
      </html>

      The break Statement


      The break statement will break the loop and continue executing the code that follows after the loop (if any).
      Example

      Code:
      <html>
      <body>
      <script type="text/javascript">
      var i=0;
      for (i=0;i<=10;i++)
        {
        if (i==3)
          {
          break;
          }
        document.write("The number is " + i);
        document.write("<br />");
        }
      </script>
      </body>
      </html>

      The continue Statement


      The continue statement will break the current loop and continue with the next value.
      Example

      Code:
      <html>
      <body>
      <script type="text/javascript">
      var i=0
      for (i=0;i<=10;i++)
        {
        if (i==3)
          {
          continue;
          }
        document.write("The number is " + i);
        document.write("<br />");
        }
      </script>
      </body>
      </html>

      JavaScript For...In Statement


      The for...in statement loops through the elements of an array or through the properties of an object.
      Syntax
      Code:
      for (variable in object)
        {
        code to be executed
        }
      Note: The code in the body of the for...in loop is executed once for each element/property.
      Note: The variable argument can be a named variable, an array element, or a property of an object.
      Example
      Use the for...in statement to loop through an array:
      Example

      Code:
      <html>
      <body>
      
      <script type="text/javascript">
      var x;
      var mycars = new Array();
      mycars[0] = "Saab";
      mycars[1] = "Volvo";
      mycars[2] = "BMW";
      
      for (x in mycars)
        {
        document.write(mycars[x] + "<br />");
        }
      </script>
      
      </body>
      </html>

      Events


      By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript.
      Every element on a web page has certain events which can trigger a JavaScript. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags.
      Examples of events:
      • A mouse click
      • A web page or an image loading
      • Mousing over a hot spot on the web page
      • Selecting an input field in an HTML form
      • Submitting an HTML form
      • A keystroke
      Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!
      For a complete reference of the events recognized by JavaScript, go to our complete JavaScript reference.
      ________________________________________

      onLoad and onUnload


      The onLoad and onUnload events are triggered when the user enters or leaves the page.
      The onLoad event is often used to check the visitor's browser type and browser version, and load the proper version of the web page based on the information.
      Both the onLoad and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user's name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: "Welcome John Doe!".
      ________________________________________

      onFocus, onBlur and onChange


      The onFocus, onBlur and onChange events are often used in combination with validation of form fields.
      Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field:
      <input type="text" size="30" id="email" onchange="checkEmail()">

      ________________________________________

      onSubmit


      The onSubmit event is used to validate ALL form fields before submitting it.
      Below is an example of how to use the onSubmit event. The checkForm() function will be called when the user clicks the submit button in the form. If the field values are not accepted, the submit should be cancelled. The function checkForm() returns either true or false. If it returns true the form will be submitted, otherwise the submit will be cancelled:
      <form method="post" action="xxx.htm" onsubmit="return checkForm()">

      ________________________________________

      onMouseOver and onMouseOut


      onMouseOver and onMouseOut are often used to create "animated" buttons.
      Below is an example of an onMouseOver event. An alert box appears when an onMouseOver event is detected:
      <a href="http://www.google.com" onmouseover="alert('An onMouseOver event');return false"><img src="w3s.gif" alt="google" /></a>


      The try...catch Statement


      The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.
      Syntax
      try
      {
      //Run some code here
      }
      catch(err)
      {
      //Handle errors here
      }
      Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error!
      Examples
      The example below is supposed to alert "Welcome guest!" when the button is clicked. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs. The catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:
      Example
      Code:
      <html>
      <head>
      <script type="text/javascript">
      var txt="";
      function message()
      {
      try
        {
        adddlert("Welcome guest!");
        }
      catch(err)
        {
        txt="There was an error on this page.\n\n";
        txt+="Error description: " + err.description + "\n\n";
        txt+="Click OK to continue.\n\n";
        alert(txt);
        }
      }
      </script>
      </head>
      
      <body>
      <input type="button" value="View message" onclick="message()" />
      </body>
      
      </html>
      The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:
      Example
      Code:
      <html>
      <head>
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