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Chapter three: Client Side Scripts
Chapter Three
Client Side Scripting Languages – Java Script
Scripting languages are used to embed programming instructions to the browser in a web page. The most widely used client-side scripting language is JavaScript. JavaScript offers several benefits including: o Simplifying some repetitive coding tasks for HTML pages o Providing functionality and interactivity not possible in HTML o Providing client functions that would otherwise require requests for additional pages and/or server-side processing
JavaScript in the HTTP Transaction The following diagram shows how the browser and its components interact with the web server and server-side resources and processing components:
Advantages of JavaScript JavaScript adds a great deal to a static web site. Here are some of the main advantages of using it: o Lightweight for fast downloading o Performs well o Minimal security risks o Works across browsers and platforms (Netscape 2.0+ and IE 3.0+)
Disadvantages JavaScript does have some weaknesses as well: o implemented differently in different browsers, and not always with backward compatibility o scripts need to be tested on all target browsers o Authoring and debugging tools are limited
JavaScript Syntax Basics JavaScript is plain ASCII text (ECMA allows for Unicode), so you can write it with any text editor (including HomeSite and ColdFusion Studio, which include a JavaScript Wizard and JavaScript Object Tree to simplify coding) o Statements terminated by semi-colon (;) or line-break (the semi-colon is optional if you have a line-break, but recommended as good coding practice) o JavaScript is case-sensitive o Comments: // Comment to end of line /* Multi-line comment */
Sample: /* Set global variables */ var browserName = navigator.appName; // browser name var browserVersion = navigator.appVersion; // version
Chapter three: Client Side Scripts
Note: a JavaScript keyword var is used to initialize variables **The
The HTML comment prevents very old browsers that do not support the SCRIPT tag from displaying the JavaScript code as plain text. The JavaScript comment before the closing bracket for the HTML comment prevents the JavaScript interpreter from trying to interpret the end of the HTML comment as JavaScript code. The language attribute of the
Example:
The code above will produce this output on an HTML page:
Hello World!
The word document.write is a standard JavaScript command for writing output to a page. By entering the document.write command between the 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:
Chapter three: Client Side Scripts
Using an External JavaScript Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page. To simplify this, 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
In addition to embedding JavaScript directly in HTML pages, you can use external JavaScript definition. The external JavaScript file should contain only JavaScript -- no HTML; it should have the extension .js to indicate the file type. Examples: form.html
Submit Form
hello.js
Note: Remember to place the script exactly where you normally would write the script!
JavaScript Variables A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value. Rules for variable names: Variable names are case sensitive They must begin with a letter or the underscore character JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME Declare a Variable You can create a variable with the var statement: var strname = some value You can also create a variable without the var statement: strname = some value
Assign a Value to a Variable You can assign a value to a variable like this: var strname = "Hege" Or like this:
Chapter three: Client Side Scripts
strname = "Hege" The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege". Lifetime of Variables When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared. If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.
Conditional Statements Conditional statements in JavaScript are used to perform different actions based on different conditions. 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 if you want to execute some code only if a specified condition is true if...else statement - use this statement if you want 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 if you want to select one of many blocks of code to be executed switch statement - use this statement if you want to select one of many blocks of code to be executed
If Statement You should use the if statement if you want to execute some code only if a specified condition is true. Syntax: 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 1
Example 2
Chapter three: Client Side Scripts
Example
The JavaScript Switch Statement Conditional statements in JavaScript are used to perform different actions based on different conditions. You should use the switch statement if you want to select one of many blocks of code to be executed.
Syntax
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: o First we have a single expression n (most often a variable), that is evaluated once. o The value of the expression is then compared with the values for each case in the structure. o If there is a match, the block of code associated with that case is executed. o Use break to prevent the code from running into the next case automatically. Example
Chapter three: Client Side Scripts
document.write("Sleepy Sunday") break default: document.write("I'm looking forward to this weekend!") }
JavaScript Operators Arithmetic Operators Operator Description Example Result + Addition x= y= x+y
- Subtraction x= y= x-y
***** Multiplication x= y= x*y
/ Division 15/ 5/
% Modulus (division remainder) 5% 10% 10%
++ Increment x= x++
x=
- - Decrement x= x--
x=
Assignment Operators Operator Example Is The Same As = x=y x=y += x+=y x=x+y
- = x-=y x=x-y = x=y x=x*y /= x/=y x=x/y %= x%=y x=x%y
Comparison Operators Operator Description Example == is equal to 5==8 returns false === is equal to (checks for both value and type) x= y="5" x==y returns true x===y returns false != is not equal 5!=8 returns true > is greater than 5>8 returns false < is less than 5<8 returns true >= is greater than or equal to 5>=8 returns false <= is less than or equal to 5<=8 returns true
Chapter three: Client Side Scripts
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")
JavaScript Functions A function is a reusable code-block that will be executed by an event, or when the function is called. To keep the browser from executing a script as soon as the page is loaded, you can write your script as a function. A function contains some code that will be executed only by an event or by a call to that function. You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file). Functions are defined at the beginning of a page, in the section.
How to Define a Function The syntax for creating a function is:
function functionname(var1,var2,...,varX) { some code }
o var1, var2, etc are variables or values passed into the function. o 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:
function functionname() { some code }
Example
o If the line: alert("Hello world!!"), in the example above had not been written within a function, it would have been executed as soon as the line was loaded. o Now, the script is not executed before the user hits the button. o We have added an onClick event to the button that will execute the function displaymessage() when the button is clicked.
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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.
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.
Example The function below should return the product of two numbers (a and b): function prod(a,b) { x=a*b return x } When you call the function above, you must pass along two parameters: product=prod(2,3) The returned value from the prod() function is 6, and it will be stored in the variable called product.
JavaScript Loops Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true. Very 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 do...while – executes at least once and 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
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 10. 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.
Chapter three: Client Side Scripts The do...while loop is a variant of the while loop. This loop will always execute a block of code ONCE, and then it will repeat the loop as long as the specified condition is true. This loop will always be executed at least once, even if the condition is false, because the code is executed before the condition is tested.
Syntax:
do { code to be executed } while (var<=endvalue)
Example
Result The number is 0
JavaScript break and continue Statements There are two special statements that can be used inside loops: break and continue. Break o The break command will break the loop and continue executing the code that follows after the loop (if any).
o Example
Result The number is 0 The number is 1 The number is 2
Continue
Chapter three: Client Side Scripts o The continue command will break the current loop and continue with the next value.
o Example
Result The number is 0 The number is 1 The number is 2 The number is 4 The number is 5 The number is 6 The number is 7 The number is 8 The number is 9 The number is 10
JavaScript For...In Statement The for...in statement is used to loop (iterate) through the elements of an array or through the properties of an object. The code in the body of the for ... in loop is executed once for each element/property.
Syntax:
for (variable in object) { code to be executed } The variable argument can be a named variable, an array element, or a property of an object.
Example: Using for...in to loop through an array:
JavaScript Events By using JavaScript, we have the ability to create dynamic web pages.
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JavaScript - Catching Errors When browsing Web pages on the internet, I guess we have all seen a JavaScript alert box, telling you there is a runtime error, and asking: "Do you wish to debug?" on some pages. Error message like that may be useful for developers, but not for the users. When users see errors, they often leave the Web page. There are two ways of catching errors in a Web page: By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6) By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3)
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!
Example 1 The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:
To take more appropriate action when an error occurs, you can add a try...catch statement. The example below contains the "Welcome guest!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, 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:
Chapter three: Client Side Scripts
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) } }
Example 2 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:
