On Thursday, February 10, 2011
0
comments
Subject Name: Programming Skills – VII (WTAD)
Subject Code: 640012
List of Practicals:
1. Write a JavaScript that shows how a variable’s type can be changed on-the-fly.
2. Write a JavaScript that demonstrates the use of +=,-=,*=,/= operators.
3. Create a Form in HTML with two fields, minimum and maximum, write JavaScript
to validate that only numeric value is entered in both, and the value entered in
minimum is less than the value entered in maximum. Download
4. Write a JavaScript that finds out multiples of 10 in 0 to 10000. On the click of button
start the timer and stop the counter after 10 seconds. Display on the screen how
many multiples of 10 are found out within stipulated time.
5. Write a JavaScript to generate two random numbers and find out maximum and
minimum out of it.
6. Write a JavaScript to remove the highest element from the array and arrange the
array in ascending order.
7. Write a JavaScript to convert Celsius to Fahrenheit.
8. Write a JavaScript to find a string from the given text. If the match is found then
replace it with another string.
9. Write a JavaScript to show a pop up window with a message Hello and background
color lime and with solid black border.
10. Write a Servlet to display “Hello World” on browser.
11. Write a Servlet to display all the headers available from request.
12. Write a Servlet to display parameters available on request.
13. Write a Servlet to display all the attributes available from request and context. | Download Now
14. Write a Servlet which displays a message and also displays how many times the
message has been displayed (how many times the page has been visited).
15. Assume that we have got three pdf files for the MCA-1 Syllabus, MCA-2 Syllabus
and MCA-3 Syllabus respectively, Now write a Servlet which displays the
appropriate PDF file to the client, by looking at a request parameter for the year (1, 2
or 3).
Subject Code: 640012
List of Practicals:
1. Write a JavaScript that shows how a variable’s type can be changed on-the-fly.
2. Write a JavaScript that demonstrates the use of +=,-=,*=,/= operators.
3. Create a Form in HTML with two fields, minimum and maximum, write JavaScript
to validate that only numeric value is entered in both, and the value entered in
minimum is less than the value entered in maximum. Download
4. Write a JavaScript that finds out multiples of 10 in 0 to 10000. On the click of button
start the timer and stop the counter after 10 seconds. Display on the screen how
many multiples of 10 are found out within stipulated time.
5. Write a JavaScript to generate two random numbers and find out maximum and
minimum out of it.
6. Write a JavaScript to remove the highest element from the array and arrange the
array in ascending order.
7. Write a JavaScript to convert Celsius to Fahrenheit.
8. Write a JavaScript to find a string from the given text. If the match is found then
replace it with another string.
9. Write a JavaScript to show a pop up window with a message Hello and background
color lime and with solid black border.
10. Write a Servlet to display “Hello World” on browser.
11. Write a Servlet to display all the headers available from request.
12. Write a Servlet to display parameters available on request.
13. Write a Servlet to display all the attributes available from request and context. | Download Now
14. Write a Servlet which displays a message and also displays how many times the
message has been displayed (how many times the page has been visited).
15. Assume that we have got three pdf files for the MCA-1 Syllabus, MCA-2 Syllabus
and MCA-3 Syllabus respectively, Now write a Servlet which displays the
appropriate PDF file to the client, by looking at a request parameter for the year (1, 2
or 3).
On
0
comments
JavaScript Introduction
JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Firefox, Chrome, Opera, and Safari.
What You Should Already Know
Before you continue you should have a basic understanding of the following:
- HTML / XHTML
If you want to study these subjects first, find the tutorials on our
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("
" + name + "
") 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
The Real Name is ECMAScript
JavaScript is an implementation of the ECMAScript language standard.
ECMAScript is developed and maintained by the
ECMA-262 is the official JavaScript standard.
The language was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all Netscape and Microsoft browsers since 1996.
The development of ECMA-262 started in 1996, and the first edition of was adopted by the ECMA General Assembly in June 1997.
The standard was approved as an international ISO (ISO/IEC 16262) standard in 1998.
The development of the standard is still in progress.
JavaScript How To
The HTML
Changing HTML Elements
The example below writes the current date into an existing
element in the HTML document:Example
|
Examples Explained
To insert a JavaScript into an HTML page, we use the tells where the JavaScript starts and ends:
The lines between the contains the JavaScript and are executed by the browser. In this case the browser will write Hello World! to the document:
Note: Without the
The two forward slashes at the end of comment line (//) is the JavaScript comment symbol. This prevents JavaScript from executing the --> tag.
JavaScripts can be put in the and in the sections of an HTML page.
JavaScript in
The example below writes the current date into an existing
element when the page loads:Example
|
Note that the JavaScript is placed at the bottom of the page to make sure it is not executed before the
element is created.JavaScript Functions and Events
JavaScripts in an HTML page will be executed when the page loads. This is not always what we want.
Sometimes we want to execute a JavaScript when an event occurs, such as when a user clicks a button. When this is the case we can put the script inside a function.
Events are normally used in combination with functions (like calling a function when an event occurs).
You will learn more about JavaScript functions and events in later chapters.
JavaScript in
The example below calls a function when a button is clicked:
Example
|
Scripts in and
You can place an unlimited number of scripts in your document, and you can have scripts in both the body and the head section at the same time.
It is a common practice to put all functions in the head section, or at the bottom of the page. This way they are all in one place and do not interfere with page content.
Using an External JavaScript
JavaScript can also be placed in external files.
External JavaScript files often contains code to be used on several different web pages.
External JavaScript files have the file extension .js.
Note: External script cannot contain the tags!
To use an external script, point to the .js file in the "src" attribute of the
JavaScript Statements
JavaScript is a sequence of statements to be executed by the browser.
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 Statements
A JavaScript statement is a command to a browser. The purpose of the command is to tell the browser what to do.
This JavaScript statement tells the browser to write "Hello Dolly" to the web page:
document.write("Hello Dolly"); |
It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the web.
The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement. Because of this you will often see examples without the semicolon at the end.
Note: Using semicolons makes it possible to write multiple statements on one line.
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 |
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 |
The example above is not very useful. It just demonstrates the use of a block. Normally a block is used to group statements together in a function or in a condition (where a group of statements should be executed if a condition is met).
You will learn more about functions and conditions in later chapters.
JavaScript Comments
JavaScript comments can be used to make the code more readable.
JavaScript Comments
Comments can be added to explain the JavaScript, or to make the code more readable.
Single line comments start with //.
The following example uses single line comments to explain the code:
Example |
JavaScript Multi-Line Comments
Multi line comments start with /* and end with */.
The following example uses a multi line comment to explain the code:
Example |
Using Comments to Prevent Execution
In the following example the comment is used to prevent the execution of a single code line (can be suitable for debugging):
Example |
In the following example the comment is used to prevent the execution of a code block (can be suitable for debugging):
Example |
Using Comments at the End of a Line
In the following example the comment is placed at the end of a code line:
Example |
Variables are "containers" for storing information.
Do You Remember Algebra From School?
Do you remember algebra from school? x=5, y=6, z=x+y
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).
JavaScript Variables
As with algebra, JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
Rules for JavaScript variable names:
- Variable names are case sensitive (y and Y are two different variables)
- Variable names must begin with a letter or the underscore character
Note: Because JavaScript is case-sensitive, variable names are case-sensitive.
Example
A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.
Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You declare JavaScript variables with the var keyword:
var x; var carname; |
After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:
var x=5; var carname="Volvo"; |
After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
Note: When you assign a text value to a variable, use quotes around the value.
Note: If you redeclare a JavaScript variable, it will not lose its value.
Local JavaScript Variables
A variable declared within a JavaScript function becomes LOCAL and can only be accessed within that function. (the variable has local scope).
You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.
Local variables are destroyed when you exit the function.
You will learn more about functions in a later chapter of this tutorial.
Global JavaScript Variables
Variables declared outside a function becomes GLOBAL, and all scripts and functions on the web page can access it.
Global variables are destroyed when you close the page.
If you declare a variable, without using "var", the variable always becomes GLOBAL.
Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be declared as global variables.
These statements:
x=5; carname="Volvo"; |
will declare the variables x and carname as global variables (if they don't already exist).
JavaScript Arithmetic
As with algebra, you can do arithmetic operations with JavaScript variables:
y=x-5; z=y+5; |
You will learn more about the operators that can be used in the next chapter of this tutorial.
JavaScript Variables
Variables are "containers" for storing information.
Do You Remember Algebra From School?
Do you remember algebra from school? x=5, y=6, z=x+y
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).
JavaScript Variables
As with algebra, JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
Rules for JavaScript variable names:
- Variable names are case sensitive (y and Y are two different variables)
- Variable names must begin with a letter or the underscore character
Note: Because JavaScript is case-sensitive, variable names are case-sensitive.
Example
A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.
Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You declare JavaScript variables with the var keyword:
var x; var carname; |
After the declaration shown above, the variables are empty (they have no values yet).
However, you can also assign values to the variables when you declare them:
var x=5; var carname="Volvo"; |
After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
Note: When you assign a text value to a variable, use quotes around the value.
Note: If you redeclare a JavaScript variable, it will not lose its value.
Local JavaScript Variables
A variable declared within a JavaScript function becomes LOCAL and can only be accessed within that function. (the variable has local scope).
You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.
Local variables are destroyed when you exit the function.
You will learn more about functions in a later chapter of this tutorial.
Global JavaScript Variables
Variables declared outside a function becomes GLOBAL, and all scripts and functions on the web page can access it.
Global variables are destroyed when you close the page.
If you declare a variable, without using "var", the variable always becomes GLOBAL.
Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be declared as global variables.
These statements:
x=5; carname="Volvo"; |
will declare the variables x and carname as global variables (if they don't already exist).
JavaScript Arithmetic
As with algebra, you can do arithmetic operations with JavaScript variables:
y=x-5; z=y+5; |
You will learn more about the operators that can be used in the next chapter of this tutorial.
JavaScript Comparison and Logical Operators
Comparison and Logical operators are used to test for true or false.
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
variablename=(condition)?value1:value2 |
Example
greeting=(visitor=="PRES")?"Dear President ":"Dear "; |
If the variable visitor has the value of "PRES", then the variable greeting will be assigned the value "Dear President " else it will be assigned "Dear".
JavaScript If...Else Statements
Conditional statements are used to perform different actions based on different conditions.
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
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 |
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
if (condition) { code to be executed if condition is true } else { code to be executed if condition is not true } |
Example |
If...else if...else Statement
Use the if....else if...else statement to select one of several blocks of code to be executed.
Syntax
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 } |
Example |
This example demonstrates a link, when you click on the link it will take you to W3Schools.com OR to RefsnesData.no. There is a 50% chance for each of them.
JavaScript Switch Statement
Conditional statements are used to perform different actions based on different conditions.
The JavaScript Switch Statement
Use the switch statement 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: 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 |
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 |
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 |
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 |
A function will be executed by an event or by a call to the function.
JavaScript Functions
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 and in the 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 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 |
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 |
The Lifetime of JavaScript Variables
If you declare a variable, using "var", 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.
Loops execute a block of code a specified number of times, or while a specified condition is true.
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
for (variable=startvalue;variable<=endvalue;variable=variable+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 |
The while loop
The while loop will be explained in the next chapter.
Loop through the six different HTML headings.
Loops execute a block of code a specified number of times, or while a specified condition is true.
The while Loop
The while loop loops through a block of code while a specified condition is true.
Syntax
while (variable<=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 |
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
do { code to be executed }while (variable<=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 |
JavaScript Break and Continue Statements
The break Statement
The break statement will break the loop and continue executing the code that follows after the loop (if any).
Example |
The continue Statement
The continue statement will break the current loop and continue with the next value.
Example |
JavaScript For...In Statement
The for...in statement loops through the properties of an object.
Syntax
for (variable in object) { code to be executed } |
Note: The code in the body of the for...in loop is executed once for each property.
Example
Looping through the properties of an object:
Example
|
Events are actions that can be detected by JavaScript.
Acting to an Event
The example below displays the date when a button is clicked:
Example
|
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
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:
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:
Example
|
JavaScript Try...Catch Statement
The try...catch statement allows you to test a block of code for errors.
JavaScript - Catching Errors
When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page.
This chapter will teach you how to catch and handle JavaScript error messages, so you don't lose your audience.
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 |
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 |
The throw Statement
The throw statement can be used together with the try...catch statement, to create an exception for the error. Learn about the throw statement in the next chapter.
The try...catch statement allows you to test a block of code for errors.
JavaScript - Catching Errors
When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page. This chapter will teach you how to catch and handle JavaScript error messages, so you don't lose your audience.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 } |
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 |
Example |
The throw Statement
The throw statement can be used together with the try...catch statement, to create an exception for the error. Learn about the throw statement in the next chapter.JavaScript Throw Statement
The throw statement allows you to create an exception.
The Throw Statement
The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.Syntax
throw exception |
Example
The example below determines the value of a variable called x. If the value of x is higher than 10, lower than 0, or not a number, we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:Example
|
Subscribe to:
Comments (Atom)