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 and CSS

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 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 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 manipulate 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 input
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

JavaScript = ECMAScript

JavaScript is an implementation of the ECMAScript language standard. ECMA-262 is the official JavaScript standard.

JavaScript was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all browsers since 1996.

The official standardization was adopted by the ECMA organization (an industry standardization association) in 1997.

The ECMA standard (called ECMAScript-262) was approved as an international ISO (ISO/IEC 16262) standard in 1998.

The development is still in progress.

JavaScript How To

The HTML <script> tag is used to insert a JavaScript into an HTML document.

The HTML "id" attribute is used to identify HTML elements.

Manipulating HTML Elements

JavaScript is typically used to manipulate existing HTML elements.

The HTML "id" attribute is used to identify HTML elements.

To access an HTML element from a JavaScript, use the document.getElementById() method.

The document.getElementById() method will access the HTML element with the specified id.

Example

Access the HTML element with the specified id, and change its content:

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>

<p id="demo">My First Paragraph</p>

<script type="text/javascript">
document.getElementById("demo").innerHTML="My First JavaScript";
</script>

</body>
</html>

Example Explained

To insert a JavaScript into an HTML page, use the <script> tag.

Inside the <script> tag use the type attribute to define the scripting language.

The <script> and </script> tells where the JavaScript starts and ends.

The lines between the <script> and </script> contain the JavaScript and are executed by the browser:

<p id="demo">My First Paragraph.</p>

<script type="text/javascript">
document.getElementById("demo").innerHTML="My First JavaScript";
</script>

In this case, the browser will access the HTML element with id="demo", and replace the content with "My First JavaScript".

Some Browsers do Not Support JavaScript

Browsers that do not support JavaScript, will display JavaScript as page content.

To prevent them from doing this, and as a part of the JavaScript standard, the HTML comment tag should be used to "hide" the JavaScript.

Just add an HTML comment tag  (end of comment) after the last JavaScript statement, like this:

<!DOCTYPE html>
<html>
<body>
<script type="text/javascript">

</script>
</body>
</html>

The two forward slashes at the end of comment line (//) is the JavaScript comment symbol. This prevents JavaScript from executing the --> tag.

Write Directly into The HTML Document

The example below writes a <p> element into the HTML document:

Example

<!DOCTYPE html>
<html>
<body>

<h1>My First Web Page</h1>

<script type="text/javascript">
document.write("<p>My First JavaScript</p>");
</script>

</body>
</html>

JavaScript Where To

JavaScripts can be put in the <body> and in the <head> sections of an HTML page.

JavaScript in <body>

The example below manipulate the content of an existing <p> element when the page loads:

Example

<!DOCTYPE html>
<html>
<body>

<p id="demo">A Paragraph</p>

</body>
</html>

Note: The JavaScript is placed at the bottom of the page to make sure it is not executed before the <p> element is created.

JavaScript Functions and Events

The JavaScript statement in the example above, is executed when the page loads, but that 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.

Then we put the script inside a function.

Functions are normally used in combination with events.

You will learn more about JavaScript functions and events in later chapters.

JavaScript Functions in <head>

The example below calls a function when a button is clicked:

Example

<!DOCTYPE html>
<html>

<head>
<script type="text/javascript">
function myFunction()
{
document.getElementById("demo").innerHTML="My First JavaScript Function";
}
</script>
</head>

<body>

<p id="demo">A Paragraph</p>

</body>
</html>

JavaScript Functions in <body>

This example also calls a function when a button is clicked, but the script is placed at the bottom of the page:

Example

<!DOCTYPE html>
<html>
<body>

<p id="demo">A Paragraph</p>

</body>
</html>

Scripts in <head> and <body>

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 contain code to be used on several different web pages.

External JavaScript files have the file extension .js.

Note: External script cannot contain the <script></script> tags!

To use an external script, point to the .js file in the "src" attribute of the <script> tag:

Example

<!DOCTYPE html>
<html>
<body>
<script type="text/javascript" src="myScript.js"></script>
</body>
</html>

Note: You can place the script in the head or body as you like.

Note: The script will behave as it was located in the document, exactly where you put it.

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 an HTML element:

document.getElementById("demo").innerHTML="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 manipulate two HTML elements:

Example

document.getElementById("demo").innerHTML="Hello Dolly";
document.getElementById("myDIV").innerHTML="How are you?";

JavaScript Blocks

JavaScript statements can be grouped together in blocks.

Blocks start with a left curly bracket {, and end with a right curly bracket }.

The purpose of a block is to make the sequence of statements execute together.

An example of statements grouped together in blocks, are JavaScript functions.

This example will run a function that will manipulate two HTML elements:

Example

function myFunction()
{
document.getElementById("demo").innerHTML="Hello Dolly";
document.getElementById("myDIV").innerHTML="How are you?";
}

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, the $ character, or the underscore character

Note: Because JavaScript is case-sensitive, variable names are case-sensitive.

Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is most often referred to as "declaring" a variable.

You declare JavaScript variables with the var keyword:

var carname;

After the declaration shown above, the variable is empty (it has no value yet).

To assign a value to the variable, use the equal (=) sign:

carname="Volvo";

However, you can also assign a value to the variable when you declare it:

var carname="Volvo";

After the execution of the statement above, the carname will hold the value Volvo.

To write the value inside an HTML element, simply refer to it by using it's variable name:

Example

var carname="Volvo";
document.getElementById("myP").innerHTML=carname;

Note: When you assign a text value to a variable, put quotes around the value.

Note: When you assign a numeric value to a variable, do not put quotes around the value, if you put quotes around a numeric value, it will be treated as text.

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 deleted as soon as the function is completed.

You will learn more about functions in a later chapter of this tutorial.

Global JavaScript Variables

Variables declared outside a function, become GLOBAL, and all scripts and functions on the web page can access it.

Global variables are deleted when you close the page.

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.

This statement:

carname="Volvo";

will declare the variable carname as a global variable (if it does not already exist).

JavaScript Arithmetic

As with algebra, you can do arithmetic operations with JavaScript variables:

Example

y=5;
x=y+2;

You will learn more about the operators that can be used in the next chapter of this tutorial.

avaScript 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.

Example

Assign values to variables and add them together:

y=5;
z=2;
x=y+z;

The result of x will be:

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 of x	Result of y	Try it
+	Addition	x=y+2	7	5	Try it »
-	Subtraction	x=y-2	3	5	Try it »
*	Multiplication	x=y*2	10	5	Try it »
/	Division	x=y/2	2.5	5	Try it »
%	Modulus (division remainder)	x=y%2	1	5	Try it »
++	Increment	x=++y	6	6	Try it »
++	Increment	x=y++	5	6	Try it »
--	Decrement	x=--y	4	4	Try it »
--	Decrement	x=y--	5	4	Try it »

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	Try it
=	x=y		x=5	Try it »
+=	x+=y	x=x+y	x=15	Try it »
-=	x-=y	x=x-y	x=5	Try it »
*=	x*=y	x=x*y	x=50	Try it »
/=	x/=y	x=x/y	x=2	Try it »
%=	x%=y	x=x%y	x=0	Try it »

The + Operator Used on Strings

The + operator can also be used to add string variables or text values together.

Example

To add two or more string variables together, use the + operator.

txt1="What a very";
txt2="nice day";
txt3=txt1+txt2;

The result of txt3 will be:

What a verynice day

To add a space between the two strings, insert a space into one of the strings:

Example

txt1="What a very ";
txt2="nice day";
txt3=txt1+txt2;

The result of txt3 will be:

What a very nice day

or insert a space into the expression:

Example

txt1="What a very";
txt2="nice day";
txt3=txt1+" "+txt2;

The result of txt3 will be:

What a very nice day

Adding Strings and Numbers

Adding two numbers, will return the sum, but adding a number and a string will return a string:

Example

x=5+5;
y="5"+5;
z="Hello"+5;

The result of x,y, and z will be:

10
55
Hello5

The rule is: If you add a number and a string, the result will be a string!

avaScript Popup Boxes

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

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
function myFunction()
{
alert("I am an alert box!");
}
</script>
</head>
<body>

<input type="button" onclick="myFunction()" 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

var r=confirm("Press a button");
if (r==true)
{
x="You pressed OK!";
}
else
{
x="You pressed Cancel!";
}

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

var name=prompt("Please enter your name","Harry Potter");
if (name!=null && name!="")
{
x="Hello " + name + "! How are you today?";
}

Line Breaks

To display line breaks inside a popup box, use a back-slash followed by the character n.

Example

alert("Hello\nHow are you?");

JavaScript Functions

A function can be executed by an event, like clicking a button.

JavaScript Functions

A function is a block of code that executes only when you tell it to execute.
It can be when an event occurs, like when a user clicks a button, or from a call within your script, or from a call within another function.
Functions can be placed both in the <head> and in the <body> section of a document, just make sure that the function exists, when the call is made.

How to Define a Function

Syntax

function functionname()
{
some code
}

The { and the } defines the start and end of the function.
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

<!DOCTYPE html>
<html>
<head>
<script type="text/javascript">
function myFunction()
{
alert("Hello World!");
}
</script>
</head>

<body>
<button onclick="myFunction()">Try it</button>
</body>
</html>

The function is executed when the user clicks the button.
You will learn more about JavaScript events in the JS Events chapter.

Calling a Function with Arguments

When you call a function, you can pass along some values to it, these values are called arguments or parameters.
Theese arguments can be used inside the function.
You can send as many arguments as you like, separated by commas (,)

myFunction(argument1,argument2)

Declare the argument, as variables, when you declare the function:

function myFunction(var1,var2)
{
some code
}

The variables and the arguments must be in the expected order. The first variable is given the value of the first passed argument etc.

Example

<button onclick="myFunction('Harry Potter','Wizard')">Try it</button>

<script type="text/javascript">
function myFunction(name,job)
{
alert("Welcome " + name + ", the " + job);
}
</script>

The function above will alert "Welcome Harry Potter, the Wizard" when the button is clicked.
The function is flexible, you can call the function using different arguments, and different welcome messages will be given:

Example

The example above will alert "Welcome Harry Potter, the Wizard" or "Welcome Bob, the Builder" depending on which button is clicked.

Functions With a Return Value

Sometimes you want your function to return a value back to where the call was made.
This is possible by using the return statement.
When using the return statement, the function will stop executing, and return the specified value.

Syntax

function myFunction()
{
var x=5;
return x;
}

The function above will return the value 5.
Note: It is not the entire JavaScript that will stop executing, only the function. JavaScript will continue executing code, where the function-call was made from.
The function-call will be replaced with the returnvalue:

var myVar=myFunction();

The variable myVar holds the value 5, which is what the function "myFunction()" returns.
You can also use the returnvalue without storing it as a variable:

document.getElementById("demo").innerHTML=myFunction();

The innerHTML of the "demo" element will be 5, which is what the function "myFunction()" returns.
You can make a returnvalue based on arguments passed into the function:

Example

Calculate the product of two numbers, and return the result:

function myFunction(a,b)
{
return a*b;
}

document.getElementById("demo").innerHTML=myFunction(4,3);

The innerHTML of the "demo" element will be:
The return statement is also used when you simply want to exit a function. The return value is optional:

function myFunction(a,b)
{
if (a>b)
{
return;
}
x=a+b
}

The function above will exit the function if a>b, and will not calculate the sum of a and b.

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.

JavaScript Events

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

<!DOCTYPE html>
<html>

<head>
<script type="text/javascript">
function displayDate()
{
document.getElementById("demo").innerHTML=Date();
}
</script>
</head>

<body>

<h1>My First Web Page</h1>

<p id="demo"></p>

<button type="button" onclick="displayDate()">Display Date</button>

</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!

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:

onMouseOver

The onmouseover event can be used to trigger a function when the user mouses over an HTML element:

Example

Mouse over the sun and the planets and see the different descriptions.

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

<!DOCTYPE html>
<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.message + "\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

<!DOCTYPE html>
<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+="Click OK to continue viewing this page,\n";
txt+="or Cancel to return to the home page.\n\n";
if(!confirm(txt))
    {
    document.location.href="http://www.w3schools.com/";
    }
}
}
</script>
</head>

<body>
<input type="button" value="View message" onclick="message()" />
</body>

</html>

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

The exception can be a string, integer, Boolean or an object.
Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!

Example

The example below determines the value of a variable called x. If the value of x is higher than 10, lower than 5, 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

<!DOCTYPE html>
<html>
<body>
<script type="text/javascript">
var x=prompt("Enter a number between 5 and 10:","");
try
{
if(x>10)
    {
    throw "Err1";
    }
else if(x<5)
    {
    throw "Err2";
    }
else if(isNaN(x))
    {
    throw "Err3";
    }
}
catch(err)
{
if(err=="Err1")
    {
    document.write("Error! The value is too high.");
    }
if(err=="Err2")
    {
    document.write("Error! The value is too low.");
    }
if(err=="Err3")
    {
    document.write("Error! The value is not a number.");
    }
}
</script>
</body>
</html>

JavaScript Special Characters

In JavaScript you can add special characters to a text string by using the backslash sign.

Insert Special Characters

The backslash (\) is used to insert apostrophes, new lines, quotes, and other special characters into a text string.

Look at the following JavaScript code:

var txt="We are the so-called "Vikings" from the north.";
document.write(txt);

In JavaScript, a string is started and stopped with either single or double quotes. This means that the string above will be chopped to: We are the so-called

To solve this problem, you must place a backslash (\) before each double quote in "Viking". This turns each double quote into a string literal:

var txt="We are the so-called \"Vikings\" from the north.";
document.write(txt);

JavaScript will now output the proper text string: We are the so-called "Vikings" from the north.

The table below lists other special characters that can be added to a text string with the backslash sign:

Code	Outputs
\'	single quote
\"	double quote
\\	backslash
\n	new line
\r	carriage return
\t	tab
\b	backspace
\f	form feed

JavaScript Guidelines

Some other important things to know when scripting with JavaScript.

JavaScript is Case Sensitive

A function named "myfunction" is not the same as "myFunction" and a variable named "myVar" is not the same as "myvar".

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

White Space

JavaScript ignores extra spaces. You can add white space to your script to make it more readable. The following lines are equivalent:

var name="Hege";
var name = "Hege";

Break up a Code Line

You can break up a code line within a text string with a backslash. The example below will be displayed properly:

document.write("Hello \
World!");

However, you cannot break up a code line like this:

document.write \
("Hello World!");

Pages

Java Script

avaScript Popup Boxes

Alert Box

Syntax

Example

Confirm Box

Syntax

Example

Prompt Box

Syntax

Example

Line Breaks

Example

JavaScript Functions

JavaScript Functions

How to Define a Function

Syntax

JavaScript Function Example

Example

Calling a Function with Arguments

Example

Example

Functions With a Return Value

Syntax

Example

The Lifetime of JavaScript Variables

JavaScript Try...Catch Statement

JavaScript - Catching Errors

The try...catch Statement

Syntax

Examples

Example

Example

The throw Statement

JavaScript Throw Statement

The Throw Statement

Syntax

Example

Example

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