JavaScript/Print version


  1. Welcome
    1. Introduction
    2. First Program
  2. Basics
    1. Placing the Code
      1. The script element
      2. Bookmarklets
    2. Lexical Structure
      1. Reserved Words
    3. Variables and Types
      1. NumbersStringsDatesArrays
    4. Operators
    5. Control Structures
    6. Functions and Objects
    7. Event Handling
    8. Program Flow
    9. Regular Expressions


JavaScript is an interpreted computer programming language formalized in the ECMAScript language standard. JavaScript engines interpret and execute JavaScript. JavaScript engines may be designed for use as standalone interpreters, embedding in applications, or both. The first JavaScript engine was created by Netscape for embedding in their Web browser. V8 is a JavaScript engine created for use in Google Chrome and may also be used as a standalone interpreter. Adobe Flash uses a JavaScript engine called ActionScript for development of Flash programs.

Relation to JavaEdit

JavaScript has no relation to Java aside from having a C-like syntax. Netscape developed JavaScript, and Sun Microsystems developed Java. The rest of this section assumes a background in programming. You may skip to the next section, if you like.

Variables have a static type (integer or string for example) that remains the same during the lifespan of a running program in Java, and have a dynamic type (Number or String for example) that can change during the lifespan of a running program in JavaScript. Variables must be declared prior to use in Java, and have a undefined value when referred to prior to assignment in JavaScript.

JavaScript engines may implement functionality beyond the ECMAScript language standard, such as the required functionality provided by V8, or the Document Object Model found in many Web browsers.

Java includes classes and object instances, and JavaScript uses prototypes.

About this bookEdit

This book is written as a tutorial, in the sense that all key concepts are explained. As such, it also contains exercises that are clearly marked as such at the end of a page or chapter. Answers for these exercises are also included.

The book can also be used as a reference. For this purpose, all keywords are mentioned and described.


This book assumes you have good knowledge and some experience in the use of computers, Web browsers, text editors, and software development environments. As you will not learn about HTML, CSS, Java, or website design in this book, consult an appropriate book to learn about these subjects.

First ProgramEdit

Here is a single JavaScript statement, which creates a pop-up dialog saying "Hello World!":

alert("Hello World!");

For the browser to execute the statement, it must be placed inside a <script> element. This element describes which section of the HTML code contains executable code, and will be described in further detail later.

  alert("Hello World!");

The <script> element should then be nested inside the <head> element of an HTML document. Assuming the page is viewed in a browser that has JavaScript enabled, the browser will execute (carry out) the statement as the page is loading.

<!DOCTYPE html>
<html lang="en">
    <title>Some Page</title>
      alert("Hello World!");
    <p>The content of the web page.</p>

This basic hello World program can then be used as a starting point for any new programs that you need to create.


Exercise 1-1Edit

Copy and paste the basic program in a file, save it on your hard disk as "exercise 1-1.html". You can run it in two ways:

  1. By going to the file with a file manager, and opening it using a web browser (e.g. in Windows Explorer it is done with a double click)
  2. By starting your browser, and then opening the file from the menu. For Firefox, that would be: Choose File in the menu, then Open File, then select the file.

What happens?


A dialog appears with the text: Hello World!

Exercise 1-2Edit

Save the file above as "exercise 1-2.html". Replace the double quotes in the line alert("Hello World!"); with single quotes, so it reads alert('Hello World!'); and save the result. If you open this file in the browser, what happens?


Nothing changes. A dialog appears with the text: Hello World! The double quotes and the single quotes are equivalent.

The SCRIPT TagEdit

The script elementEdit

All JavaScript, when placed in an HTML document, needs to be within a script element. A script element is used to link to an external JavaScript file, or to contain inline scripting (script snippets in the HTML file). A script element to link to an external JavaScript file looks like:

<script src="script.js"></script>

while a script element that contains inline JavaScript looks like:

  // JavaScript code here

Inline scripting has the advantage that both your HTML and your JavaScript are in one file, which is convenient for quick development and testing. Having your JavaScript in a separate file is recommended for JavaScript functions that can potentially be used in more than one page, and also to separate content from behaviour.

Inline JavaScriptEdit

Using inline JavaScript allows you to easily work with HTML and JavaScript within the same page. This is commonly used for temporarily testing out some ideas, and in situations where the script code is specific to that one page.

  // Write your JavaScript code here

Inline HTML comment markersEdit

The inline HTML comments are to prevent older browsers that do not understand the script element from displaying the script code in plain text.

  // JavaScript code here
  // -->

Older browsers that do not understand the script element will interpret the entire content of the script element above as one single HTML comment, beginning with "<!--" and ending with "-->", effectively ignoring the script completely. If the HTML comment was not there, the entire script would be displayed in plain text to the user by these browsers.

Current browsers that know about the script element will ignore the first line of a script element, if it starts with "<!--". In the above case, the first line of the actual JavaScript code is therefore the line "// JavaScript code here".

The last line of the script, "// -->", is a one line JavaScript comment that prevents the HTML end comment tag "-->" from being interpreted as JavaScript.

The use of comment markers is rarely required nowadays, as the browsers that do not recognize the script element are virtually non-existent. These early browsers were Mosaic, Netscape 1, and Internet Explorer 2. From Netscape 2.0 in December 1995 and Internet Explorer 3.0 in August 1996 onward, those browsers were able to interpret JavaScript.[1] Any modern browser that doesn't support JavaScript will still recognize the <script> tag and not display it to the user.

Inline XHTML JavaScriptEdit

In XHTML, the method is somewhat different:

  // <![CDATA[
  // [Todo] JavaScript code here!
  // ]]>

Note that the <![CDATA[ tag is commented out. The // prevents the browser from mistakenly interpreting the <![CDATA[ as a JavaScript statement. (That would be a syntax error).

Linking to external scriptsEdit

JavaScript is commonly stored in a file so that it may be used by many web pages on your site. This method is considered a best practice. This is because it separates a page's behavior (JavaScript) from its content (HTML), and it makes it easier to update code. If several pages all link to the same JavaScript file, you only have to change the code in one place.

Add src="script.js" to the opening script tag. This means that the JavaScript code for this page will be located in a file called "script.js" that is in the same directory as the web page. If the JavaScript file is located somewhere else, you must change the src attribute to that path. For example, if your JavaScript file is named "script.js" and is located in a directory called "js", your src would be "js/script.js".

Location of script elementsEdit

The script element may appear almost anywhere within the HTML file.

A standard location is within the head element. Placement within the body, however, is allowed.

<!DOCTYPE html>
  <title>Web page title</title>
<!-- HTML code here -->

There are, however, some best practices for speeding up a web site [2] suggested by the Yahoo! Developer Network that specify a different placement for script tags: to put scripts at the bottom, just before the </body> tag. This speeds up downloading, and also allows for direct manipulation of the Document Object Model (DOM) while the page is loading. It is also a good practice to separate HTML documents from CSS code for easier management.

<!DOCTYPE html>
  <title>Web page title</title>
<!-- HTML code here -->
<script src="script.js"></script>

Controlling external script evaluation and parser blockingEdit

By default, JavaScript execution is "parser blocking". When the browser encounters a script in the document, it must pause DOM construction, hand over control to the JavaScript runtime, and let the script execute before proceeding with DOM construction.[3]

As an alternative to placing scripts at the bottom of the document body, loading and execution of external scripts may be controlled using async or defer attributes. Asynchronous external scripts are loaded and executed in parallel with document parsing. The script will be executed as soon as it is available.[4]

<!DOCTYPE html>
  <title>Web page title</title>
  <script async src="script.js"></script>
<!-- HTML code here -->

Deferred external scripts are loaded in parallel with document parsing, but script execution is deferred until after the document is fully parsed.[5]

<!DOCTYPE html>
  <title>Web page title</title>
  <script defer src="script.js"></script>
<!-- HTML code here -->


  1. w:JavaScript#History and naming
  2. Yahoo: best practices for speeding up your web site
  3. Google: Adding Interactivity with JavaScript
  4. Mozilla: The Script element
  5. Mozilla: The Script element


Bookmarklets are one line scripts stored in the URL field of a bookmark. Bookmarklets have been around for a long time so they will work in older browsers.

JavaScript URI schemeEdit

You should be familiar with URL that start with schemes like http and ftp, e.g. There is also the JavaScript scheme, which is used to start every bookmarklet.

JavaScript:alert('Hello, World!');

Example usesEdit

Media controlsEdit

The values in these examples can be adapted as desired. One may replace video with audio where applicable, meaning where an <audio> tag is embedded.

Loop the video
javascript:document.getElementsByTagName("video")[0].loop=true; // also works

Can be switched off using 0 or false.

Jump to ten minutes (using multiplication)
Jump forward by one minute (sixty seconds)
Jump back by half a minute (using division)
Get duration of a video on the page in console
Alert the duration
javascript:alert('This video is '+document.getElementsByTagName("video")[0].duration+' seconds long.')
Alert the playback time
javascript:alert('The current position of the video is at '+document.getElementsByTagName("video")[0].currentTime+' seconds.')
Set audio volume to 50%
Mute audio
javascript:document.getElementsByTagName("video")[0].muted=1 // "true" works as well

Unmute using 0 or false.

Double the playback speed
Ask for playback speed
javascript:document.getElementsByTagName("video")[0].playbackRate= parseFloat( prompt("How fast should it play?") );

parseFloat is necessary to prevent setting the value to zero if the dialogue window is closed without user input.

Ask for playback position in seconds
javascript:document.getElementsByTagName("video")[0].currentTime=parseFloat( prompt("Jump to playback position in seconds:") );
Ask for playback position in minutes
javascript:document.getElementsByTagName("video")[0].currentTime=60*parseFloat( prompt("Jump to playback position in minutes:") );
Ask for playback position in percentage (0 to 100)
javascript:document.getElementsByTagName("video")[0].currentTime=document.getElementsByTagName("video")[0].duration/100*parseFloat( prompt("Jump to playback position in percents:") );

Using multiple lines of codeEdit

Since you cannot have line breaks in bookmarklets you must use a semicolon at the end of each code statement instead.

JavaScript:name=prompt('What is your name?'); alert('Hello, ' + name);

The JavaScript Protocol in LinksEdit

The JavaScript protocol can be used in links. This may be considered bad practice, as it prevents access for or confuses users who have disabled JavaScript. See Best Practices.

<a href="JavaScript:document.bgColor='#0000FF'">blue background</a>


A large quantity of links may be found on, which show a variety of features that can be performed within JavaScript.

Lexical StructureEdit

Case SensitivityEdit

JavaScript is case-sensitive. This means that Hello() is not the same as HELLO(), hello(), or even hEllo().


Whitespace includes spaces, tabs, and, line breaks.[note 1] JavaScript ignores it, but it makes the code easier for people to read.[2]

The following is JavaScript with very little whitespace.

function filterEmailKeys(event){
var charCode=event.charCode||event.keyCode;
var char=String.fromCharCode(charCode);
return true;
return false;

The following is the same JavaScript with a typical amount of whitespace.

function filterEmailKeys(event) {
  event = event || window.event;
  var charCode = event.charCode || event.keyCode;
  var char = String.fromCharCode(charCode);
  if (/[a-zA-Z0-9_\-\.@]/.exec(char)) {
    return true;
  return false;

The following is the same JavaScript with a lot of whitespace.

function filterEmailKeys( evt )
    evt = evt || window.event;

    var charCode = evt.charCode || evt.keyCode;
    var char = String.fromCharCode ( charCode );

    if ( /[a-zA-Z0-9_\-\.@]/.exec ( char ) )
        return true;

    return false;


Comments are also ignored when executing.

Comments allow you to leave notes in your code to help other people understand it. They also allow you to comment out code that you want to hide from the parser, but you don't want to delete.

Single-line comments

A double slash, //, turns all of the following text on the same line into a comment that will not be processed by the JavaScript interpreter.

// Shows a welcome message
alert("Hello, World!")
Multi-line comments

Multi-line comments start with slash asterisk, /*, and end with the reverse asterisk slash, */. Multi-line comments don't nest.

Here is an example of how to use the different types of commenting techniques.

/* This is a multi-line comment
that contains multiple lines
of commented text. */
var a = 1;
/* commented out to perform further testing
a = a + 2;
a = a / (a - 3); // is something wrong here?
alert('a: ' + a);

/* This comment has two /* but they're both cancelled out by */


In many programming languages, semicolons are required at the end of each code statement. In JavaScript the use of semicolons is optional, as a new line indicates the end of the statement (with some exceptions). This is called automatic semicolon insertion, and the exceptions can be quite surprising.[3] Automatic semicolon insertion can create hard to debug problems.

a = b + c
(d + e).print()

The above code is not interpreted as two statements. Because of the parentheses on the second line, JavaScript interprets the above as if it were

a = b + c(d + e).print();

when instead, you may have meant it to be interpreted as

a = b + c;
(d + e).print();

Even though semicolons are optional, it's preferable to end statements with a semicolon to prevent any misunderstandings from taking place.


A literal is a hard coded value. Literals provide a means of expressing specific values in your script. For example, to the right of the equals sign:

var myLiteral = "a fixed value";

There are several types of literals available. The most common are the string literals[citation needed], but there are also numeric literals, booleans, undefined, null, regex literals, array literals, and object literals.

Example of an object literal:

var myObject = { name:"value", anotherName:"anotherValue" };

Details of these different types are covered in Variables and Types.


An identifier is a name for a piece of data such as a variable, array, or function. There are rules:

  • Letters, dollar signs, underscores, and numbers are allowed in identifiers.
  • The first character cannot be a number.

Examples of valid identifiers:

  • u
  • $hello
  • _Hello
  • hello90

1A2B3C is an invalid identifier, as it starts with a number.

Naming variablesEdit

When naming variables there are some rules that must be obeyed:

  • Upper case and lower case letters of the alphabet, underscores, and dollar signs can be used
  • Numbers are allowed after the first character
  • Some other characters like "á" can technically be used in variable names,[note 2] but special characters are not allowed.
  • Variable names are case sensitive: different case means a different name
  • A variable may not be a reserved word


  1. Technically vertical tab, zero width non-breaking space, and any unicode character with the "Space Separator" category also count as whitespace.[1]
  2. as long as they have the unicode properties "ID_Start" or "ID_Continue", for the start or rest of the name respectively.[4]


  1. "ECMA262 12th Edition". ECMAScript Language Specification, Chapter 12.2 - White Space. 
  2. "Readable Code: Video". Khan Academy. 
  3. ECMA-262 12th Edition ECMAScript Language Specification, Chapter 12.9 - Automatic Semicolon Insertion
  4. "ECMA262 12th Edition". ECMAScript Language Specification, the IdentifierName production. 

Reserved WordsEdit

This page contains a list of reserved words in JavaScript, which cannot be used as names of variables, functions or other objects.

Reserved words used in JavaScriptEdit

Current list of keywords used in JavaScript Version 5.1:[1][2]

abstract arguments boolean break byte
case catch char class* const
continue debugger default delete do
double else enum* eval export*
extends* final finally float for
function goto if implements import*
in instanceof int interface let
long native new package private
protected public return short static
super* switch synchronized this throw
throws transient try typeof var
void volatile while with yield

(*) new reserved words in ECMAScript5

A few other reserved words used in JavaScript represent literal values:[1]

false null true

Words reserved for JavaScript in the futureEdit

Some words have been reserved according to the ECMAScript specification so that they cannot be used as variable names, although currently, they do not have any functionality. These keywords may or may not be reserved words for some ECMAScript specification, and are grouped according to their condition of being reserved.[3]

Words that are always reservedEdit

await enum

Words that are reserved in strict modeEdit

implements private static
interface protected
package public

Words that were reserved in ECMAScript standards 1-3Edit

abstract[citation needed] extends native[citation needed]
boolean[citation needed] final[citation needed] short[citation needed]
byte[citation needed] float[citation needed] super
char[citation needed] goto[citation needed] synchronized[citation needed]
class import throws[citation needed]
const int[citation needed] transient[citation needed]
double[citation needed] let volatile[citation needed]
export long[citation needed]


  1. a b "ECMA-262 5.1: ECMAScript Language Specification" 2011, Section 7.6.1: Reserved Words, (keywords, the two Boolean literals, the null literal, and future reserved words).
  2. "JavaScript Reserved Words". Retrieved 2016-05-24. 

Variables and TypesEdit

JavaScript is a loosely typed language. This means that you can use the same variable for different types of information, but you may also have to check what type a variable is yourself, if the differences matter. For example, if you wanted to add two numbers, but one variable turned out to be a string, the result wouldn't necessarily be what you expected.

Variable declarationEdit

Variables are commonly explicitly declared by the var statement, as shown below:

var c;

Doing so is obligatory, if the code includes the string comment "use strict;". The above variable is created, but has the default value of undefined. To be of value, the variable needs to be initialized:

var c = 0;

After being declared, a variable may be assigned a new value that will replace the old one:

c = 1;

But make sure to declare a variable with var before (or while) assigning to it; otherwise you will create a "scope bug."

Primitive typesEdit

Primitive types are types provided by the system, in this case by JavaScript. Primitive type for JavaScript are Booleans, numbers and text. In addition to the primitive types, users may define their own classes.

The primitive types are treated by JavaScript as value types and when passed to a function, they are passed as values. Some types, such as string, allow method calls.

Boolean typeEdit

Boolean variables can only have two possible values, true or false.

var mayday = false;
var birthday = true;

Numeric typesEdit

You can use an integer and double types on your variables, but they are treated as a numeric type.

var sal = 20;
var pal = 12.1;

In the ECMA JavaScript specification, your number literals can go from 0 to -+1.79769e+308. And because 5e-324 is the smallest infinitesimal you can get, anything smaller is rounded to 0.

String typesEdit

The String and char types are all strings, so you can build any string literal that you wished for.

var myName = "Some Name";
var myChar = 'f';

Complex typesEdit

A complex type is an object, be it either standard or custom made. Its home is the heap and is always passed by reference.

Array typeEdit

Main page: JavaScript/Arrays

In JavaScript, all Arrays are untyped, so you can put everything you want in an Array and worry about that later. Arrays are objects, they have methods and properties you can invoke at will. For example, the .length property indicates how many items are currently in the array. If you add more items to the array, the value of the .length gets larger. You can build yourself an array by using the statement new followed by Array, as shown below.

var myArray = new Array(0, 2, 4);
var myOtherArray = new Array();

Arrays can also be created with the array notation, which uses square brackets:

var myArray = [0, 2, 4];
var myOtherArray = [];

Arrays are accessed using the square brackets:

myArray[2] = "Hello";
var text = myArray[2];

There is no limit to the number of items that can be stored in an array.

Object typesEdit

An object within JavaScript is created using the new operator:

var myObject = new Object();

Objects can also be created with the object notation, which uses curly braces:

var myObject = {};

JavaScript objects can implement inheritance and support overriding, and you can use polymorphism. There are no scope modifiers, with all properties and methods having public access. More information on creating objects can be found in Object Oriented Programming.

You can access browser built-in objects and objects provided through browser JavaScript extensions.


In JavaScript, the scope is the current context of the code. It refers to the accessibility of functions and variables, and their context. There exists a global and a local scope. The understanding of scope is crucial to writing good code. Whenever the code accesses this, it accesses the object that "owns" the current scope.

Global scopeEdit

An entity like a function or a variable has global scope, if it is accessible from everywhere in the code.

var a = 99;

function hello() {
  alert("Hello, " + a + "!");
hello();    // prints the string "Hello, 99!"
alert(a);   // prints the number 99
console.log("a = " + a); // prints "a = 99" to the console of the browser

Here, the variable a is in global scope and accessible both in the main code part and the function hello() itself. If you want to debug your code, you may use the console.log(...) command that outputs to the console window in your browser. This can be opened under the Windows OS with the F12 key.

Local scopeEdit

A local scope exists when an entity is defined in a certain code part, like a function.

var a = 99;

function hello() {
  var x = 5;
  alert("Hello, " + (a + x) + "!");
hello();    // prints the string "Hello, 104!"
alert(a);   // prints the number 99
alert(x);   // throws an exception

If you watch the code on a browser (on Google Chrome, this is achieved by pressing F12), you will see an Uncaught ReferenceError: x is not defined for the last line above. This is because x is defined in the local scope of the function hello and is not accessible from the outer part of the code.

See alsoEdit

Further readingEdit


JavaScript implements numbers as floating point values, that is, they're attaining decimal values as well as whole number values.

Basic useEdit

To make a new number, a simple initialization suffices:

var foo = 0; // or whatever number you want

After you have made your number, you can then modify it as necessary. Numbers can be modified or assigned using the operators defined within JavaScript.

foo = 1; //foo = 1
foo += 2; //foo = 3 (the two gets added on)
foo -= 2; //foo = 1 (the two gets removed)

Number literals define the number value. In particular:

  • They appear as a set of digits of varying length.
  • Negative literal numbers have a minus sign before the digits.
  • Floating point literal numbers contain one decimal point, and may optionally use the e notation with the character e.
  • An integer literal may be prepended with "0" to indicate that a number is in base-8. (8 and 9 are not octal digits, and if found, cause the integer to be read in the normal base-10).
  • An integer literal may also be found with prefixed "0x" to indicate a hexadecimal number.

The Math objectEdit

Unlike strings, arrays, and dates, numbers in JavaScript aren't objects, so they don't contain any methods that can be accessed by the normal dot notation. Instead the static Math object provides usual numeric functions and constants as its methods and properties. As a static object, the Math object does not need to be instantiated in order to use its methods. The methods and properties of the Math object are referenced using the dot operator in the usual way, for example:

var varOne = Math.ceil(8.5);
var varPi = Math.PI;
var sqrt3 = Math.sqrt(3);



Returns the least integer greater than the number passed as an argument.

var myInt = Math.ceil(90.8);
document.write(myInt); //91;


Returns the greatest integer less than the number passed as an argument.

var myInt = Math.floor(90.8);
document.write(myInt); //90;

max(int1, int2)Edit

Returns the highest number from the two numbers passed as arguments.

var myInt = Math.max(8, 9);
document.write(myInt); //9

min(int1, int2)Edit

Returns the lowest number from the two numbers passed as arguments.

var myInt = Math.min(8, 9);
document.write(myInt); //8


Generates a pseudo-random number.

var myInt = Math.random();


Returns the closest integer to the number passed as an argument.

var myInt = Math.round(90.8);
document.write(myInt); //91;

parseInt() and parseFloat()Edit

parseInt() and parseFloat() are JavaScript functions that convert strings into numbers. If these functions are given a string of alphabetic characters (A-Z or a-z) to convert, then they both return NaN, meaning Not a Number.

var x = parseInt("7.5") + 4;
console.log(x);  //7 + 4 = 11.

var y = parseFloat("2.8") + 3;
console.log(y); //2.8 + 3 = 5.8.

var z = parseInt("Cat");
console.log(z); // NaN


Properties of the Math object are most commonly used constants or functions:

  • E: Returns the constant e.
  • PI: Returns the value of pi.
  • LN10: Returns the natural logarithm of 10.
  • LN2: Returns the natural logarithm of 2.
  • SQRT2: Returns the square root of 2.

Further readingEdit


A string is a type of variable that stores a string (chain of characters).

Basic useEdit

To make a new string, you can make a variable and give it a value of new String().

var foo = new String();

But, most developers skip that part and use a string literal:

var foo = "my string";

After you have made your string, you can edit it as you like:

foo = "bar";		// foo = "bar"
foo = "barblah";	// foo = "barblah"
foo += "bar";		// foo = "barblahbar"

A string literal is normally delimited by the '  or "  character, and can normally contain almost any character. Common convention differs on whether to use single quotes or double quotes for strings. Some developers are for single quotes (Crockford, Amaram, Sakalos, Michaux), while others are for double quotes (NextApp, Murray, Dojo). Whichever method you choose, try to be consistent in how you apply it.

Due to the delimiters, it's not possible to directly place either the single or double quote within the string when it's used to start or end the string. In order to work around that limitation, you can either switch to the other type of delimiter for that case, or place a backslash before the quote to ensure that it appears within the string:

foo = 'The cat says, "Meow!"';
foo = "The cat says, \"Meow!\"";
foo = "It's \"cold\" today.";
foo = 'It\'s "cold" today.';

Properties and methods of the String() objectEdit

As with all objects, Strings have some methods and properties.


The concat() function joins two strings.

var foo = "Hello";
var bar = foo.concat(" World!")
alert(bar);	// Hello World!


Returns the length as an integer.

var foo = "Hello!";
alert(foo.length);		// 6


Returns the first occurrence of a string inside of itself, starting with 0. If the search string cannot be found, -1 is returned. The indexOf() method is case sensitive.

var foo = "Hello, World! How do you do?";
alert(foo.indexOf(' '));	// 6

var hello = "Hello world, welcome to the universe.";
alert(hello.indexOf("welcome"));      // 13


Returns the last occurrence of a string inside of itself, starting with index 0.. If the search string cannot be found, -1 is returned.

var foo = "Hello, World! How do you do?";
alert(foo.lastIndexOf(' '));	// 24

replace(text, newtext)Edit

The replace() function returns a string with content replaced. Only the first occurrence is replaced.

var foo = "foo bar foo bar foo";
var newString = foo.replace("bar", "NEW!")
alert(foo);		// foo bar foo bar foo
alert(newString);	// foo NEW! foo bar foo

As you can see, the replace() function only returns the new content and does not modify the 'foo' object.

slice(start[, end])Edit

Slice extracts characters from the start position.

"hello".slice(1);	// "ello"

When the end is provided, they are extracted up to, but not including the end position.

"hello".slice(1, 3);	// "el"

Slice allows you to extract text referenced from the end of the string by using negative indexing.

"hello".slice(-4, -2);	// "el"

Unlike substring, the slice method never swaps the start and end positions. If the start is after the end, slice will attempt to extract the content as presented, but will most likely provide unexpected results.

"hello".slice(3, 1);	// ""

substr(start[, number of characters])Edit

substr extracts characters from the start position, essentially the same as slice.

"hello".substr(1);	// "ello"

When the number of characters is provided, they are extracted by count.

"hello".substr(1, 3);	// "ell"

substring(start[, end])Edit

substring extracts characters from the start position.

"hello".substring(1);	// "ello"

When the end is provided, they are extracted up to, but not including the end position.

"hello".substring(1, 3);	// "el"

substring always works from left to right. If the start position is larger than the end position, substring will swap the values; although sometimes useful, this is not always what you want; different behavior is provided by slice.

"hello".substring(3, 1);	// "el"


This function returns the current string in lower case.

var foo = "Hello!";
alert(foo.toLowerCase());	// hello!


This function returns the current string in upper case.

var foo = "Hello!";
alert(foo.toUpperCase());	// HELLO!

Escape SequencesEdit

Escape sequences are very useful tools in editing your code in order to style your output of string objects, this improves user experience greatly.[1]

  • \b backspace (U+0008 BACKSPACE)
  • \f: form feed (U+000C FORM FEED)
  • \n: line feed (U+000A LINE FEED)
  • \r: carriage return (U+000D CARRIAGE RETURN)
  • \t: horizontal tab (U+0009 CHARACTER TABULATION)
  • \v: vertical tab (U+000B LINE TABULATION)
  • \0: null character (U+0000 NULL) (only if the next character is not a decimal digit; else it’s an octal escape sequence)
  • \': single quote (U+0027 APOSTROPHE)
  • \": double quote (U+0022 QUOTATION MARK)
  • \\: backslash (U+005C REVERSE SOLIDUS)

var myString = "Double quote: ""; // produces: Double quote:
var myString = "Double quote: \""; // produces: Double quote: "
var my5tring = 'On Scratch, a user is called 'German'' \\ produces: On Scratch, a user is called
var my5tring = 'On Scratch, a user is called \'German\'' \\ produces: On Scratch, a user is called 'German'
var myS7ring = '"' \\ produces: "
var myS7ring = "'" \\ produces: '

Further readingEdit


A Date is an object that contains a given time to millisecond precision.

Unlike strings and numbers, the date must be explicitly created with the new operator.

var date = new Date(); // Create a new Date object with the current date and time.

The Date object may also be created using parameters passed to its constructor. By default, the Date object contains the current date and time found on the computer, but can be set to any date or time desired.

var time_before_2000 = new Date(1999, 12, 31, 23, 59, 59, 999);

The date can also be returned as an integer. This value can be used to "seed" a PRNG (Pseudo Random Number Generator) method, for example.

var integer_date = +new Date; // Returns a number, like 1362449477663.

The date object normally stores the value within the local time zone. If UTC is needed, there are a set of functions available for that use.

The Date object does not support non-CE epochs, but can still represent almost any available time within its available range.

Properties and methodsEdit

Properties and methods of the Date() object:

  • getDate(): Returns the day of the month. [1 - 31][2]
  • getDay(): Returns the day of the week within the object. [0 - 6]. Sunday is 0, with the other days of the week taking the next value.
  • getFullYear(): Retrieves the full 4-digit year within the Date object.
  • getMonth(): Returns the current month. [0 - 11][2]
  • getHours(): Returns hours based on a 24 hour clock.[2]
  • getMinutes():Returns minutes based on [0 - 59][2]
  • getSeconds():Returns seconds based on [0 - 59][2]
  • getTime(): Gets the time in milliseconds since January 1, 1970 (Epoch).
  • parse(text): Reads the string text, and returns the number of milliseconds since January 1, 1970 (Epoch).
  • setFullYear(year): Stores the full 4-digit year within the Date object.
  • setMonth(month, day): Sets the month within the Date object, and optionally the day within the month. [0 - 11]. The Date object uses 0 as January instead of 1.
  • Gets the current date and time.

Further ReadingEdit



An array is a type of variable that stores a collection of variables. Arrays in JavaScript are zero-based - they start from zero. (instead of foo[1], foo[2], foo[3], JavaScript uses foo[0], foo[1], foo[2].)


JavaScript arrays at a glance:

animals = ["cat", "dog", "tiger"]          // Initialization
fruits = [["apple", "orange"], ["pear"]]   // Initialization of a nested array
cat = animals[0]                           // Indexed access
apple = fruits[0][0]                       // Indexed access, nested
animals[0] = "fox"                         // Write indexed access

for (var i = 0; i < animals.length; ++i)
  item = animals[i]                        // For each loop

animals = animals.concat("mouse")          // Append
animals = animals.concat(["horse", "ox"])  // Expand
animals.pop()                              // Yields "ox", removing it from animals
animals.push("ox")                         // Push ox back to the end
animals.shift()                            // Yields "fox", removing it from animals
animals.unshift("fox")                     // Place fox back to the beginning
mytext = [1, 2, 3].join("-")               // Yields "1-2-3" via join
items = mytext.split("-")                  // Splits

Basic useEdit

To make a new array, make a variable and give it a value of new Array().

var foo = new Array()

After defining it, you can add elements to the array by using the variable's name, and the index of the array element in square brackets.

foo[0] = "foo";
foo[1] = "fool";
foo[2] = "food";

You can call an element in an array the same way.

//outputs "food"

You can define and set the values for an array with shorthand notation.

var foo = ["foo", "fool", "food"];


Make an array with "abc" as one of the elements, and then make an alert box using that element.

Nested arraysEdit

You can also put an array within an array.

The first step is to simply make an array. Then make an element (or more) of it an array.

var foo2 = new Array();
foo2[0] = new Array();
foo2[1] = new Array();

To call/define elements in a nested array, use two sets of square brackets.

foo2[0][0] = "something goes here";
foo2[0][1] = "something else";
foo2[1][0] = "another element";
foo2[1][1] = "yet another";
alert(foo2[0][0]); //outputs "something goes here"

You can use shorthand notation with nested arrays, too.

var foo2 = [ ["something goes here", "something else"], ["another element", "yet another"] ];

So that they're easier to read, you can spread these shorthand notations across multiple lines.

var foo2 = [
    ["something goes here", "something else"],
    ["another element", "yet another"]

Properties and methods of the Array() objectEdit


The concat() method returns the combination of two or more arrays. To use it, first you need two or more arrays to combine.

var arr1 = ["a","b","c"];
var arr2 = ["d","e","f"];

Then, make a third array and set its value to arr1.concat(arr2).

var arr3 = arr1.concat(arr2) //arr3 now is: ["a","b","c","d","e","f"]

Note that in this example the new arr3 array contains the contents of both the arr1 array and the arr2 array.

join() and split()Edit

The Array Object's join() method returns a single string which contains all of the elements of an array — separated by a specified delimiter. If the delimiter is not specified, it is set to a comma. The String object's split() method returns an array in which the contents of the supplied string become the array elements — each element separated from the others based on a specified string delimiter.

To use join(), first make an array.

var abc = ["a","b","c"];

Then, make a new variable and set it to abc.join().

var a = abc.join(); //"a,b,c"

You can also set a delimiter.

var b = abc.join("; "); //"a; b; c"

To convert it back into an array with the String object's split() method.

var a2 = a.split(","); //["a","b","c"]
var b2 = b.split("; "); //["a";"b";"c"]

pop() and shift()Edit

The Array pop() method removes and returns the last element of an array. The Array shift() method removes and returns the first element of an array. The length property of the array is changed by both the pop and shift methods.

(note: The shift() method also changes all the index numbers of the array. For example, arr[0] is removed, arr[1] becomes arr[0], arr[2] becomes arr[1], and so on.)

First, make an array.

var arr = ["0","1","2","3"];

Then use pop() or shift().

alert(arr); //outputs "0,1,2,3"
alert(arr.pop()); //outputs "3"
alert(arr); //outputs "0,1,2"
alert(arr.shift()); //outputs "0"
alert(arr); //outputs "1,2"

push() and unshift()Edit

The push() and unshift() methods reverse the effect of pop() and shift(). The push() method adds an element to the end of an array and returns its new length. The unshift() method does the same with the beginning of the array (and like shift(), also adjusts the indexes of the elements.)

arr.unshift("0"); //"0,1,2"
arr.push("3"); //"0,1,2,3"

Further readingEdit


Arithmetic operatorsEdit

JavaScript has the arithmetic operators +, -, *, /, and %. These operators function as the addition, subtraction, multiplication, division, and modulus operators, and operate very similarly to other languages. Multiplication and division operators will be calculated before addition and subtraction. Operations in parenthesis will be calculated first.

var a = 12 + 5;    // 17
var b = 12 - 5;    // 7
var c = 12*5;      // 60
var d = 12/5;      // 2.4 - division results in floating point numbers.
var e = 12%5;      // 2 - the remainder of 12/5 in integer math is 2.
var f = 5 -2 * 4   // -3 - multiplication is calculated first.
var g = (2+2) / 2  // 2 - Parenthesis are calculated first.

Some mathematical operations, such as dividing by zero, cause the returned variable to be one of the error values - for example, infinity, or NaN.

The return value of the modulus operator maintains the sign of the first operand.

The + and - operators also have unary versions, where they operate only on one variable. When used in this fashion, + returns the number representation of the object, while - returns its negative counterpart.

var a = "1";
var b = a;     // b = "1": a string
var c = +a;    // c = 1: a number
var d = -a;    // d = -1: a number

+ is also used as the string concatenation operator: If any of its arguments is a string or is otherwise not a number, any non-string arguments are converted to strings, and the 2 strings are concatenated. For example, 5 + [1, 2, 3] evaluates to the string "51, 2, 3". More usefully, str1 + " " + str2 returns str1 concatenated with str2, with a space between.

All other arithmetic operators will attempt to convert their arguments into numbers before evaluating. Note that unlike C or Java, the numbers and their operation results are not guaranteed to be integers.

Bitwise operatorsEdit

There are seven bitwise operators: &, |, ^, ~, >>, <<, and >>>.

These operators convert their operands to integers (truncating any floating point towards 0), and perform the specified bitwise operation on them. The logical bitwise operators, &, |, and ^, perform the and, or, and xor on each individual bit and provides the return value. The ~ (not operator) inverts all bits within an integer, and usually appears in combination with the logical bitwise operators.

Two bit shift operators, >>, <<, move the bits in one direction that has a similar effect to multiplying or dividing by a power of two. The final bit-shift operator, >>>, operates the same way, but does not preserve the sign bit when shifting.

These operators are kept for parity with the related programming languages, but are unlikely to be used in most JavaScript programs.

Assignment operatorsEdit

The assignment operator = assigns a value to a variable. Primitive types, such as strings and numbers are assigned directly, however function and object names are just pointers to the respective function or object. In this case, the assignment operator only changes the reference to the object rather than the object itself. For example, after the following code is executed, "0, 1, 0" will be alerted, even though setA was passed to the alert, and setB was changed. This is, because they are two references to the same object.

setA = [ 0, 1, 2 ];
setB = setA;
setB[2] = 0;

Similarly, after the next bit of code is executed, x is a pointer to an empty array.

z = [5];
x = z;

All the above operators have corresponding assignment operators of the form operator=. For all of them, x operator= y is just a convenient abbreviation for x = x operator y.

Arithmetic Logical Shift
+= &= >>=
-= |= <<=
*= ^= >>>=

For example, a common usage for += is in a for loop

var element = document.getElementsByTagName('h2');
var i;
for (i = 0; i < element .length; i += 1) {
  // do something with element [i]

Increment operatorsEdit

There are also the increment and decrement operators, ++ and --. a++ increments a and returns the old value of a. ++a increments a and returns the new value of a. The decrement operator functions similarly, but reduces the variable instead.

As an example, the last four lines all perform the same task:

var a = 1;
a = a + 1;
a += 1;

Pre and post-increment operatorsEdit

Increment operators may be applied before or after a variable. When they are applied before or after a variable, they are pre-increment or post-increment operators, respectively. The choice of which to use changes how they affect operations.

// increment occurs before a is assigned to b
var a = 1;
var b = ++a; // a = 2, b = 2;

// increment occurs to c after c is assigned to d
var c = 1;
var d = c++; // c = 2, d = 1;

Due to the possibly confusing nature of pre and post-increment behaviour, code can be easier to read, if the increment operators are avoided.

// increment occurs before a is assigned to b
var a = 1;
a += 1;
var b = a; // a = 2, b = 2;

// increment occurs to c after c is assigned to d
var c = 1;
var d = c;
c += 1; // c = 2, d = 1;

Comparison operatorsEdit

The comparison operators determine, if the two operands meet the given condition.

Operator Returns Notes
== true, if the two operands are equal May ignore operand's type
(e.g. a string as an integer)
=== true, if the two operands are identical Does not ignore operands' types, and only
returns true if they are the same type and value
!= true, if the two operands are not equal May ignore an operand's type
(e.g. a string as an integer)
!== true, if the two operands are not identical Does not ignore the operands' types, and only
returns false if they are the same type and value.
> true, if the first operand is greater than the second one
>= true, if the first operand is greater than or equal to the second one
< true, if the first operand is less than the second one
<= true, if the first operand is less than or equal to the second one

Be careful when using == and !=, as they may ignore the type of one of the terms being compared. This can lead to strange and non-intuitive situations, such as:

0 == '' // true
0 == '0' // true
false == 'false' // false; (''Boolean to string'')
false == '0' // true (''Boolean to string'')
false == undefined // false
false == null // false (''Boolean to null'')
null == undefined // true

For stricter compares use === and !==

0 === '' // false
0 === '0' // false
false === 'false' // false
false === '0' // false
false === undefined // false
false === null // false
null === undefined // false

Logical operatorsEdit

  • && - and
  • || - or
  • ! - not

The logical operators are and, or, and not. The && and || operators accept two operands and provides their associated logical result, while the third accepts one, and returns its logical negation. && and || are short circuit operators. If the result is guaranteed after evaluation of the first operand, it skips evaluation of the second operand.

Technically, the exact return value of these two operators is also equal to the final operand that it evaluated. Due to this, the && operator is also known as the guard operator, and the || operator is also known as the default operator.

function handleEvent(event) {
    event = event || window.event;
    var target = || event.srcElement;
    if (target && target.nodeType === 1 && target.nodeName === 'A') {
        // ...

The ! operator determines the inverse of the given value, and returns the boolean: true values become false, or false values become true.

Note: JavaScript represents false by either a Boolean false, the number 0, NaN, an empty string, or the built in undefined or null type. Any other value is treated as true.

Other operatorsEdit

? :Edit

The ? : operator (also called the "ternary" operator).

var target = (a == b) ? c : d;

Be cautious though in its use. Even though you can replace verbose and complex if/then/else chains with ternary operators, it may not be a good idea to do so. You can replace

if (p && q) {
    return a;
} else {
    if (r != s) {
        return b;
    } else {
        if (t || !v) {
            return c;
        } else {
            return d;


return (p && q) ? a
  : (r != s) ? b
  : (t || !v) ? c
  : d

The above example is a poor coding style/practice. When other people edit or maintain your code, (which could very possibly be you,) it becomes much more difficult to understand and work with the code.

Instead, it is better to make the code more understandable. Some of the excessive conditional nesting can be removed from the above example.

if (p && q) {
    return a;
if (r != s) {
    return b;
if (t || !v) {
    return c;
} else {
    return d;


delete x unbinds x.

The delete keyword deletes a property from an object. The delete keyword deletes both the value of the property and the property itself. After deletion, the property cannot be used before it is added back again. The delete operator is designed to be used on object properties. It has no effect on variables or functions.[1]


new cl creates a new object of type cl. The cl operand must be a constructor function.


o instanceof c tests whether o is an object created by the constructor c.


typeof x returns a string describing the type of x. Following values may be returned:[2]

Type returns
boolean "boolean"
number "number"
string "string"
function "function"
undefined "undefined"
null "object"
others "object"
  1. W3Schools: JavaScript Object Properties
  2. "typeof" (in English) (HTML). Mozilla Corporation. 2014-11-18. Archived from the original on 2014-11-18. Retrieved 2015-03-05. 

Control StructuresEdit

The control structures within JavaScript allow the program flow to change within a unit of code or function. These statements can determine whether or not given statements are executed - and provide the basis for the repeated execution of a block of code.

Most of the statements listed below are so-called conditional statements that can operate either on a statement or on a block of code enclosed with braces ({ and }). The structure provided by the use of conditional statements utilizes Booleans to determine whether or not a block gets executed. In this use of Booleans, any defined variable that is neither zero nor an empty string will be evaluated as true.

control structuresEdit


The if statement is straightforward — if the given expression is true, the statement or statements will be executed. Otherwise, they are skipped.

if (a === b) {
  document.body.innerHTML += "a equals b";

The if statement may also consist of multiple parts, incorporating else and else if sections. These keywords are part of the if statement, and identify the code blocks that are executed, if the preceding condition is false.

if (a === b) {
  document.body.innerHTML += "a equals b";
} else if (a === c) {
  document.body.innerHTML += "a equals c";
} else {
  document.body.innerHTML += "a does not equal either b or c";


The while statement executes a given statement as long as a given expression is true. For example, the code block below will increase the variable c to 10:

while (c < 10) {
  c += 1;
  // …

This control loop also recognizes the break and continue keywords. The break keyword causes the immediate termination of the loop, allowing for the loop to terminate from anywhere within the while block.

The continue keyword finishes the current iteration of the while block or statement, and checks the condition to see, if it is true. If it is true, the loop commences again.

do … whileEdit

The do … while statement executes a given statement as long as a given expression is true - however, unlike the while statement, this control structure will always execute the statement or block at least once. For example, the code block below will increase the variable c to 10:

do {
  c += 1;
} while (c < 10);

As with while, break and continue are both recognized and operate in the same manner. In other words, break exits the loop, and continue checks the condition before attempting to restart the loop.


The for statement allows greater control over the condition of iteration. While it has a conditional statement, it also allows a pre-loop statement, and post-loop increment without affecting the condition. The initial expression is executed once, and the conditional is always checked at the beginning of each loop. At the end of the loop, the increment statement executes before the condition is checked once again. The syntax is:

for (<initial expression>;<condition>;<final expression>)

The for statement is usually used for integer counters:

var c;
for (c = 0; c < 10; c += 1) {
  // …

While the increment statement is normally used to increase a variable by one per loop iteration, it can contain any statement, such as one that decreases the counter.

break and continue are both recognized. The continue statement will still execute the increment statement before the condition is checked.

A second version of this loop is the for .. in statement that has following form:

for (element in object) {
  // …

The order of object elements accessed by this version is arbitrary. For instance, this structure can be used to loop through all the properties of an object instance. It should not be used when the object is of Array type


The switch statement evaluates an expression, and determines flow control based on the result of the expression:

switch(i) {
case 1:
  // …
case 2:
  // …
  // …

When i gets evaluated, its value is checked against each of the case labels. These case labels appear in the switch statement and, if the value for the case matches i, continues the execution at that point. If none of the case labels match, execution continues at the default label (or skips the switch statement entirely, if none is present.)

Case labels may only have constants as part of their condition.

The break keyword exits the switch statement, and appears at the end of each case in order to prevent undesired code from executing. While the break keyword may be omitted (for example, you want a block of code executed for multiple cases), it may be considered bad practice doing so.

The continue keyword does not apply to switch statements.

A slightly different usage of the switch statement can be found at the following link:

Omitting the break can be used to test for more than one value at a time:

switch(i) {
case 1:
case 2:
case 3:
  // …
case 4:
  // …
  // …

In this case the program will run the same code in case i equals 1, 2 or 3.


The with statement is used to extend the scope chain for a block[1] and has the following syntax:

with (expression) {
  // statement


The with statement can help to

  • reduce file size by reducing the need to repeat a lengthy object reference, and
  • relieve the interpreter of parsing repeated object references.

However, in many cases, this can be achieved by using a temporary variable to store a reference to the desired object.


The with statement forces the specified object to be searched first for all name lookups. Therefore

  • all identifiers that aren't members of the specified object will be found more slowly in a 'with' block and should only be used to encompass code blocks that access members of the object.
  • with makes it difficult for a human or a machine to find out which object was meant by searching the scope chain.
  • Used with something else than a plain object, with may not be forward-compatible.

Therefore, the use of the with statement is not recommended, as it may be the source of confusing bugs and compatibility issues.


var area;
var r = 10;

with (Math) {
  a = PI*r*r;       // == a = Math.PI*r*r
  x = r*cos(PI);    // == a = r*Math.cos(Math.PI);
  y = r*sin(PI/2);  // == a = r*Math.sin(Math.PI/2);

Functions and ObjectsEdit


A function is an action to take to complete a goal, objective, or task. Functions allow you to split a complex goal into simpler tasks, which make managing and maintaining scripts easier. Parameters or arguments can be used to provide data, which is passed to a function to effect the action to be taken. The Parameters or arguments are placed inside the parentheses, then the function is closed with a pair of curly braces. The block of code to be executed is placed inside the curly braces. Functions can be passed zero or more arguments. A function is executed when a call to that function is made anywhere within the script, the page, an external page, or by an event. Functions are always guaranteed to return some value when executed. The data passed to a function when executed is known as the function's input and the value returned from an executed function is known as the function's output.

A JavaScript function is a code-block that can be reused. The function can be called via an event, or by manual calling.

Functions can be constructed in three main ways. We begin with three "Hello, World!" examples:

Way 1 Way 2 Way 3
function hello() {
  alert("Hello, World!");
var hello = function() {
  alert("Hello, World!");
var hello = new Function(
  'alert("Hello, World!");'

Each function:

  • can be called with hello()
  • does not expect any arguments
  • performs an action to alert the user with a message
  • undefined is returned when execution is finished

The hello function can be changed to allow you to say hello to someone specific through the use of arguments:

Way 1 Way 2 Way 3
function hello(who) {
  alert("Hello, " + who + "!");
var hello = function(who) {
  alert("Hello, " + who + "!");
var hello = new Function('who',
  'alert("Hello, " + who + "!");'

Each function:

  • can be called with hello()
  • expects one argument to be passed
  • performs an action to alert the user with a message
  • undefined is returned when execution is finished

Each function can be called in several ways:

Way 1 Way 2 Way 3
hello("you");, "you");
hello.apply(window, ["you"]);


Basic exampleEdit

function myFunction(string) {
  document.innerHTML += string;

The example would first:

  • Define the myFunction function
  • Call the myFunction function with argument "hello"

The result:

  • An alert message with 'hello'
  • The string 'hello' being added to the end of the document's/page's HTML.

"Hello World!"Edit

Let's put together the "Hello World!" code from above on a sample Web page. The page calls the function once when the page is loaded, and whenever a button is clicked.

  <head><title>Some Page</title></head>
    <button id="msg">greeting</button>
    <script type="text/javascript">
      function hello() {
        alert("Hello World!");

      document.getElementById("msg").onclick = hello;


Extended "Hello World!"Edit

In the following example, the function hello does "understand" whether it is called with a parameter or not, and returns the greeting with this parametre, or with the word "World":

<!DOCTYPE html>
  <head><title>Extended Hello World!</title></head>
    <button id="msg">greeting</button>
    <script type="text/javascript">
      function hello(who) {
        if ((who == null)
         || (who.toString().search("object") >= 0)) {

          who = "World";
        alert("Hello " + who + "!");
      document.getElementById("msg").onclick = hello;

Functions with argumentsEdit

Let's start with a quick example, then we will break it down.

function stepToFive(number) {
  if (number > 5) {
    number -= 1;
  if (number < 5) {
    number += 1;
  return number;

This program takes a number as an argument. If the number is larger than 5, it subtracts one. If it's smaller than five it adds one. Let's get down and dirty and look at this piece by piece.

function stepToFive(number) { 

This is similar to what we've seen before. We now have number following the function name. This is where we define our arguments for later use, which is similar to defining variables, except in this case the variables are only valid inside of the function.

  if (number > 5) { 

If statements. If the condition is true, execute the code inside the curly brackets.

  number -= 1;

Assuming that JavaScript is your first language, you might not know what this means. This takes one off from the variable number. You can think of it as a useful shorthand for number = number - 1;.

  number += 1;

This is similar to the last one, except that it adds one instead of subtracting it.

  return number;

This returns the value of number from the function. This will be covered in more depth later on.

Here is an example of calling a function within an html page.

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">
<html lang="en">
  <title>Some Page</title>
    <script type="text/javascript">
      function stepToFive(number) {
        if (number > 5) {
          number -= 1;
        if (number < 5) {
          number += 1;
        return number;
      <script type="text/javascript">
        var num = stepToFive(6);

There are a few more things to cover here.

var num = stepToFive(6);

This is where the return statement in the function comes in handy. num here gets assigned the number 5, since that is what stepToFive will return when it is called with an argument of 6.

See AlsoEdit

Event HandlingEdit

Event HandlersEdit

An event that can be handled is something happening in a browser window, including a document loading, the user clicking a mouse button, the user pressing a key, and the browser screen changing size. When a function is assigned to handle an event type, that function is run when an event of the event type occurs.

An event handler can be assigned in the following ways:

  1. Via an element attribute directly in HTML: <body onload="alert('Hello World!');">
  2. Via JavaScript, by assigning the event type to an element attribute: document.onclick = clickHandler;
  3. Via JavaScript by a direct call to the addEventListener() method of an element.

A handler that is assigned from a script uses the syntax '[element].[event] = [function];', where [element] is a page element, [event] is the name of the selected event and [function] is the name of the function that is called when the event takes place.

For example:

document.onclick = clickHandler;

This handler will cause the function clickHandler() to be executed whenever the user clicks the mouse anywhere on the screen. Note that when an event handler is assigned, the function name does not end with parentheses. We are just pointing the event to the name of the function. The clickHandler() function is defined like this:

function clickHandler(event) {
  //some code here

In some browsers the event must be explicitly passed to the function, so as a precaution it's often best to include a conditional to test that the event variable has been passed, and if it hasn't then to use an alternative method that works on those other browsers:

function clickHandler(event) {
  event = event || window.event;
  //some code here

Elements within a document can also be assigned event handlers. For example:

document.getElementsByTagName('a')[0].onclick = linkHandler;

This will cause the linkHandler() function to be executed when the user clicks the first link on the page.

Keep in mind that this style of handler assignment depends on the link's position inside the page. If another link tag is added before this one, it will take over the handler from the original link. A best practice is to maintain the separation of code and page structure by assigning each link an identifier by using the id attribute.

<a id="faqLink" href="faq.html">Faq</a>

A handler assignment can then work regardless of where the element is positioned.

document.getElementById('faqLink').onclick = linkHandler;

Events are actions that can be detected by JavaScript, and the event object gives information about the event that has occurred. Sometimes we want to execute a JavaScript when an event occurs, such as when a user clicks a button. Events are normally used in combination with functions, and the function will not be executed before the event occurs! JavaScript event handlers are divided into two types:

  1. Interactive event handlers - depend on user interaction with the HTML page; ex. clicking a button
  2. Non-Interactive event handlers - do not need user interaction; ex. on load

Event AttributesEdit

Below are the event attributes that can be inserted into different HTML elements to define event actions. IE: Internet Explorer, F: Firefox, O: Opera, W3C: W3C Standard.

Attribute The event occurs when... IE F O W3C
onblur An element loses focus 3 1 9 Yes
onchange The content of a field changes 3 1 9 Yes
onclick Mouse clicks an object 3 1 9 Yes
ondblclick Mouse double-clicks an object 4 1 9 Yes
onerror An error occurs when loading
a document or an image
4 1 9 Yes
onfocus An element gets focus 3 1 9 Yes
onkeydown A keyboard key is pressed 3 1 No Yes
onkeypress A keyboard key is pressed
or held down
3 1 9 Yes
onkeyup A keyboard key is released 3 1 9 Yes
onload A page or image has
finished loading
3 1 9 Yes
onmousedown A mouse button is pressed 4 1 9 Yes
onmousemove The mouse is moved 3 1 9 Yes
onmouseout The mouse is moved
off an element
4 1 9 Yes
onmouseover The mouse is moved
over an element
3 1 9 Yes
onmouseup A mouse button is released 4 1 9 Yes
onresize A window or frame is resized 4 1 9 Yes
onselect Text is selected 3 1 9 Yes
onunload The user exits the page 3 1 9 Yes

Mouse/Keyboard Attributes:

Property Description IE F O W3C
altKey Returns whether or not the "ALT"
key was pressed when an event
was triggered
6 1 9 Yes
button Returns which mouse button was
clicked when an event was triggered
6 1 9 Yes
clientX Returns the horizontal coordinate of
the mouse pointer when an event was triggered
6 1 9 Yes
clientY Returns the vertical coordinate of the
mouse pointer when an event was triggered
6 1 9 Yes
ctrlKey Returns whether or not the "CTRL" key
was pressed when an event was triggered
6 1 9 Yes
metaKey Returns whether or not the "meta" key
was pressed when an event was triggered
6 1 9 Yes
relatedTarget Returns the element related to the
element that triggered the event
No 1 9 Yes
screenX Returns the horizontal coordinate of the
mouse pointer when an event was triggered
6 1 9 Yes
screenY Returns the vertical coordinate of the mouse
pointer when an event was triggered
6 1 9 Yes
shiftKey Returns whether or not the "SHIFT" key was
pressed when an event was triggered
6 1 9 Yes

Other Event Attributes:

Property Description IE F O W3C
bubbles Returns a Boolean value that indicates
whether or not an event is a bubbling event
No 1 9 Yes
cancellable Returns a Boolean value that indicates
whether or not an event can have
its default action prevented
No 1 9 Yes
currentTarget Returns the element whose event
listeners triggered the event
No 1 9 Yes
Returns the element that triggered the event No 1 9 Yes
timeStamp Returns the time stamp, in milliseconds
from the epoch (system start or event trigger)
No 1 9 Yes

Standard event handlersEdit

Attribute Trigger
onabort Loading of image was interrupted
onblur Element loses focus
onchange Element gets modified
onclick Element gets clicked
ondblclick Element gets double clicked
onerror An error occurred loading an element
onfocus An element received focus
onkeydown A key was pressed when an element has focus
onkeypress A keystroke was received by the element
onkeyup A key was released when the element has focus
onload An element was loaded
onmousedown The mouse button was pressed on the element
onmousemove The mouse pointer moves while inside the element
onmouseout The mouse pointer was moved outside the element
onmouseover The mouse pointer was moved onto the element
onmouseup The mouse button was released on the element.
onreset The form's reset button was clicked
onresize The containing window or frame was resized
onselect Text within the element was selected
onsubmit A form is being submitted
onunload The content is being unloaded (e.g. window being closed)
onscroll The user scrolls (in any direction and with any means).

Event Handlers as HTML attributesEdit

In HTML, JavaScript events can be included within any specified attribute - for example, a body tag can have an onload event:

<body onload="alert('Hello World!');">

The content of the HTML event attributes is JavaScript code that is interpreted when the event is triggered, and works very similarly to the blocks of JavaScript. This form of code is used when you want to have the JavaScript attached directly to the tag in question.

This type of technique is called inline JavaScript, and can be seen as being a less desirable technique than other unobtrusive JavaScript techniques that have previously been covered. The use of inline JavaScript can be considered to be similar in nature to that of using inline CSS, where HTML is styled by putting CSS in style attributes. This is a practice that is best avoided in favour of more versatile techniques.


This method adds an event handler to an element for an event type without dropping existing handlers.



This methods removes a particular event handler from an element, given event type.



keyCode property of a keyboard event contains a number indicating which key was pressed.

For keydown event, key codes include 65 for A through 90 for Z, and 48 for 0 through 57 for 9.


document.addEventListener("keydown", function(evt) {
  alert("Key pressed; key code: " + evt.keyCode);


Further readingEdit

Regular ExpressionsEdit


JavaScript implements regular expressions (regex for short) when searching for matches within a string. As with other scripting languages, this allows searching beyond a simple letter-by-letter match, and can even be used to parse strings in a certain format.

Unlike strings, regular expressions are delimited by the slash (/) character, and may have some options appended.

Regular expressions most commonly appear in conjunction with the string.match() and string.replace() methods.

At a glance, by example:

strArray = "Hello world!".match(/world/); // Singleton array; note the slashes
strArray = "Hello!".match(/l/g); // Matched strings are returned in a string array
"abc".match(/a(b)c/)[1] === "b" // Matched subgroup is the 2nd item (index 1)
str1 = "Hey there".replace(/Hey/g, "Hello");
str2 = "N/A".replace(/\//g, ","); // Slash is escaped with \
str3 = "Hello".replace(/l/g, "m").replace(/H/g, "L").replace(/o/g, "a"); // Pile
if (str3.match(/emma/)) { console.log("Yes"); }
if (str3.match("emma")) { console.log("Yes"); } // Quotes work as well
"abbc".replace(/(.)\1/g, "$1") === "abc" // Backreference


JavaScript's set of regular expressions follows the extended set. While copying a Regex pattern from JavaScript to another location may work as expected, some older programs may not function as expected.

  • In the search term, \1 is used to back reference a matched group, as in other implementations.
  • In the replacement string, $1 is substituted with a matched group in the search, instead of \1.
    • Example: "abbc".replace(/(.)\1/g, "$1") => "abc"
  • | is magic, \| is literal
  • ( is magic, \( is literal
  • The syntaxes (?=...), (?!...), (?<=...), and (?<!...) are not available.


  • Matching
    • string = "Hello world!".match(/world/);
    • stringArray = "Hello world!".match(/l/g); // Matched strings are returned in a string array
    • "abc".match(/a(b)c/)[1] => "b" // Matched subgroup is the second member (having the index "1") of the resulting array
  • Replacement
    • string = string.replace(/expression without quotation marks/g, "replacement");
    • string = string.replace(/escape the slash in this\/way/g, "replacement");
    • string = string.replace( ... ).replace ( ... ). replace( ... );
  • Test
    • if (string.match(/regexp without quotation marks/)) {


Single-letter modifiers:

g Global. The list of matches is returned in an array.
i Case-insensitive search
m Multiline. If the operand string has multiple lines, ^ and $ match the beginning and end of each line within the string, instead of matching the beginning and end of the whole string only:
"a\nb\nc".replace(/^b$/g,"d") === "a\nb\nc"
"a\nb\nc".replace(/^b$/gm,"d") === "a\nd\nc"



Operator Effect
\b Matches boundary of a word.
\w Matches an alphanumeric character, including "_".
\W Negation of \w.
\s Matches a whitespace character (space, tab, newline, formfeed)
\S Negation of \s.
\d Matches a digit.
\D Negation of \d.

Function callEdit

For complex operations, a function can process the matched substrings. In the following code, we are capitalizing all the words. It can't be done by a simple replacement, as each letter to capitalize is a different character:

var capitalize = function(matchobj) {
  var group1 = matchobj.replace(/^(\W)[a-zA-Z]+$/g, "$1");
  var group2 = matchobj.replace(/^\W([a-zA-Z])[a-zA-Z]+$/g, "$1");
  var group3 = matchobj.replace(/^\W[a-zA-Z]([a-zA-Z]+)$/g, "$1");
  return group1 + group2.toUpperCase() + group3;

var classicText = "To be or not to be?";

var changedClassicText = classicText.replace(/\W[a-zA-Z]+/g, capitalize);

console.log(changedClassicText==="To Be Or Not To Be?");

The function is called for each substring. Here is the signature of the function:

function (<matchedSubstring>[, <capture1>, ...<captureN>, <indexInText>, <entireText>]) {
return <stringThatWillReplaceInText>;


  • The first parameter is the substring that matches the pattern.
  • The next parameters are the captures in the substrings. There are as many parameters as there are captures.
  • The next parameter is the index of the beginning of the substring starting from the beginning of the text.
  • The last parameter is a remainder of the entire text.
  • The return value will be put in the text instead of the matching substring.

See alsoEdit

External linksEdit



Editor's note
The following is only an outline, to be fleshed out.

JavaScript optimizationEdit

Optimization TechniquesEdit

  • High Level Optimization
    • Algorithmic Optimization (Mathematical Analysis)
    • Simplification
  • Low Level Optimization
    • Loop Unrolling
    • Strength Reduction
    • Duff's Device
    • Clean Loops
  • External Tools & Libraries for speeding/optimizing/compressing JavaScript code

Common Mistakes and MisconceptionsEdit

String concatenationEdit

Strings in JavaScript are immutable objects. This means that once you create a string object, to modify it, another string object must theoretically be created.

Now, suppose you want to perform a ROT-13 on all the characters in a long string. Supposing you have a rot13() function, the obvious way to do this might be:

var s1 = "the original string";
var s2 = "";

for (i = 0; i < s1.length; i++) {
  s2 += rot13(s1.charAt(i));

Especially in older browsers like Internet Explorer 6, this will be very slow. This is because, at each iteration, the entire string must be copied before the new letter is appended.

One way to make this script faster might be to create an array of characters, then join it:

var s1 = "the original string";
var a2 = new Array(s1.length);
var s2 = "";

for (i = 0; i < s1.length; i++) {
  a2[i] = rot13(s1.charAt(i));
s2 = a2.join('');

Internet Explorer 6 will run this code faster. However, since the original code is so obvious and easy to write, most modern browsers have improved the handling of such concatenations. On some browsers the original code may be faster than this code.

A second way to improve the speed of this code is to break up the string being written to. For instance, if this is normal text, a space might make a good separator:

var s1 = "the original string";
var c;
var st = "";
var s2 = "";

for (i = 0; i < s1.length; i++) {
  c = rot13(s1.charAt(i));
  st += c;
  if (c == " ") {
    s2 += st;
    st = "";
s2 += st;

This way the bulk of the new string is copied much less often, because individual characters are added to a smaller temporary string.

A third way to really improve the speed in a for loop, is to move the [array].length statement outside the condition statement. In face, every occurrence, the [array].length will be re-calculate For a two occurrences loop, the result will not be visible, but (for example) in a five thousand occurrence loop, you'll see the difference. It can be explained with a simple calculation :

// we assume that myArray.length is 5000
for (x = 0;x < myArray.length;x++){
// doing some stuff

"x = 0" is evaluated only one time, so it's only one operation.

"x < myArray.length" is evaluated 5000 times, so it is 10,000 operations (myArray.length is an operation and compare myArray.length with x, is another operation).

"x++" is evaluated 5000 times, so it's 5000 operations.

There is a total of 15 001 operation.

// we assume that myArray.length is 5000
for (x = 0, l = myArray.length; x < l; x++){
// doing some stuff

"x = 0" is evaluated only one time, so it's only one operation.

"l = myArray.length" is evaluated only one time, so it's only one operation.

"x < l" is evaluated 5000 times, so it is 5000 operations (l with x, is one operation).

"x++" is evaluated 5000 times, so it's 5000 operations.

There is a total of 10002 operation.

So, in order to optimize your for loop, you need to make code like this :

var s1 = "the original string";
var c;
var st = "";
var s2 = "";

for (i = 0, l = s1.length; i < l; i++) {
  c = rot13(s1.charAt(i));
  st += c;
  if (c == " ") {
    s2 += st;
    st = "";
s2 += st;


JavaScript DebuggersEdit


  • Firebug is a powerful extension for Firefox that has many development and debugging tools including JavaScript debugger and profiler.

Venkman JavaScript DebuggerEdit

Internet Explorer debuggingEdit

Safari debuggingEdit

Safari includes a powerful set of tools that make it easy to debug, tweak, and optimize a website for peak performance and compatibility. To access them, turn on the Develop menu in Safari preferences. These include Web Inspector, Error Console, disabling functions, and other developer features. The Web Inspector gives you quick and easy access to the richest set of development tools ever included in a browser. From viewing the structure of a page to debugging JavaScript to optimizing performance, the Web Inspector presents its tools in a clean window designed to make developing web applications more efficient. To activate it, choose Show Web Inspector from the Develop menu. The Scripts pane features the powerful JavaScript Debugger in Safari. To use it, choose the Scripts pane in the Web Inspector and click Enable Debugging. The debugger cycles through your page’s JavaScript, stopping when it encounters exceptions or erroneous syntax. The Scripts pane also lets you pause the JavaScript, set breakpoints, and evaluate local variables.[2]

JTF: JavaScript Unit Testing FarmEdit

  • JTF is a collaborative website that enables you to create test cases that will be tested by all browsers. It's the best way to do TDD and to be sure that your code will work well on all browsers.


built-in debugging toolsEdit

Some people prefer to send debugging messages to a "debugging console" rather than use the alert() function[2][3][4]. Following is a brief list of popular browsers and how to access their respective consoles/debugging tools.

  • Firefox: Ctrl+Shift+K opens an error console.
  • Opera (9.5+): Tools >> Advanced >> Developer Tools opens Dragonfly.
  • Chrome: Ctrl+Shift+J opens chrome's "Developer Tools" window, focused on the "console" tab.
  • Internet Explorer: F12 opens a firebug-like Web development tool that has various features including the ability to switch between the IE8 and IE7 rendering engines.
  • Safari: Cmd+Alt+C opens the WebKit inspector for Safari.

Common MistakesEdit

  • Carefully read your code for typos.
  • Be sure that every "(" is closed by a ")" and every "{" is closed by a "}".
  • Trailing commas in Array and Object declarations will throw an error in Microsoft Internet Explorer but not in Gecko-based browsers such as Firefox.
  // Object
  var obj = {
    'foo'   : 'bar',
    'color' : 'red', //trailing comma

  // Array
  var arr = [
    'bar', //trailing comma
  • Remember that JavaScript is case sensitive. Look for case related errors.
  • Don't use Reserved Words as variable names, function names or loop labels.
  • Escape quotes in strings with a "\" or the JavaScript interpreter will think a new string is being started, i.e:
alert('He's eating food'); should be
alert('He\'s eating food'); or
alert("He's eating food");
  • When converting strings to numbers using the parseInt function, remember that "08" and "09" (e.g. in datetimes) indicate an octal number, because of the prefix zero. Using parseInt using a radix of 10 prevents wrong conversion. var n = parseInt('09',10);
  • Remember that JavaScript is platform independent, but is not browser independent. Because there are no properly enforced standards, there are functions, properties and even objects that may be available in one browser, but not available in another, e.g. Mozilla / Gecko Arrays have an indexOf() function; Microsoft Internet Explorer does not.

Debugging MethodsEdit

Debugging in JavaScript doesn't differ very much from debugging in most other programming languages. See the article at Computer Programming Principles/Maintaining/Debugging.

Following Variables as a Script is RunningEdit

The most basic way to inspect variables while running is a simple alert() call. However some development environments allow you to step through your code, inspecting variables as you go. These kind of environments may allow you to change variables while the program is paused.

Browser BugsEdit

Sometimes the browser is buggy, not your script. This means you must find a workaround.

Browser bug reports

browser-dependent codeEdit

Some advanced features of JavaScript don't work in some browsers.

Too often our first reaction is: Detect which browser the user is using, then do something the cool way if the user's browser is one of the ones that support it. Otherwise skip it.

Instead of using a "browser detect", a much better approach is to write "object detection" JavaScript to detect if the user's browser supports the particular object (method, array or property) we want to use.[5] [6]

To find out if a method, property, or other object exists, and run code if it does, we write code like this:

var el = null;
if (document.getElementById) {
  // modern technique
  el = document.getElementById(id);
} else if (document.all) {
  // older Internet Explorer technique
  el = document.all[id];
} else if (document.layers) {
  // older Netscape Web browser technique
  el = document.layers[id];


  1. Sheppy, Shaver et al. (2014-11-18). "with" (in English) (HTML). Mozilla. Archived from the original on 2014-11-18. Retrieved 2015-03-18. 
  2. "Safari - The best way to see the sites." (in English) (HTML). Apple. Retrieved 2015-03-09. 

Further readingEdit


Previous: Introduction to the Document Object Model (DOM) Index Next: Runtime document manipulation

DHTML (Dynamic HTML) is a combination of JavaScript, CSS and HTML.

alert messagesEdit

<script type="text/javascript">
    alert('Hello World!');

This will give a simple alert message.

<script type="text/javascript">
    prompt('What is your name?');

This will give a simple prompt message.

<script type="text/javascript">
confirm('Are you sure?');

This will give a simple confirmation message.

Javascript Button and Alert Message Example:Edit

Sometimes it is best to dig straight in with the coding. Here is an example of a small piece of code:

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">
<html lang="en">
  <title>"THE BUTTON" - Javascript</title>
    <script type="text/javascript">
      x = 'You have not pressed "THE BUTTON"'
      function bomb() {
        alert('O-GOD NOOOOO, WE ARE ALL DOOMED!!');
        alert('Have a nice day. :-)');
        x = 'You pressed "THE BUTTON" and I told you not to!';
    <style type="text/css">
      body {
      <input type="button" value="THE BUTTON - Don't Click It" onclick="bomb()"><br />
      <input type="button" value="Click Here And See If You Have Clicked ''THE BUTTON''" onclick="alert(x)">
      This script is dual-licensed under both, <a href="">GFDL</a> and <a href="GNU General Public License">GPL</a>. See <a href="">Wikibooks</a>

What does this code do? When it loads it tells what value the variable 'x' should have. The next code snippet is a function that has been named "bomb". The body of this function fires some alert messages and changes the value of 'x'.

The next part is mainly HTML with a little javascript attached to the INPUT tags. The "onclick" property tells its parent what has to be done when clicked. The bomb function is assigned to the first button, the second button just shows an alert message with the value of x.

Javascript if() - else ExampleEdit

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">
<html lang="en">
    <title>The Welcome Message - Javascript</title>
    <script type="text/javascript">
      function wlcmmsg() {
        name = prompt('What is your name?', '');
        correct = confirm('Are you sure your name is ' + name + ' ?');
        if (correct == true) {
          alert('Welcome ' + name);
        } else {
    <style type="text/css">
      body {
  <body onload="wlcmmsg()" onunload="alert('Goodbye ' + name)">
      This script is dual-licensed under both, <a href="">GFDL</a> and <a href="GNU General Public License">GPL</a>. See <a href="">Wikibooks</a>

Two ScriptsEdit

Now, back to the first example. We have modified the script adding a different welcome message. This version requests the user to enter a name. They are also asked if they want to visit the site. Some CSS has also been added to the button.

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">
<html lang="en">
    <title>"THE BUTTON" - Javascript</title>
    <script type="text/javascript">
      // global variable x
      x = 'You have not pressed "THE BUTTON"';

      function bomb() {
        alert('O-GOD NOOOOO, WE ARE ALL DOOMED!!');
        alert('Have a nice day. :-)');
        x = 'You pressed "THE BUTTON" and I told you not too!';
    <style type="text/css">
      body {
  <body onload="welcome()">
    <script type="text/javascript">
      function welcome() {
        var name = prompt('What is your name?', '');
        if (name == "" || name == "null") { 
          alert('You have not entered a name');
          return false;
        var visit = confirm('Do you want to visit this website?')
        if (visit == true) {
          alert('Welcome ' + name);
        } else {
      <input type="button" value="THE BUTTON - Don't Click It" onclick="bomb()" STYLE="color: #ffdd00; background-color: #ff0000"><br>
      <input type="button" value="Click Here And See If You Have Clicked ''THE BUTTON''" onclick="alert(x)">
      This script is dual-licensed under both, <a href="">GFDL</a> and <a href="GNU General Public License">GPL</a>. See <a href="">Wikibooks</a>,

Simple CalculatorEdit

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "">
<html lang="en">
    <script type="text/javascript"> 
    function multi() { 
      var a = document.Calculator.no1.value;
      var b = document.Calculator.no2.value;
      var p = (a*b);
      document.Calculator.product.value = p;

    function divi() {
      var d = document.Calculator.dividend.value;
      var e = document.Calculator.divisor.value;
      var q = (d/e);
      document.Calculator.quotient.value = q;

    function circarea() { 
      var r = document.Calculator.radius.value;
      pi = 3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095505822317253594081284811174502841027019385211055596446229489549303819644288109756659334461284756
      var a = pi*(r*r);
      document.Calculator.area.value = a;
      var c = 2*pi*r;
      document.Calculator.circumference.value = c;
    <style type="text/css">
      body {

      label {
    <form name="Calculator" action="">
        <input type="text" name="no1"> × <input type="text" name="no2"> 
        <input type="button" value="=" onclick="multi()">
        <input type="text" name="product">
        <input type="text" name="dividend"> ÷ <input type="text" name="divisor"> 
        <input type="button" value="=" onclick="divi()">
        <input type="text" name="quotient">
        <legend>Area and Circumfrence of Circle</legend>
        <p>(Uses pi to 240 d.p)</p>
          <label for="radius">Type in radius</label> <input type="text" name="radius" id="radius" value=""> 
          <input type="button" value="=" onclick="circarea()">
          <label for="area">Area</label> <input type="text" name="area" id="area" value="">
          <label for="circumference">Circumference</label> <input type="text" name="circumference" id="circumference" value="">
    <p>Licensed under the <a href="">GNU GPL</a>.</p>
Previous: Introduction to the Document Object Model (DOM) Index Next: Runtime document manipulation

Finding ElementsEdit

Simple UseEdit

Let's say, on a page, we have:

<div id="myDiv">content</div>

A simple way of finding this element in JavaScript would be:

var myDiv = document.getElementById("myDiv"); // Would find the DIV element by its ID, which in this case is 'myDiv'.

Use of getElementsByTagNameEdit

Another way to find elements on a web page is by the getElementsByTagName(name) method. It returns an array of all name elements in the node.

Let's say, on a page, we have:

<div id="myDiv">
  <p>Paragraph 1</p>
  <p>Paragraph 2</p>
  <h1>An HTML header</h1>
  <p>Paragraph 3</p>

Using the getElementsByTagName method we can get an array of all <p> elements inside the div:

var myDiv = document.getElementById("myDiv"); // get the div
var myParagraphs = myDiv.getElementsByTagName('P'); //get all paragraphs inside the div

// for example you can get the second paragraph (array indexing starts from 0)
var mySecondPar = myParagraphs[1]

Adding ElementsEdit

Basic UsageEdit

Using the Document Object Module we can create basic HTML elements. Let's create a div.

var myDiv = document.createElement("div");

Now, the element is created, but it exists nowhere on the actual page so far, only in memory.

What if we want the div to have an ID, or a class?

var myDiv = document.createElement("div"); = "myDiv";
myDiv.class = "main";

And we want it added into the page? Let's use the DOM again…

var myDiv = document.createElement("div"); = "myDiv";
myDiv.class = "main";

Further UseEdit

So let's have a simple HTML page…

<body bgcolor="white" text="blue">
  <h1> A simple Javascript created button </h1>
  <div id="button"></div>

Where the div which has the id of button, let's add a button.

var myButton = document.createElement("input");
myButton.type = "button";
myButton.value = "my button";
placeHolder = document.getElementById("button");

All together the HTML code looks like:

<body bgcolor="white" text="blue">
  <h1> A simple Javascript created button </h1>
  <div id="button"></div>
myButton = document.createElement("input");
myButton.type = "button";
myButton.value = "my button";
placeHolder = document.getElementById("button");

The page will now have a button on it which has been created via JavaScript.

Changing ElementsEdit

In JavaScript you can change elements by using the following syntax:

element.attribute="new value"

Here, the srcattribute of an image is changed so when the script is called, it changes the picture from myPicture.jpg to otherPicture.jpg.

//The HTML
<img id="example" src="myPicture.jpg">
//The JavaScript

In order to change an element, you use its argument name for the value you wish to change. For example, let's say we have a button, and we wish to change its value.

<input type="button" id="myButton" value="I'm a button!">

Later on in the page, with JavaScript, we could do the following to change that button's value:

myButton = document.getElementById("myButton"); //searches for and detects the input element from the 'myButton' id
myButton.value = "I'm a changed button"; //changes the value

To change the type of input it is (button to text, for example), use:

myButton.type = "text"; //changes the input type from 'button' to 'text'.

Another way to change or create an attribute is to use a method like element.setAttribute("attribute", "value") or element.createAttribute("attribute", "value"). Use setAttribute to change an attribute that has been defined before.

//The HTML
<div id="div2"></div> //Make a div with an id of div2 (we also could have made it with JavaScript)
//The Javascript
var e = document.getElementById("div2"); //Get the element
e.setAttribute("id", "div3"); //Change id to div3

But use createAttribute(), if you want to set a value that hasn't been defined before.

var e = document.createElement('div'); //Make a div element (we also could have made it with HTML)
e.createAttribute("id", "myDiv"); //Set the id to "myDiv"

Removing ElementsEdit

In JavaScript, an element can only be deleted from its parent. To delete one, you have to get the element, find its parent, and delete it using the removeChild method.[1]

For example, in a HTML document that looks like

<div id="parent">
    <p id="child">I'm a child!</p>

The code you would need to delete the element with the ID "child" would be

// get elements
var child = document.getElementById("child");
var parent = document.getElementById("parent");

// Delete child

Instead of getting the parent element manually, you can use the parentNode property of the child to find its parent automatically. The code for this on the above HTML document would look like

// Get the child element node
var child = document.getElementById("child");

// Remove the child element from the document


Code StructuringEdit


To do:
Flesh out. I have deleted what was a complete POV, entered by an anonymous.



Featured weblinks:

Discussion forums, bulletin boards:

More Web sites:

Useful Software ToolsEdit

A list of useful tools for JavaScript programmers.

Editors / IDEsEdit

  • Adobe Brackets: Another browser-based editor by Adobe
  • Eclipse: The Eclipse IDE includes an editor and debugger for JavaScript
  • Notepad++: A Great tool for editing any kind of code, includes syntax highlighting for many programming languages.
  • Programmers' Notepad: A general tool for programming many languages.
  • Scripted: An open source browser-based editor by Spring Source
  • Sublime Text: One of the most used editors for HTML/CSS/JavaScript editing
  • Web Storm or IntelliJ IDEA: both IDEs include and editor and debugger for JavaScript, IDEA also includes a Java development platform

Engines and other toolsEdit