JavaScript

 

  Suggested Reading:

    1. JavaScript Language Library (Code Archives KnowledgeBase)

    2. ECMA 3 Browser Support JavaScript Library (Code Archives KnowledgeBase)
 


 
Paradigm Multi-paradigm: scripting, prototype-based, imperative, functional
Appeared in 1995
Designed by Brendan Eich
Developer Netscape Communications Corporation, Mozilla Foundation
Stable release 1.8.2 (June 22, 2009; 20 months ago (2009-06-22))
Preview release 1.8.5 (July 27, 2010; 7 months ago (2010-07-27))
Typing discipline dynamic, weak, duck
Major implementations KJS, Rhino, SpiderMonkey, V8, WebKit
Influenced by C, Scheme, Java, Perl, Python, Self
Influenced JScript, JScript .NET, Objective-J, TIScript

 

 
Filename extension .js
Internet media type application/javascript, text/javascript
Uniform Type Identifier com.netscape.javascript-?source
Type of format Scripting language

JavaScript, also known as ECMAScript is a prototype-based object-oriented scripting language that is dynamic, weakly typed and has first-class functions. It is also considered a functional programming language like Scheme and OCaml because it has closures and supports higher-order functions.

JavaScript is an implementation of the ECMAScript language standard and is primarily used in the form of client-side JavaScript, implemented as part of a web browser in order to provide enhanced user interfaces and dynamic websites. This enables programmatic access to computational objects within a host environment.

JavaScript's use in applications outside web pages - for example in PDF-documents, site-specific browsers and desktop widgets - is also significant. Newer and faster Javascript VMs and frameworks built upon them (particularly Node.js) have also increased the popularity of Javascript for server-side web apps.

JavaScript uses syntax influenced by that of C. JavaScript copies many names and naming conventions from Java, but the two languages are otherwise unrelated and have very different semantics. The key design principles within JavaScript are taken from the Self and Scheme programming languages.

History

JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. LiveScript was the official name for the language when it first shipped in beta releases of Netscape Navigator 2.0 in September 1995, but it was renamed JavaScript in a joint announcement with Sun Microsystems on December 4, 1995 when it was deployed in the Netscape browser version 2.0B3.

The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web browser. The final choice of name caused confusion, giving the impression that the language was a spin-off of the Java programming language, and the choice has been characterized by many as a marketing ploy by Netscape to give JavaScript the cachet of what was then the hot new web-programming language. It has also been claimed that the language's name is the result of a co-marketing deal between Netscape and Sun, in exchange for Netscape bundling Sun's Java runtime with their then-dominant browser.

JavaScript very quickly gained widespread success as a client-side scripting language for web pages. As a consequence, Microsoft developed a compatible dialect of the language, naming it JScript to avoid trademark issues. JScript added new date methods to fix the Y2K-problematic methods in JavaScript, which were based on Java's java.util.Date class. JScript was included in Internet Explorer 3.0, released in August 1996. The dialects are perceived to be so similar that the terms "JavaScript" and "JScript" are often used interchangeably. Microsoft, however, notes dozens of ways in which JScript is not ECMA-compliant.

In November, 1996 Netscape announced that it had submitted JavaScript to Ecma International for consideration as an industry standard, and subsequent work resulted in the standardized version named ECMAScript.

JavaScript has become one of the most popular programming languages on the web. Initially, however, many professional programmers denigrated the language because its target audience was web authors and other such "amateurs", among other reasons. The advent of Ajax returned JavaScript to the spotlight and brought more professional programming attention. The result was a proliferation of comprehensive frameworks and libraries, improved JavaScript programming practices, and increased usage of JavaScript outside of web browsers, as seen by the proliferation of server-side JavaScript platforms.

In January 2009 the CommonJS project was founded with the goal of specifying a common standard library mainly for JavaScript development outside the browser.

Trademark

"JavaScript" is a trademark of Oracle Corporation. It is used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.

Features

The following features are common to all conforming ECMAScript implementations, unless explicitly specified otherwise.

Imperative and structured

JavaScript supports all the structured programming syntax in C (e.g., if statements, while loops, switch statements, etc.). One partial exception is scoping: C-style block-level scoping is not supported (instead, JavaScript has function-level scoping). JavaScript 1.7, however, supports block-level scoping with the let keyword. Like C, JavaScript makes a distinction between expressions and statements. One syntactic difference from C is automatic semicolon insertion, in which the semicolons that terminate statements can be omitted.

Dynamic

dynamic typing
As in most scripting languages, types are associated with values, not with variables. For example, a variable x could be bound to a number, then later rebound to a string. JavaScript supports various ways to test the type of an object, including duck typing.
object based
JavaScript is almost entirely object-based. JavaScript objects are associative arrays, augmented with prototypes (see below). Object property names are string keys: obj.x = 10 and obj["x"] = 10 are equivalent, the dot notation being syntactic sugar. Properties and their values can be added, changed, or deleted at run-time. Most properties of an object (and those on its prototype inheritance chain) can be enumerated using a for...in loop. JavaScript has a small number of built-in objects such as Function and Date.
run-time evaluation
JavaScript includes an eval function that can execute statements provided as strings at run-time.

Functional

first-class functions
Functions are first-class; they are objects themselves. As such, they have properties and methods, such as length and call(); and they can be assigned to variables, passed as arguments, returned by other functions, and manipulated like any other object. Any reference to a function allows it to be invoked using the () operator.
nested functions
'Inner' or 'nested' functions are functions defined within another function. They are created each time the outer function is invoked. In addition to that, the scope of the outer function, including any constants, local variables and argument values, become part of the internal state of each inner function object, even after execution of the outer function concludes.
closures
JavaScript allows nested functions to be created, with the lexical scope in force at their definition, and has a () operator to invoke them now or later. This combination of code that can be executed outside the scope in which it is defined, with its own scope to use during that execution, is called a closure in computer science.

Prototype-based

prototypes
JavaScript uses prototypes instead of classes for inheritance. It is possible to simulate many class-based features with prototypes in JavaScript.
functions as object constructors
Functions double as object constructors along with their typical role. Prefixing a function call with new creates a new object and calls that function with its local this keyword bound to that object for that invocation. The constructor's prototype property determines the object used for the new object's internal prototype. JavaScript's built-in constructors, such as Array, also have prototypes that can be modified.
functions as methods
Unlike many object-oriented languages, there is no distinction between a function definition and a method definition. Rather, the distinction occurs during function calling; a function can be called as a method. When a function is called as a method of an object, the function's local this keyword is bound to that object for that invocation.

Miscellaneous

run- time environment
JavaScript typically relies on a run-time environment (e.g. in a web browser) to provide objects and methods by which scripts can interact with "the outside world". In fact, it relies on the environment to provide the ability to include/import scripts (e.g. HTML <script> elements). (This is not a language feature per se, but it is common in most JavaScript implementations.)
variadic functions
An indefinite number of parameters can be passed to a function. The function can access them through formal parameters and also through the local arguments object.
array and object literals
Like many scripting languages, arrays and objects (associative arrays in other languages) can each be created with a succinct shortcut syntax. In fact, these literals form the basis of the JSON data format.
regular expressions
JavaScript also supports regular expressions in a manner similar to Perl, which provide a concise and powerful syntax for text manipulation that is more sophisticated than the built-in string functions.

Vendor-specific extensions

JavaScript is officially managed by Mozilla Foundation, and new language features are added periodically. However, only some non-Mozilla JavaScript engines support these new features:

Syntax and semantics

As of 2009, the latest version of the language is JavaScript 1.8.1. It is a superset of ECMAScript (ECMA-262) Edition 3. Extensions to the language, including partial E4X (ECMA-357) support and experimental features considered for inclusion into future ECMAScript editions, are documented here.

Simple examples

A simple recursive function:

function factorial(n) {

    if (n === 0) {
        return 1;

    }
    return n * factorial(n - 1); 


}

A simple personalised greeting script:

var name = prompt("What is your name?");
alert("Welcome "+name);

Anonymous function (or lambda) syntax:

function add (i, j) {

    var add_pri = function (x, y) {

        return x + y;
    };
 
    return add_pri(i, j);

}

Closures:

function showclosure () {

    var inc = makeinc(1);

 
    inc(); // 1

    inc(); // 2

    inc(); // 3
}

 
function makeinc (initialValue) {

    var count = initialValue;

 
    return function () {

        return count++;
    };

}

Variadic function demonstration. This will alert with 1 then 2 then 3. arguments is a special variable.

function unlimited_args () {

    for (var i = 0; i < arguments.length; i++) {

        alert(arguments[i]);
    }
}
 

unlimited_args(1, 2, 3);

More advanced example

This sample code showcases various JavaScript features.

/* Finds the lowest common multiple of two numbers */
function LCMCalculator (x, y) { // constructor function

    var checkInt = function (x) { // inner function

        if (x % 1 !== 0) {
            throw new TypeError(x + "is not an integer"); // exception throwing

        }
        return x;
    };
    this.a = checkInt(x)

    // ^ semicolons are optional (but beware since this may cause consecutive lines to be
    //erroneously treated as a single statement)
    this.b = checkInt(y);

}
// The prototype of object instances created by a constructor is 
// that constructor's "prototype" property.
LCMCalculator.prototype = { // object literal

    constructor : LCMCalculator, // when reassigning a prototype, set the constructor property appropriately
    gcd : function () { // method that calculates the greatest common divisor

        // Euclidean algorithm:
        var a = Math.abs(this.a), b = Math.abs(this.b), t;

        if (a < b) {
            // swap variables
            t = b; 
            b = a; 
            a = t; 
        }

        while (b !== 0) {
            t = b;

            b = a % b;
            a = t;

        }
        // Only need to calculate gcd once, so "redefine" this method.
        // (Actually not redefinition - it's defined on the instance itself,
        // so that this.gcd refers to this "redefinition" instead of LCMCalculator.prototype.gcd.)

        // Also, 'gcd' == "gcd", this['gcd'] == this.gcd
        this['gcd'] = function () { 
            return a; 
        };

        return a;
    },
    "lcm" /* can use strings here */: function () {

        // Variable names don't collide with object properties, e.g. |lcm| is not |this.lcm|.
        // not using |this.a * this.b| to avoid FP precision issues 
        var lcm = this.a / this.gcd() * this.b; 
        // Only need to calculate lcm once, so "redefine" this method.

        this.lcm = function () { 
            return lcm; 
        };

 
        return lcm;
    },
    toString : function () {

        return "LCMCalculator: a = " + this.a + ", b = " + this.b;

    }
};
 
// Note: Array's map() and forEach() are predefined in JavaScript 1.6.
// They are currently not available in the JScript engine built into
// Microsoft Internet Explorer, but are implemented in Firefox, Chrome, etc. 
// They are used here to demonstrate JavaScript's inherent functional nature.

 
[[25, 55],[21, 56],[22, 58],[28, 56]].map(function (pair) { // array literal + mapping function

    return new LCMCalculator(pair[0], pair[1]);

}).sort(function (a, b) { // sort with this comparative function

    return a.lcm() - b.lcm();

}).forEach(function (obj) {

    /* Note: print() is a JS builtin function available in Mozilla's js CLI;
     * It is functionally equivalent to Java's System.out.println().
     * Within a web browser, print() is a very different function 
     * (opens the "Print Page" dialog),
     * so use something like document.write() or alert() instead.
     */

    // print       (obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());

    // alert       (obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());

    document.write(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm() + "<br>");

});

The following output should be displayed in the browser window.

LCMCalculator: a = 28, b = 56, gcd = 28, lcm = 56
LCMCalculator: a = 21, b = 56, gcd = 7, lcm = 168
LCMCalculator: a = 25, b = 55, gcd = 5, lcm = 275
LCMCalculator: a = 22, b = 58, gcd = 2, lcm = 638

If Internet Explorer is used, the example will generate an error. Hence the example illustrates the point that the JScript interpreter in Internet Explorer executes code differently from the JavaScript and ECMAScript interpreters in other browsers. (See comments in the source code for details on the relevant differences for this example.)

Use in web pages

The primary use of JavaScript is to write functions that are embedded in or included from HTML pages and that interact with the Document Object Model (DOM) of the page. Some simple examples of this usage are:

Because JavaScript code can run locally in a user's browser (rather than on a remote server), the browser can respond to user actions quickly, making an application more responsive. Furthermore, JavaScript code can detect user actions which HTML alone cannot, such as individual keystrokes. Applications such as Gmail take advantage of this: much of the user-interface logic is written in JavaScript, and JavaScript dispatches requests for information (such as the content of an e-mail message) to the server. The wider trend of Ajax programming similarly exploits this strength.

A JavaScript engine (also known as JavaScript interpreter or JavaScript implementation) is an interpreter that interprets JavaScript source code and executes the script accordingly. The first JavaScript engine was created by Brendan Eich at Netscape Communications Corporation, for the Netscape Navigator web browser. The engine, code-named SpiderMonkey, is implemented in C. It has since been updated (in JavaScript 1.5) to conform to ECMA-262 Edition 3. The Rhino engine, created primarily by Norris Boyd (formerly of Netscape; now at Google) is a JavaScript implementation in Java. Rhino, like SpiderMonkey, is ECMA-262 Edition 3 compliant.

A web browser is by far the most common host environment for JavaScript. Web browsers typically use the public API to create "host objects" responsible for reflecting the DOM into JavaScript. The web server is another common application of the engine. A JavaScript webserver would expose host objects representing an HTTP request and response objects, which a JavaScript program could then manipulate to dynamically generate web pages.

Because JavaScript is the only language that the most popular browsers share support for, it has become a target language for many frameworks in other languages, even though JavaScript was never intended to be such a language. Despite the performance limitations inherent to its dynamic nature, the increasing speed of JavaScript engines has made the language a surprisingly feasible compilation target.

Example - use in web pages

A minimal example of a standards-conforming web page containing JavaScript (using HTML 4.01 syntax) would be the following:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>

  <head><title>simple page</title></head>
  <body>
    <script type="text/javascript">

      document.write('Hello World!');
    </script>
    <noscript>
<p>Your browser either does not support JavaScript, or you have JavaScript turned off.</p>
    </noscript>

  </body>
</html>

Compatibility considerations

Since JavaScript runs in widely varying environments, an important part of testing and debugging it is testing across browsers.

The DOM interfaces for manipulating web pages are not part of the ECMAScript standard, or of JavaScript itself. Officially, they are defined by a separate standardization effort by the W3C; in practice, browser implementations differ from the standards and from each other, and not all browsers execute JavaScript.

To deal with these differences, JavaScript authors can attempt to write standards-compliant code which will also be executed correctly by most browsers; failing that, they can write code that checks for the presence of certain browser features and behaves differently if they are not available. In some cases, two browsers may both implement a feature but with different behavior, and authors may find it practical to detect what browser is running and change their script's behavior to match. Programmers may also use libraries or toolkits which take browser differences into account.

Furthermore, scripts may not work for some users. For example, a user may:

To support these users, web authors can try to create pages which degrade gracefully on user agents (browsers) which do not support the page's JavaScript. In particular, the page should remain usable albeit without the extra features that the JavaScript would have added. An alternative approach that many find preferable is first to author content using basic technologies that work in all browsers, then to enhance it for users with JavaScript enabled, testing for feature support before adding the enhancements. This is known as progressive enhancement.

Accessibility

Assuming that the user has not disabled its execution, client-side web JavaScript should be written to enhance the experiences of visitors with visual or physical disabilities, and certainly should avoid denying information to these visitors.

Screen readers, used by the blind and partially sighted, can be JavaScript-aware and so may access and read the page DOM after the script has altered it. The HTML should be as concise, navigable and semantically rich as possible whether the scripts have run or not. JavaScript should not be totally reliant on mouse-specific events so as to deny its benefits to users who either cannot use a mouse or who choose to favor the keyboard for whatever reason. Equally, although hyperlinks and webforms can be navigated and operated from the keyboard, accessible JavaScript should not require keyboard events either. There are device-independent events such as onfocus and onchange that are preferable in most cases.

JavaScript should not be used in a way that is confusing or disorientating to any web user. For example, using script to alter or disable the normal functionality of the browser, such as by changing the way the back-button or the refresh event work, is usually best avoided. Equally, triggering events that the user may not be aware of reduces the user's sense of control as do unexpected scripted changes to the page content.

Often the process of making a complex web page as accessible as possible becomes a nontrivial problem where issues become matters of debate and opinion, and where compromises are necessary in the end. However, user agents and assistive technologies are constantly evolving and new guidelines and relevant information are continually being published on the web.

Security

JavaScript and the DOM provide the potential for malicious authors to deliver scripts to run on a client computer via the web. Browser authors contain this risk using two restrictions. First, scripts run in a sandbox in which they can only perform web-related actions, not general-purpose programming tasks like creating files. Second, scripts are constrained by the same origin policy: scripts from one web site do not have access to information such as usernames, passwords, or cookies sent to another site. Most JavaScript-related security bugs are breaches of either the same origin policy or the sandbox.

Cross-site vulnerabilities

A common JavaScript-related security problem is cross-site scripting, or XSS, a violation of the same-origin policy. XSS vulnerabilities occur when an attacker is able to cause a target web site, such as an online banking website, to include a malicious script in the webpage presented to a victim. The script in this example can then access the banking application with the privileges of the victim, potentially disclosing secret information or transferring money without the victim's authorization. A solution to XSS vulnerabilities is to use HTML escaping whenever displaying untrusted data.

Some browsers include partial protection against reflected XSS attacks, in which the attacker provides a URL including malicious script. However, even users of those browsers are vulnerable to other XSS attacks, such as those where the malicious code is stored in a database. Only correct design of Web applications on the server side can fully prevent XSS.

XSS vulnerabilities can also occur because of implementation mistakes by browser authors.

Another cross-site vulnerability is cross-site request forgery or CSRF. In CSRF, code on an attacker's site tricks the victim's browser into taking actions the user didn't intend at a target site (like transferring money at a bank). It works because, if the target site relies only on cookies to authenticate requests, then requests initiated by code on the attacker's site will carry the same legitimate login credentials as requests initiated by the user. In general, the solution to CSRF is to require an authentication value in a hidden form field, and not only in the cookies, to authenticate any request that might have lasting effects. Checking the HTTP Referrer header can also help.

"JavaScript hijacking" is a type of CSRF attack in which a <script> tag on an attacker's site exploits a page on the victim's site that returns private information such as JSON or JavaScript. Possible solutions include requiring an authentication token in the POST and GET parameters for any response that returns private JSON (even if it has no side effects); using POST and never GET for requests that return private JSON; and modifying the response so that it can't be used via a <script> tag (by, for example, wrapping the JSON in a JavaScript comment).

Misplaced trust in the client

Client-server applications, whether they involve JavaScript or not, must recognize that untrusted clients may be under the control of attackers. Thus any secret embedded in JavaScript could be extracted by a determined adversary, and the application author cannot assume that his JavaScript runs as intended, or at all. Some implications:

Browser and plugin coding errors

JavaScript provides an interface to a wide range of browser capabilities, some of which may have flaws such as buffer overflows. These flaws can allow attackers to write scripts which would run any code they wish on the user's system.

These flaws have affected major browsers including Firefox, Internet Explorer, and Safari.

Plugins, such as video players, Adobe Flash, and the wide range of ActiveX controls enabled by default in Microsoft Internet Explorer, may also have flaws exploitable via JavaScript, and such flaws have been exploited in the past.

In Windows Vista, Microsoft has attempted to contain the risks of bugs such as buffer overflows by running the Internet Explorer process with limited privileges. Google Chrome similarly limits page renderers in its own "sandbox."

Sandbox implementation errors

Web browsers are capable of running JavaScript outside of the sandbox, with the privileges necessary to, for example, create or delete files. Of course, such privileges aren't meant to be granted to code from the web.

Incorrectly granting privileges to JavaScript from the web has played a role in vulnerabilities in both Internet Explorer and Firefox. In Windows XP Service Pack 2, Microsoft demoted JScript's privileges in Internet Explorer.

Microsoft Windows allows JavaScript source files on a computer's hard drive to be launched as general-purpose, non-sandboxed programs. This makes JavaScript (like VBScript) a theoretically viable vector for a Trojan horse, although JavaScript Trojan horses are uncommon in practice. (See Windows Script Host.)

Uses outside web pages

In addition to web browsers and servers, JavaScript interpreters are embedded in a number of tools. Each of these applications provides its own object model which provides access to the host environment, with the core JavaScript language remaining mostly the same in each application.

Embedded scripting language

Scripting engine

Application platform

Development tools

Within JavaScript, access to a debugger becomes invaluable when developing large, non-trivial programs. Because there can be implementation differences between the various browsers (particularly within the Document Object Model) it is useful to have access to a debugger for each of the browsers that a web application targets.

Script debuggers are available for Internet Explorer, Firefox, Safari, Google Chrome, and Opera.

Three debuggers are available for Internet Explorer: Microsoft Visual Studio is the richest of the three, closely followed by Microsoft Script Editor (a component of Microsoft Office), and finally the free Microsoft Script Debugger which is far more basic than the other two. The free Microsoft Visual Web Developer Express provides a limited version of the JavaScript debugging functionality in Microsoft Visual Studio. Internet Explorer has included developer tools since version 8 (reached by pressing the F12 key).

Web applications within Firefox can be debugged using the Firebug add-on, or the older Venkman debugger. Firefox also has a simpler built-in Error Console, which logs and evaluates JavaScript. It also logs CSS errors and warnings.

Opera includes a set of tools called DragonFly.

WebKit's Web Inspector includes a JavaScript debugger in Safari and Google Chrome.

Some debugging aids are themselves written in JavaScript and built to run on the Web. An example is the program JSLint, developed by Douglas Crockford, currently senior JavaScript architect at Yahoo! who has written extensively on the language. JSLint scans JavaScript code for conformance to a set of standards and guidelines. Web development bookmarklets and Firebug Lite provide variations on the idea of the cross-browser JavaScript console.

MiniME is an open source JavaScript minifier, obfuscator and code checking tool for the .NET platform.

Versions

Version Release date Equivalent to Netscape
Navigator
Mozilla
Firefox
Internet
Explorer
Opera Safari Google
Chrome
1.0 March 1996   2.0   3.0      
1.1 August 1996   3.0          
1.2 June 1997   4.0-4.05          
1.3 October 1998 ECMA-262 1st edition / ECMA-262 2nd edition 4.06-4.7x   4.0      
1.4     Netscape
Server
         
1.5 November 2000 ECMA-262 3rd edition 6.0 1.0 5.5 (JScript 5.5),
6 (JScript 5.6),
7 (JScript 5.7),
8 (JScript 5.8)
6.0-11.0 3.0-5 1.0-10.0.666
1.6 November 2005 1.5 + Array extras + Array and String generics + E4X   1.5        
1.7 October 2006 1.6 + Pythonic generators + Iterators + let   2.0        
1.8 June 2008 1.7 + Generator expressions + Expression closures   3.0        
1.8.1   1.8 + Native JSON support + Minor Updates   3.5        
1.8.2 June 22, 2009 1.8.1 + Minor updates   3.6        
1.8.5 July 27, 2010 1.8.1 + ECMAScript 5 Compliance   4 9      

Related languages and features

Objective-J is a superset of JavaScript that compiles to standard JavaScript. It adds traditional inheritance and Smalltalk/Objective-C style dynamic dispatch and optional pseudo-static typing to JavaScript.

CoffeeScript is an alternate syntax for JavaScript intended to be more concise and readable and adding features like array comprehensions and pattern matching. Like Objective-J, it compiles to JavaScript. Ruby and Python have been cited as influential on Coffeescript syntax.

jQuery and Prototype are popular JavaScript libraries designed to simplify DOM-oriented client-side HTML scripting.

TIScript is a superset of JavaScript that adds classes, namespaces and lambda expressions.

JSON, or JavaScript Object Notation, is a general-purpose data interchange format that is defined as a subset of JavaScript.

Mozilla browsers currently support LiveConnect, a feature that allows JavaScript and Java to intercommunicate on the web. However, Mozilla-specific support for LiveConnect is scheduled to be phased out in the future in favor of passing on the LiveConnect handling via NPAPI to the Java 1.6+ plug-in (not yet supported on the Mac as of March 2010). Most browser inspection tools, such as FireBug in Firefox, include JavaScript interpreters that can act on the visible page's DOM.

JavaScript and Java

A common misconception is that JavaScript is similar or closely related to Java. It is true that both have a C-like syntax, the C language being their most immediate common ancestor language. They are both object-oriented, typically sandboxed (when used inside a browser), and are widely used in client-side Web applications. In addition, JavaScript was designed with Java's syntax and standard library in mind. In particular, all Java keywords were reserved in original JavaScript, JavaScript's standard library follows Java's naming conventions, and JavaScript's Math and Date objects are based on classes from Java 1.0.

However, the similarities end there. Java has static typing; JavaScript's typing is dynamic (meaning a variable can hold an object of any type and cannot be restricted). Java is loaded from compiled bytecode; JavaScript is loaded as human-readable source code. Java's objects are class-based; JavaScript's are prototype-based. JavaScript also has many functional features based on the Scheme language.