JavaScript

JavaScript is a prototype-based object-oriented scripting language used to enable programmatic access to computational objects within a host environment. Although also used in other applications, it is primarily used in the form of client-side JavaScript, implemented as part of a web browser, providing enhanced user interfaces and dynamic websites. JavaScript is a dialect of the ECMAScript standard and is characterized as a dynamic, weakly typed, prototype-based language with first-class functions. JavaScript was influenced by many languages and was designed to look like Java, but to be easier for non-programmers to work with.

History

JavaScript was originally developed by Brendan Eich of Netscape under the name Mocha, which was later renamed to LiveScript, and finally to JavaScript. The change of name from LiveScript to JavaScript roughly coincided with Netscape adding support for Java technology in its Netscape Navigator web browser. JavaScript was first introduced and deployed in the Netscape browser version 2.0B3 in December 1995. The naming has caused confusion, giving the impression that the language is a spin-off of Java, and it 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.

JavaScript, despite the name, is essentially unrelated to the Java programming language even though the two do have superficial similarities. Both languages use syntaxes influenced by that of C syntax, and JavaScript copies many Java names and naming conventions. 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.[citation needed] The key design principles within JavaScript are inherited from the Self and Scheme programming languages.

“JavaScript” is a trademark of Sun Microsystems. It was used under license for technology invented and implemented by Netscape Communications and current entities such as the Mozilla Foundation.

Due to the widespread success of JavaScript as a client-side scripting language for web pages, Microsoft developed a compatible dialect of the language, naming it JScript to avoid trademark issues. JScript added new date methods to fix the non-Y2K-friendly methods in JavaScript, which were based on java.util.Date. 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.

Netscape submitted JavaScript to Ecma International for standardization resulting 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

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 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 can be passed around and interacted with like any other object.

inner functions and closures

Inner functions (functions defined within other functions) are created each time the outer function is invoked, and variables of the outer functions for that invocation continue to exist as long as the inner functions still exist, even after that invocation is finished (e.g. if the inner function was returned, it still has access to the outer function’s variables) — this is the mechanism behind closures within JavaScript.

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:

  • property getter and setter functions (also supported by WebKit, Opera, ActionScript, and Rhino)
  • conditional catch clauses
  • iterator protocol adopted from Python
  • shallow generators/coroutines also adopted from Python
  • array comprehensions and generator expressions also adopted from Python
  • proper block scope via new let keyword
  • array and object destructuring (limited form of pattern matching)
  • concise function expressions (function(args) expr)
  • E4X

Syntax and semantics

Main article: JavaScript syntax

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.

Sample code showcasing various JavaScript features:

/* Finds the lowest common multiple of two numbers */
function LCMCalculator(x, y) { // constructor function
    function checkInt(x) { // inner function
        if (x % 1 != 0)
            throw new TypeError(x + " is not an integer"); // exception throwing
        return x;
    }
    //semicolons are optional (but beware since this may cause consecutive lines to be
    //erroneously treated as a single statement)
    this.a = checkInt(x)
    this.b = checkInt(y)
}
// The prototype of object instances created by a constructor is
// that constructor's "prototype" property.
LCMCalculator.prototype = { // object literal
    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) {
            t = b; b = a; a = t; // swap variables
        }
        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;
    }
};
[[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's 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() instead.
     */
    print(obj + ", gcd = " + obj.gcd() + ", lcm = " + obj.lcm());
});
// Note: Array's map() and forEach() are predefined in JavaScript 1.6.
// They are currently not available in all major JavaScript engines (including Internet Explorer's),
// but are shown here to demonstrate JavaScript's inherent functional nature.

The output is:

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

Use in web pages

Main article: Client-side JavaScript

See also: JavaScript engine and Ajax (programming)

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

  • Opening or popping up a new window with programmatic control over the size, position, and attributes of the new window (e.g. whether the menus, toolbars, etc. are visible).
  • Validation of web form input values to make sure that they will be accepted before they are submitted to the server.
  • Changing images as the mouse cursor moves over them: This effect is often used to draw the user’s attention to important links displayed as graphical elements.

Because JavaScript code can run locally in a user’s browser (rather than on a remote server) it can respond to user actions quickly, making an application feel 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.

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

<!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>

Your browser either does not support JavaScript, or you have JavaScript turned off.
    </noscript>
  </body>
</html>

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.

Compatibility considerations

Main articles: Web Interoperability and Web accessibility

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 will not work for all users. For example, a user may:

  • use an old or rare browser with incomplete or unusual DOM support,
  • use a PDA or mobile phone browser which cannot execute JavaScript,
  • have JavaScript execution disabled as a security precaution,
  • or be visually or otherwise disabled and use a speech browser

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.

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

Main articles: Cross-site scripting and Cross-site request forgery

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.

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 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).

Modern web browsers now integrate features to prevent XSS attacks.

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 can’t assume that his JavaScript runs as intended, or at all. Some implications:

  • Web site authors cannot perfectly conceal how their JavaScript operates, because the code is sent to the client, and obfuscated code can be reverse-engineered.
  • JavaScript form validation only provides convenience for users, not security. If a site verifies that the user agreed to its terms of service, or filters invalid characters out of fields that should only contain numbers, it must do so on the server, not only the client.
  • Scripts can be selectively disabled, so JavaScript can’t be relied on to prevent operations such as “save image”.
  • It would be extremely bad practice to embed a password in JavaScript (where it can be extracted by an attacker), then have JavaScript verify a user’s password and pass “password_ok=1” back to the server (since the “password_ok=1” response is easy to forge).

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 to an operating-system-enforced “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

Outside the web, 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.

  • ActionScript, the programming language used in Adobe Flash, is another implementation of the ECMAScript standard.
  • Apple’s Dashboard Widgets, Microsoft’s Gadgets, Yahoo! Widgets, Google Desktop Gadgets, Serence Klipfolio are implemented using JavaScript.
  • The Mozilla platform, which underlies Thunderbird, Firefox and some other web browsers, uses JavaScript to implement the graphical user interface (GUI) of its various products.
  • Adobe’s Acrobat and Adobe Reader (formerly Acrobat Reader) support JavaScript in PDF files.
  • Tools in the Adobe Creative Suite, including Photoshop, Illustrator, Dreamweaver and InDesign, allow scripting through JavaScript.
  • Microsoft’s Active Scripting technology supports the JavaScript-compatible JScript as an operating system scripting language.
  • The Java programming language, in version SE 6 (JDK 1.6), introduced the javax.script package, including a JavaScript implementation based on Mozilla Rhino. Thus, Java applications can host scripts that access the application’s variables and objects, much like web browsers host scripts that access the browser’s Document Object Model (DOM) for a webpage.
  • The Qt C++ toolkit includes a QtScript module to interpret JavaScript, analogous to javax.script.
  • OpenOffice.org office application suite allows for JavaScript as one of its scripting languages.
  • Adobe Integrated Runtime is a JavaScript runtime that allows developers to create desktop applications.
  • The interactive music signal processing software Max/MSP released by Cycling ’74, offers a JavaScript model of its environment for use by developers. It allows much more precise control than the default GUI-centric programming model.
  • Late Night Software’s JavaScript OSA (aka JavaScript for OSA, or JSOSA), is a freeware alternative to AppleScript for Mac OS X. It is based on the Mozilla 1.5 JavaScript implementation, with the addition of a MacOS object for interaction with the operating system and third-party applications.
  • ECMAScript was included in the VRML97 standard for scripting nodes of VRML scene description files.
  • Some high-end Philips universal remote panels, including TSU9600 and TSU9400, can be scripted using JavaScript.
  • Sphere is an open source and cross platform computer program designed primarily to make role-playing games that use JavaScript as a scripting language.
  • The open-source Re-Animator framework allows developing 2D sprite-based games using JavaScript and XML.
  • Methabot is a web crawler that uses JavaScript as scripting language for custom filetype parsers and data extraction using E4X.
  • The game engine Unity supports three scripting languages: JavaScript, C#, and Boo.
  • DX Studio (3D engine) uses the SpiderMonkey implementation of JavaScript for game and simulation logic.
  • webOS uses the WebKit implementation of JavaScript as a part of its application framework.
  • CA, Inc.’s AutoShell cross-application scripting environment is built on JavaScript/SpiderMonkey with preprocessor like extensions for command definitions and custom classes for various system related tasks like file i/o, operation system command invocation and redirection and COM scripting.
  • Maxwell Render provides an ECMA standard based scripting engine for tasks automation.
  • Google Docs Spreadsheet has a script editor which allows users to create custom formulas, automate repetitive tasks and also interact with other Google products such as Gmail.

Furthermore, it is possible to use JavaScript not as a scripting language, but as a general-purpose application programming language—as in GNOME Shell, the shell for the GNOME 3 desktop environment. The Seed, Gjs (from Gnome) and kjscmd/kjsembed (from KDE) packages are aimed to utilize that needs.

Debugging

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.

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 Apple’s Safari.

Some debugging aids are themselves bits of JavaScript code built to run on the Web. JSlint scans code for violations of a standard coding style. Web development bookmarklets and Firebug Lite provide variations on the idea of the cross-browser JavaScript console.

Since JavaScript is interpreted, loosely-typed, and may be hosted in varying environments, each incompatible with the others, a programmer has to take extra care to make sure the code executes as expected in as wide a range of circumstances as possible, and that functionality degrades gracefully when it does not.

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