Lets Learn About JavaScript - HadNur

JavaScript is a high-level, dynamic, untyped, and interpreted programming language. It has been standardized in the ECMAScript language specification. Alongside HTML and CSS, it is one of the three essential technologies of World Wide Web content production; the majority of websites employ it and it is supported by all modern web browsers without plug-ins. JavaScript is prototype-based with first-class functions, making it a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles. It has an API for working with text, arrays, dates and regular expressions, but does not include any I/O, such as networking, storage or graphics facilities, relying for these upon the host environment in which it is embedded.

Despite some naming, syntactic, and standard library similarities, JavaScript and Java are otherwise unrelated and have very different semantics. The syntax of JavaScript is actually derived from C, while the semantics and design are influenced by the Self and Scheme programming languages.

JavaScript is also used in environments that are not web-based, such as PDF documents, site-specific browsers, and desktop widgets. Newer and faster JavaScript virtual machines (VMs) and platforms built upon them have also increased the popularity of JavaScript for server-side web applications. On the client side, JavaScript has been traditionally implemented as an interpreted language, but more recent browsers perform just-in-time compilation. It is also used in game development, the creation of desktop and mobile applications, and server-side network programming with runtime environments such as Node.js.

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 consisted of 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 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.

With the rise of the single-page web app and JavaScript-heavy sites, it is increasingly being used as a compile target for source-to-source compilers from both dynamic languages and static languages. In particular, Emscripten and highly optimized JIT compilers, in tandem with asm.js that is friendly to AOT compilers like OdinMonkey, have enabled C and C++ programs to be compiled into JavaScript and execute at near-native speeds, causing JavaScript to be considered the “assembly language of the web”,[34] according to its creator and others.

The most common use of JavaScript is to add client-side behavior to HTML pages, a.k.a. Dynamic HTML (DHTML). Scripts 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:

  • Loading new page content or submitting data to the server via AJAX without reloading the page (for example, a social network might allow the user to post status updates without leaving the page)
  • Animation of page elements, fading them in and out, resizing them, moving them, etc.
  • Interactive content, for example games, and playing audio and video
  • Validating input values of a Web form to make sure that they are acceptable before being submitted to the server.
  • Transmitting information about the user’s reading habits and browsing activities to various websites. Web pages frequently do this for web analytics, ad tracking, personalization or other purposes.

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 that 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 create “host objects” to represent the Document Object Model (DOM) in JavaScript. The web server is another common host environment. A JavaScript web server would typically expose host objects representing HTTP request and response objects, which a JavaScript program could then interrogate and 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.

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. There are subsets of general JavaScript — ADsafe, Secure ECMA Script (SES) — that provide greater level of security, especially on code created by third parties (such as advertisements). Content Security Policy is the main intended method of ensuring that only trusted code is executed on a web page.

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 integrated within Internet Explorer, Firefox, Safari, Google Chrome, Opera and Node.js

In addition to the native Internet Explorer Developer Tools, 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 that 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).

In comparison to Internet Explorer, Firefox has a more comprehensive set of developer tools, which include a debugger as well. Old versions of Firefox without these tools used a Firefox addon called Firebug, or the older Venkman debugger. Also, WebKit’s Web Inspector includes a JavaScript debugger, which is used in Safari. A modified version called Blink DevTools is used in Google Chrome. Node.js has node-inspector, an interactive debugger that integrates with the Blink DevTools, available in Google Chrome. Last but not least, Opera includes a set of tools called Dragonfly.

In addition to the native computer software, there are online JavaScript IDEs, debugging aids are themselves written in JavaScript and built to run on the Web. An example is the program JSLint, developed by Douglas Crockford who has written extensively on the language. JSLint scans JavaScript code for conformance to a set of standards and guidelines. Many libraries for JavaScript, such as three.js, provide links to demonstration code that can be edited by users. They are also used as a pedagogical tool by institutions such as Khan Academy to allow students to experience writing code in an environment where they can see the output of their programs, without needing any setup beyond a web browser. On October 6, 2008 Remy Sharp release jsbin, one of the earliest environments to support live updates of JavaScript, CSS, and HTML. He credits John Resig’s Learning App, as inspiration.

JSON, or JavaScript Object Notation, is a general-purpose data interchange format that is defined as a subset of JavaScript’s object literal syntax. Like much of JavaScript (regexps and anonymous functions as 1st class elements, closures, flexible classes, ‘use strict’), JSON, except for replacing Perl’s key-value operator ‘=>’ by an RFC 822 inspired ‘:’, is syntactically pure Perl.

jQuery is a popular JavaScript library designed to simplify DOM-oriented client-side HTML scripting along with offering cross-browser compatibility because various browsers respond differently to certain vanilla JavaScript code. Underscore.js is a utility JavaScript library for data manipulation that is used in both client-side and server-side network applications.

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.

asm.js is a subset of JavaScript that can be run in any JavaScript engine or run faster in an ahead-of-time (AOT) compiling engine.

 

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