JavaScript was originally created in order to make web-pages “live”. Programs in this language are called scripts. In the browser, they connect directly to HTML and, as soon as the page is loaded – immediately executed. JavaScript programs are plain text. They do not require any special training. In this respect, JavaScript is very different from another language, which is called Java[1-2].
Why JavaScript?
When the JavaScript language was created, it originally had a different name: “LiveScript”. But then the Java language was very popular, and marketers decided that a similar name would make the new language more popular. It was planned that JavaScript would be such a junior brother of Java. However, the story has its own way, JavaScript has grown strongly, and now it is a completely independent language, with its specification called ECMAScript, and has nothing to do with Java.
JavaScript can be executed not only in the browser, but anywhere, you need only a special program – the interpreter. The process of executing a script is called an “interpretation”.
What can JavaScript do?
Modern JavaScript is a “safe” programming language for general use. It does not provide low-level tools for working with memory, the processor, since it was initially focused on browsers that do not need it. As for other features, they depend on the environment in which JavaScript is running. In the browser, JavaScript can do everything related to manipulating the page, interacting with the visitor and, to some extent, with the server[1]:
• Create new HTML tags, delete existing ones, change element styles, hide, show items, etc.
• Respond to visitor actions, process mouse clicks, move the cursor, press a keyboard, and so on.
• Send requests to the server and download data without reloading the page (this technology is called “AJAX”).
• Receive and set up cookies, request data, display messages.
What does not JavaScript?
JavaScript is a fast and powerful language, but the browser imposes some restrictions on its execution.
This is done for the security of users, so that an attacker could not use JavaScript to obtain personal data or somehow damage the user’s computer.
These restrictions are not present where JavaScript is used outside the browser, for example, on a server. In addition, modern browsers provide their own mechanisms for installing plug-ins and extensions that have advanced capabilities, but require special installation actions from the user
Most JavaScript features in the browser are limited to the current window and page[3-5]:
• JavaScript can not read / write arbitrary files to the hard disk, copy them or call programs. It does not have direct access to the operating system.
Modern browsers can work with files, but this possibility is limited to a specially allocated directory – “sandbox”. Opportunities for access to devices are also worked out in modern standards and partially available in some browsers.
• JavaScript, which works in one tab, can not communicate with other tabs and windows, except when it opened this window or multiple tabs from the same source (the same domain, port, protocol).
There are ways to get around this, and they are covered in the tutorial, but they require special code for both documents that are in different tabs or windows. Without it, for security reasons, you can not climb from one tab to another with JavaScript.
• From JavaScript, you can easily send requests to the server from which the page came. A request for another domain is also possible, but less convenient, since there are security restrictions.
What is the uniqueness of JavaScript?
There are at least three remarkable JavaScript features:
• Full integration with HTML / CSS.
• Simple things are done simply.
• Supported by all popular browsers and enabled by default.
These three things at the same time are no longer in any browser technology.
Therefore, JavaScript is the most common means of creating browser interfaces.
References
- Введение в JavaScript. [Электронный ресурс]. URL: https://learn.javascript.ru/intro#что-такое-javascript (дата обращения 22.04.2017).
- JavaScript: от начала до конца. [Электронный ресурс]. URL: https://habrahabr.ru/post/165649/ (дата обращения 22.04.2017).
- JavaScript Blog. [Электронный ресурс]. URL: http://dailyjs.com/ (дата обращения 22.04.2017).
- Learn JavaScript. [Электронный ресурс]. URL: https://www.sitepoint.com/javascript/ (дата обращения 22.04.2017).
- What is JavaScript? [Электронный ресурс]. URL: http://www.computerhope.com/jargon/j/javascri.htm (дата обращения 22.04.2017).
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