Javascript was created to make web pages responsive. With Javascript you’ll be able to create reponsive web pages, games, animated 2D and 3D graphics, comprehensive database-driven apps, and much more. Today, JavaScript can execute not only in the browser, but also on the server, or actually on any device that has a special program called  the JavaScript engine. The browser has an embedded engine sometimes called a “JavaScript virtual machine”.

JavaScript virtual machine is complicated. But the basics are easy.

1. The engine (embedded if it’s a browser) reads (“parses”) the script.
2. Then it converts (“compiles”) the script to the machine language.
3. And then the machine code runs, pretty fast.

The engine applies optimizations at each step of the process. It even watches the compiled script as it runs, analyzes the data that flows through it, and further optimizes the machine code based on that knowledge.

There are at least three great things about JavaScript which is the only browser technology that combines these three things.

• Full integration with HTML/CSS.
• Simple things are done simply.
• Support by all major browsers and enabled by default.

 What can JavaScript do in browser? 

Modern JavaScript is a “safe” programming language. It does not provide low-level access to memory or CPU, because it was initially created for browsers which do not require it.

JavaScript’s capabilities greatly depend on the environment it’s running in. For instance,  Node.js  supports functions that allow JavaScript to read/write to disk, perform network requests, etc.

In-browser JavaScript can do everything related to webpage manipulation, interaction with the user, and the webserver.

For instance, in-browser JavaScript is able to:

• Add new HTML to the page, change the existing content, modify styles.
• React to user actions, run on mouse clicks, pointer movements, key presses.
• Send requests over the network to remote servers, download and upload files (so-called  AJAX  and  COMET  technologies).
• Get and set cookies, ask questions to the visitor, show messages.
• Remember the data on the client-side (“local storage”).

 What can’t JavaScript do in browser? 

JavaScript’s abilities in the browser are limited for the sake of the user’s safety. The aim is to prevent an evil webpage from accessing private information or harming the user’s data.

Examples of such restrictions include:

• JavaScript on a webpage may not read/write arbitrary files on the hard disk, copy them or execute programs. It has no direct access to OS functions.Modern browsers allow it to work with files, but the access is limited and only provided if the user does certain actions, like “dropping” a file into a browser window or selecting it via an input tag.There are ways to interact with camera/microphone and other devices, but they require a user’s explicit permission. So a JavaScript-enabled page may not sneakily enable a web-camera, observe the surroundings and send the information to the  NSA .
• Different tabs/windows generally do not know about each other. Sometimes they do; for example when one window uses JavaScript to open the other one. But even in this case, JavaScript from one page may not access the other if they come from different sites (from a different domain, protocol or port).This is called the “Same Origin Policy”. To work around that, both pages must agree for data exchange and contain a special JavaScript code that handles it. We’ll cover that in the tutorial.This limitation is, again, for the user’s safety. A page from http://anysite.com which a user has opened must not be able to access another browser tab with the URL http://gmail.com and steal information from there.
• JavaScript can easily communicate over the net to the server where the current page came from. But its ability to receive data from other sites/domains is crippled. Though possible, it requires explicit agreement (expressed in HTTP headers) from the remote side. Once again, that’s a safety limitation.