The JavaScript language Notes PDF Download

The JavaScript language Notes PDF Download

What is JavaScript?

javaScript was initially created to “make web pages alive”.
The programs in this language are called scripts. They can be written right in a web page’s HTML
and run automatically as the page loads.
Scripts are provided and executed as plain text. They don’t need special preparation or
compilation to run.
In this aspect, JavaScript is very different from another language called Java  .
Why is it called JavaScript?
When JavaScript was created, it initially had another name: “LiveScript”. But Java was very
popular at that time, so it was decided that positioning a new language as a “younger brother”
of Java would help.
But as it evolved, JavaScript became a fully independent language with its own specification
called ECMAScript  , and now it has no relation to Java at all.
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”.
Different engines have different “codenames”. For example:
V8  – in Chrome and Opera.
SpiderMonkey  – in Firefox.
…There are other codenames like “Trident” and “Chakra” for different versions of IE,
“ChakraCore” for Microsoft Edge, “Nitro” and “SquirrelFish” for Safari, etc.
The terms above are good to remember because they are used in developer articles on the
internet. We’ll use them too. For instance, if “a feature X is supported by V8”, then it probably
works in Chrome and Opera

What CAN’T in-browser JavaScript do?
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 system 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 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,

What can in-browser JavaScript do?

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 which a
user has opened must not be able to access another browser tab with the URL 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.

Such limits do not exist if JavaScript is used outside of the browser, for example on a server.
Modern browsers also allow plugin/extensions which may ask for extended permissions.


The JavaScript language Notes PDF Download

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