Introduction of Java Script

JavaScript is a cross-platform, object-oriented scripting language used to make webpages interactive (e.g., having complex animations, clickable buttons, popup menus, etc.). There are also more advanced server side versions of JavaScript such as Node.js, which allow you to add more functionality to a website than downloading files (such as realtime collaboration between multiple computers). Inside a host environment (for example, a web browser), JavaScript can be connected to the objects of its environment to provide programmatic control over them.

JavaScript was introduced in 1995 as a way to add programs to web pages in the Netscape Navigator browser. The language has since been adopted by all other major graphical web browsers. It has made modern web applications possible—applications with which you can interact directly without doing a page reload for every action. JavaScript is also used in more traditional websites to provide various forms of interactivity and cleverness.

Java Script vs Java

It is important to note that JavaScript has almost nothing to do with the programming language named Java. The similar name was inspired by marketing considerations rather than good judgment. When JavaScript was being introduced, the Java language was being heavily marketed and was gaining popularity. Someone thought it was a good idea to try to ride along on this success. Now we are stuck with the name.

What Java Script 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 arbitrary files, 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”).

Variables in Java Script

A variable is a “named storage” for data. We can use variables to store goodies, visitors, and other data.

There are two limitations on variable names in JavaScript:

1. The name must contain only letters, digits, or the symbols $ and _.
2. The first character must not be a digit.

We can declare variables to store data by using the var, let, or const keywords.

• let – is a modern variable declaration. When variable values are changing in code at that time use let variable.
• var – is an old-school variable declaration. Normally we don’t use it at all, but we’ll cover subtle differences from let in the chapter The old "var", just in case you need them.
• const – is like let, but the value of the variable can’t be changed.

Variables should be named in a way that allows us to easily understand what’s inside them.

Conditional Branching Statements in Java Script

if statement

The if(...) statement evaluates a condition in parentheses and, if the result is true, executes a block of code.

if else statement

The if statement may contain an optional “else” block. It executes when the condition is falsy.

Functions in Java Script

Quite often we need to perform a similar action in many places of the script.

For example, we need to show a nice-looking message when a visitor logs in, logs out and maybe somewhere else.

Functions are the main “building blocks” of the program. They allow the code to be called many times without repetition.

We’ve already seen examples of built-in functions, like alert(message), prompt(message, default) and confirm(question). But we can create functions of our own as well.