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 simply 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 contains a standard library of objects, such as Array, Date, and Math, and a core set of language elements such as operators, control structures, and statements. Core JavaScript can be extended for a variety of purposes by supplementing it with additional objects; for example:

• Client-side JavaScript extends the core language by supplying objects to control a browser and its Document Object Model (DOM). For example, client-side extensions allow an application to place elements on an HTML form and respond to user events such as mouse clicks, form input, and page navigation.


• Server-side JavaScript extends the core language by supplying objects relevant to running JavaScript on a server. For example, server-side extensions allow an application to communicate with a database, provide continuity of information from one invocation to another of the application, or perform file manipulations on a server.

This guide assumes you have the following basic background:

• A general understanding of the Internet and the World Wide Web (WWW).


• Good working knowledge of HyperText Markup Language (HTML).


• Some programming experience. If you are new to programming, try one of the tutorials linked on the main page about JavaScript.

JavaScript and Java are similar in some ways but fundamentally different in some others. The JavaScript language resembles Java but does not have Java's static typing and strong type checking. JavaScript follows most Java expression syntax, naming conventions and basic control-flow constructs which was the reason why it was renamed from LiveScript to JavaScript.

In contrast to Java's compile-time system of classes built by declarations, JavaScript supports a runtime system based on a small number of data types representing numeric, Boolean, and string values. JavaScript has a prototype-based object model instead of the more common class-based object model. The prototype-based model provides dynamic inheritance; that is, what is inherited can vary for individual objects. JavaScript also supports functions without any special declarative requirements. Functions can be properties of objects, executing as loosely typed methods.

JavaScript is a very free-form language compared to Java. You do not have to declare all variables, classes, and methods. You do not have to be concerned with whether methods are public, private, or protected, and you do not have to implement interfaces. Variables, parameters, and function return types are not explicitly typed.

Java is a class-based programming language designed for fast execution and type safety. Type safety means, for instance, that you can't cast a Java integer into an object reference or access private memory by corrupting Java bytecodes. Java's class-based model means that programs consist exclusively of classes and their methods. Java's class inheritance and strong typing generally require tightly coupled object hierarchies. These requirements make Java programming more complex than JavaScript programming.

In contrast, JavaScript descends in spirit from a line of smaller, dynamically typed languages such as HyperTalk and dBASE. These scripting languages offer programming tools to a much wider audience because of their easier syntax, specialized built-in functionality, and minimal requirements for object creation.

To get started with writing JavaScript, open the Web Console in multi-line mode, and write your first "Hello world" JavaScript code:

        (function(){ "use strict"; /* Start of your code */ function
          greetMe(yourName) { alert('Hello ' + yourName); } greetMe('World'); /*
          End of your code */ })();

You use variables as symbolic names for values in your application. The names of variables, called identifiers, conform to certain rules.

A JavaScript identifier must start with a letter, underscore (_), or dollar sign ($). Subsequent characters can also be digits (0–9).

Because JavaScript is case sensitive, letters include the characters "A" through "Z" (uppercase) as well as "a" through "z" (lowercase).

You can use most of ISO 8859-1 or Unicode letters such as å and ü in identifiers. (For more details, see this blog post.) You can also use the Unicode escape sequences as characters in identifiers.

Some examples of legal names are Number_hits, temp99, $credit, and _name.

You can declare a variable in two ways:

• With the keyword var. For example, var x = 42. This syntax can be used to declare both local and global variables, depending on the execution context.


• With the keyword const or let. For example, let y = 13. This syntax can be used to declare a block-scope local variable. (See Variable scope below.)

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a localvariable, because it is available only within that function.

JavaScript before ECMAScript 2015 does not have block statement scope. Rather, a variable declared within a block is local to the function (or global scope) that the block resides within.

For example, the following code will log 5, because the scope of x is the global context (or the function context if the code is part of a function). The scope of x is not limited to the immediate if statement block.

        if (true) { var x = 5; } console.log(x); // x is 5
      

This behavior changes when using the let declaration (introduced in ECMAScript 2015).

        if (true) { let y = 5; } console.log(y); // ReferenceError: y is not
          defined

Global variables are in fact properties of the global object.

In web pages, the global object is window, so you can set and access global variables using the window.variable syntax.

Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a document, you can refer to this variable from an iframe as parent.phoneNumber.

All the documentation in this page is taken from MDN