Introduction

JavaScript is a cross-platform, object-oriented scripting language. It is a small and lightweight language. 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.

JavaScript and Java

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.

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.

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.

Hello world

To get started with writing JavaScript, open the Scratchpad and write your first "Hello world" JavaScript code:

function greetMe(yourName) { alert("Hello " + yourName); }
          greetMe("World");

Select the code in the pad and hit Ctrl+R to watch it unfold in your browser!

Declaring variables

You can declare a variable in three ways: With the keyword var. For example,

var x = 42.

This syntax can be used to declare both local and global variables. By simply assigning it a value. For example,

x = 42.

This always declares a global variable. It generates a strict JavaScript warning. You shouldn't use this variant. With the keyword let. For example,

let y = 13.

This syntax can be used to declare a block scope local variable. See Variable scope below.

Variable Scope

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 function (or global context) within which x is declared, not the block, which in this case is an if statement.

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