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.

What you should already know

This guide assumes you have the following basic background:

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.

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.

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");

Variables

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

Declaring Variables

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

var x = 42.

Variable Scope

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 local variable, 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 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

Global Variables

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.

Constants

You can create a read-only, named constant with the const keyword. The syntax of a constant identifier is the same as for a variable identifier: it must start with a letter, underscore or dollar sign and can contain alphabetic, numeric, or underscore characters.

const PI = 3.14;

Data types

The latest ECMAScript standard defines seven data types:

if...else statement

Use the if statement to execute a statement if a logical condition is true. Use the optional else clause to execute a statement if the condition is false. An if statement looks as follows:

if (condition) { statement_1; } else { statement_2; }