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.

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.

console.log("Hello, World!"); let message = "Welcome to JavaScript";
What you should already know

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.

<div>HTML Structure</div>
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.

class MyClass { constructor() { } }
Hello world

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

function greet() { console.log("Hello, world!"); }

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

This simple example demonstrates the basic syntax of a JavaScript function and how to output text to the console.

Variables

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) and the characters "a" through "z" (lowercase).

You can use ISO 8859-1 or Unicode letters such as å and ü in identifiers. You can also use the Unicode escape sequences as characters in identifiers.

let x = 42; const PI = 3.14159;
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 and 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.

var oldWay = "function scoped"; let newWay = "block scoped";
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.

if (true) { let blockScoped = "inside"; }
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.

A constant cannot change value through assignment or be re-declared while the script is running. It must be initialized to a value.

The scope rules for constants are the same as those for let block scope variables. If the const keyword is omitted, the identifier is assumed to represent a variable.

const COMPANY_NAME = "Tech Corp";
Data types

The latest ECMAScript standard defines seven data types:

Six data types that are primitives: Boolean, null, undefined, Number, String, and Symbol (new in ECMAScript 2015).

and Object - a complex data type that allows you to store collections of data.

let isActive = true; // Boolean let name = "JavaScript"; // String
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; } condition can be any expression that evaluates to true or false.

You may also compound the statements using else if to have multiple conditions tested in sequence, as follows:

if (score >= 90) { grade = 'A'; } else if (score >= 80) { grade = 'B'; } else { grade = 'F'; }