JS Documentation Page

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.

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.

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.

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.

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!

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).

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.

With the keyword let. For example,

let y = 13.

Variable scope

When you declare a variable outside of any function, it is called a global variable. When you declare a variable within a function, it is called a local variable.

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

Using the let declaration introduced in ECMAScript 2015:

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

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.

const PI = 3.14;

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

const MY_OBJECT = {"key": "value"};
MY_OBJECT.key = "otherValue";

Data types

The latest ECMAScript standard defines seven data types:

Boolean. true and false.
null. A special keyword denoting a null value.
undefined. A value that is not defined.
Number. 42 or 3.14159.
String. "Howdy"
Symbol. Unique and immutable instances.
Object

if...else statement

Use the if statement to execute a statement if a logical condition is true.

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

while statement

A while statement executes its statements as long as a specified condition evaluates to true.

while (condition)
  statement

Example loop:

var n = 0;
var x = 0;
while (n < 3) {
  n++;
  x += n;
}

Function declarations

A function definition consists of the function keyword, followed by the name, arguments, and statements.

function square(number) {
  return number * number;
}

Reference

All documentation on this page is sourced from MDN.